Supraventricular tachycardia in children
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《美国医学杂志》
4C2 WMC Health Sciences Centre, Division of Pediatric Cardiology, Department of Pediatrics, University of Alberta, Edmonton, Alberta, T6G 2B7, Canada
Abstract
Several different mechanisms are responsible for paroxysmal supraventricular tachycardia in children. Different forms of tachycardia occur at different age. Atrio-ventricular reentry tachycardia results from the presence of congenital atrio-ventricular bypass tracts and is frequently encountered at all ages. Infants may present with ectopic atrial tachycardia or atrial flutter. Atrio-ventricular node reentry tachycardia becomes more frequent in adolescence. Atrial scarring resulting from open heart surgery predisposes to complex intra-atrial reentry. Certain forms of congenital and acquired heart disease are associated with specific types of arrhythmia. Many children with paroxysmal supraventricular tachycardia do not require any therapy. The decision to proceed with treatment should be based on the frequency and severity of symptoms and on the effect of arrhythmia on the quality of life. Infants require medical treatment because of the difficulty to recognize symptoms of tachycardia and a risk of heart failure. Patients with Wolff-Parkinson-White syndrome as well as those with significant heart disease are at risk of sudden death. Syncope in children with paroxysmal tachycardia may indicate a severe fall in cardiac output from extremely rapid heart rate. Patients with potentially life-threatening arrhythmia should not participate in competitive physical activities. Treatment options have undergone significant evolution over the past decade. Indications for the use of specific antiarrhythmic medications have been refined. Contemporary catheter ablation procedures employ different forms of energy allowing for safe and effective procedures. Catheter ablation is the treatment of choice for symptomatic paroxysmal tachycardia in school children and in some infants who failed medical treatment. Surgery is the preferred treatment in few selected cases. The goal of this review is to present the state of the art approach to the diagnosis and management of paroxysmal supraventricular tachycardia in infants, children and adolescents.
Keywords: Supraventricular tachycardia; Arrhythmia; Heart failure; Children
Paroxysmal supraventricular tachycardia (PSVT) is the most common symptomatic arrhythmia in young patients. It affects children of all ages. Its prevalence is estimated at more than one in 500 children. A pediatrician may see several new patients with PSVT in a year. A child with new onset PSVT is likely to present with the same paroxysmal tachycardia for at least 10 subsequent years. Different forms of PSVT present at different patient age.[1]
The goal of this review is to provide practical guidelines regarding diagnosis and management of supraventricular tachycardia in infants, children and adolescents. The review explains pathophysiology of different forms of PSVT and risks of heart failure or sudden death.
Presentation of PSVT in infants
Symptoms of PSVT in infancy differ from those in childhood and adolescence. A newborn may present with history of fetal tachycardia or with signs of left ventricular dysfunction from tachycardia in fetal life. Hydrops fetalis represents severe heart failure from persistent rapid tachycardia. Frequently, PSVT becomes less frequent and easier to manage after birth.[2] Conversely, neonates with no history of fetal tachycardia may present with incessant and difficult to treat pathological tachycardia after birth. PSVT may occur in the first days of life or somewhat later in the first year of life. Most infants with PSVT have structurally normal hearts. In 15% of infants, paroxysmal tachycardia is associated with heart disease, drug administration or a febrile illness. Incessant tachycardia at a rate of more than 200/min leads to progressive left ventricular dysfunction. Persistent very rapid tachycardia at a rate of 250/min or more may produce life threatening heart failure within few days. Symptoms of PSVT in infants are inconspicuous and include irritability, poor feeding, tachypnea, diaphoresis and poor color. When counseling parents about risks of recurrent PSVT, it is important to stress that symptoms of tachycardia masquerade those of many other common illnesses in infancy. It is prudent to advise parents to check infant's heart rate when the child is quiet or asleep. Parents may verify heart rate at the time of feeding by placing hand on the child's chest or with an inexpensive stethoscope. In infants, atrio-ventricular reentry tachycardia (AVRT) resulting from the presence of an accessory atrio-ventricular (A-V) bypass tract accounts for approximately 80% of all cases of PSVT. Permanent junctional reciprocating tachycardia (PJRT) and automatic atrial tachycardia are much less common but they frequently pose a therapeutic challenge. Atrial flutter may occur in newborns and usually resolves within few days or with administration of digoxin. When tachycardia is recognized, it is important to obtain an echocardiogram in order to rule out structural heart disease and to assess ventricular function.
Presentation of PSVT in children and adolescents
While obtaining clinical history from children, physicians have to inquire about the sensation produced by arrhythmia, how frequently symptoms occur, how long they persist each time and what other symptoms accompany arrhythmia. It is important to learn in detail about each episode of rhythm disturbance. Physicians may choose to inquire about the most recent bout of arrhythmia first and then ask about each previous episode. Frequently physicians have to put much effort into prompting their patients to report everything they can remember regarding presentation and circumstances surrounding each attack of arrhythmia. It is essential to inquire if there is any family history of arrhythmia or any family history of sudden death or a convulsive disorder which may represent life threatening arrhythmia.
Typically, PSVT will produce impression of a very rapid and regular heart beat that is very difficult or outright impossible to count. Patients may describe that the heart is fluttering in the throat. A sensation of somewhat accelerated pounding (painful) heart beat in the chest usually indicates sinus tachycardia. In general, PSVT presents with heart rates well in excess of 150/min while sinus tachycardia at rest does not exceed such rate even in a stressful situation.
As a rule, atrio-ventricular node reentry tachycardia (AVNRT) is triggered by vigorous physical activity while AVRT mediated by an A-V bypass may also occur at rest. Patients may recognize a sudden onset of PSVT even during sinus tachycardia. They report that the heart rate suddenly accelerates and does not slow down for several minutes or longer even though they stop physical activity and lie down. Typical PSVT also terminates suddenly as if turned off with a switch. Gradual acceleration and deceleration of tachycardia usually indicate sinus tachycardia or ectopic (automatic) atrial tachycardia. Infrequently, children affected by paroxysmal tachycardia of truly sudden onset and offset may still report that the tachycardia starts suddenly but terminates gradually or it begins and wanes gradually.
The majority of older children and adolescents provide very precise description of their symptoms allowing for a secure diagnosis of PSVT (although it is always preferred to record a rhythm strip with arrhythmia). Even children as young as 3 years of age may recognize symptoms of PSVT. "My heart is racing, beeping, skipping, hurting, jumping up and down" - these are only few of many descriptions that young children may use to explain their sensation of tachycardia. Sometimes, children are able to give merely nebulous history of vague palpitations especially if they cannot recognize when and how their arrhythmia starts or what it feels like. Preschool children may only say that their heart hurts. At times, children may complain of general malaise or indistinct pressure or discomfort in the throat. Some children may complain of isolated headaches. Few patients might deny any symptoms.
Tachycardia may be accompanied by fatigue, chest discomfort, shortness of breath or lightheadedness. Severe cardiovascular compromise produces more pronounced dyspnea and lightheadedness. Syncope is unusual and may indicate life threatening arrhythmia which necessitates rapid diagnosis and treatment. Older children and adolescents may present with two or more forms of arrhythmia and it is important to distinguish what symptoms represent each heart rhythm disturbance such as paroxysmal tachycardia, accelerated sinus rhythm or premature contractions. It is very important to remember that clinical symptoms of relatively benign PSVT are not different from those produced by life threatening ventricular tachycardia.
Risk of Cardiac Arrest
The risk of cardiac arrest from PSVT in children and adolescents is very low. Three general groups of children at risk of life threatening heart failure or sudden death are neonates with PSVT, patients with Wolff-Parkinson-White (WPW) syndrome and children with heart disease.
Nonspecific presentation of PSVT in infants makes it difficult to recognize. It is essential to remember that cardiac reserve in neonates is very small and typical supraventricular tachycardia with heart rate exceeding 200/min may lead to life threatening myocardial dysfunction within several days.[3] Neonates may already present with subclinical heart failure from tachycardia before birth. Every neonate with PSVT should be evaluated with an echocardiogram in order to rule out structural heart disease and to assess ventricular function. Administration of intravenous adenosine in order to terminate PSVT or to make precise diagnosis of tachycardia is safe. It is important to remember that all antiarrhythmic medications with the exception of digoxin exert a negative inotropic effect on the myocardium and therefore may produce significant hypotension and lead to cardiac arrest. Even oral administration of a beta blocker in an infant with impaired ventricular function may lead to life threatening arterial hypotension. All newborns should be hospitalized and their vital signs should be closely monitored during initial therapy with all antiarrhythmic agents.
The risk of sudden death in adult patients with WPW syndrome is well recognized. Cardiac arrest is usually precipitated by atrial fibrillation which is transmitted into ventricles through a rapid conducting A-V bypass tract. Atrial fibrillation is uncommon in children, nevertheless sudden death is a well known complication of WPW syndrome in young patients as well Figure1A, B and Figure2. Patients at high risk are those in whom the shortest interval between two subsequent preexcited ventricular beats during atrial fibrillation is less than 220 milliseconds.[4], [5] This measurement could be taken from an ECG or a rhythm strip recorded at the time of atrial fibrillation or during electrophysiologic testing of the A-V bypass tract. Syncope in patients with WPW syndrome may indicate high risk of sudden death although sudden death may be the very first presentation of life-threatening arrhythmia. Even a single attack of syncope in a child with WPW syndrome should lead to electrophysiology studies and ablation of the bypass tract. Successful ablation of the A-V bypass tract eliminates the risk of sudden death from WPW syndrome.[6] Strenuous physical activities shorten the refractory period of the bypass tract and may precipitate atrial fibrillation or flutter. Sometimes, ventricular preexcitation resolves during physical activity, for instance during an exercise stress test. It is not known, however, if these patients are at low risk of cardiac arrest. Competitive sports should be discouraged in children with ventricular preexcitation on a surface ECG.
Many forms of congenital and acquired heart disease may impair ventricular function or disturb coronary circulation and thus increase risks associated with PSVT. Rapid tachycardia shortens filling time of ventricles and decreases stroke volume and cardiac output. Coronary blood flow is reduced. Patients with borderline left ventricular function are at particularly high risk of life threatening arterial hypotension from PSVT. Few examples of heart disease leading to ventricular dysfunction in children include myocarditis, dilated cardiomyopathy, congenital defects producing large shunts (in infants and small children) and complex heart anomalies. Stenotic and regurgitant lesions of heart valves significantly decrease stroke volume which is already compromised during tachycardia because of abnormally short ventricular filling and ejection times. PSVT in patients with pulmonary hypertension is life threatening. Physicians have to keep in mind that many patients with operated heart disease are left with considerable ventricular dysfunction even though parents may inform them that the heart was "fixed".
Certain forms of heart disease predispose to the occurrence of certain types of PSVT. Patients with hypertrophic cardiomyopathy are at risk of atrial fibrillation which produces a major reduction in the left ventricular stroke volume in a setting of a thick, non-compliant myocardium and leads to myocardial ischemia. Rheumatic disease of the mitral valve is also associated with paroxysmal atrial fibrillation and a risk of thromboembolic events. Children with the Ebstein anomaly of the tricuspid valve present with AVRT which may produce significant hemodynamic derangement in a setting of severe tricuspid insufficiency. Children with Kawasaki disease and coronary artery abnormalities are at risk of myocardial ischemia and infarction. Children and adults with operated congenital or acquired heart disease may present with incisional atrial reentry tachycardia (IART). This rapid tachycardia at atrial rates of 180 - 250 resembles atrial flutter although it is more difficult to manage. Detailed discussion of risks associated with PSVT in patients with heart disease falls outside of the scope of this review.
Presence of severe non-cardiac disease may augment symptoms associated with PSVT.
Diagnosis
The diagnosis of PSVT is based on thorough clinical history and electrocardiograms (ECG's) or rhythm strips recorded at the time of tachycardia and during sinus rhythm. It is very important to encourage patients and their care givers to go to the emergency room or to their clinic in order to document prolonged attacks of arrhythmia so that their physician can make a proper diagnosis. An ECG has to be repeated after tachycardia resolves. Resting ECGs have to be recorded on several occasions since ventricular preexcitation may be intermittent and may not be obvious on each tracing. Different types of PSVT present differently on a 12 lead ECG. An ECG recording of the tachycardia allows a cardiologist to design the best possible treatment. A single lead tracing should be recorded only if a complete 12 lead ECG cannot be done. Clinicians should attempt to identify P waves representing atrial activation during tachycardia. Short PR tachycardia is such PSVT when the PR interval is shorter than the RP interval. Short PR tachycardia includes sinus tachycardia, ectopic atrial tachycardia, PJRT and an atypical AVNRT. Short RP tachycardia is such PSVT when the RP interval is short and the PR interval is long. The P wave is usually superimposed on the T wave and very close to the preceding QRS complex. The P wave may be recognizable in few leads only and usually in limb lead II and in precordial leads V1 and V2. Short RP tachycardia includes AVRT mediated by an A-V bypass tract (WPW syndrome) and typical AVNRT.
An exercise stress test may induce tachycardia and allow for its documentation. Ambulatory Holter monitoring may be successful in recording paroxysmal tachycardia in patients with frequent recurrences of PSVT. Holter is not likely to document PSVT in patients with very infrequent bouts of tachycardia although it may show intermittent ventricular preexcitation pointing to WPW syndrome. Clinicians are more likely to document heart rhythm responsible for sporadic palpitations and other symptoms suggestive arrhythmia if they choose ambulatory event recorders which patients may keep at home for several weeks.
Brief and infrequent attacks of palpitations even accompanied by significant symptoms are very difficult to record. Symptoms usually resolve by the time patients arrive at the emergency room or in an ambulatory clinic. Clinicians may choose to teach patients or their parents how to take pulse rate or how to count heart rate with a stethoscope. Patients may want to start a diary indicating the date of their symptoms, their activity when palpitations or tachycardia occurred, the pulse rate or the heart rate as well as accompanying symptoms and duration of symptoms.
PSVT with wide QRS complexes
Infrequently, an electrocardiogram recorded during PSVT may show tachycardia with wide QRS complexes. This may be the case in the following situations:
Antidromic reciprocating tachycardia in patients with WPW syndrome Figure3.
PSVT with right or left bundle branch block (aberrant ventricular conduction).
PSVT with bystander participation of an accessory A-V bypass tract
Electrocardiograms recorded during tachycardia and in sinus rhythm frequently allow a clinician to establish proper diagnosis. Presence of ventricular preexcitation during sinus rhythm may point to antidromic reciprocating tachycardia or bystander participation of an accessory A-V bypass tract in patients with WPW syndrome. The term "bystander participation of a bypass tract" implies that a bypass tract transmits rapid atrial activation to the ventricles but the mechanism of tachycardia does not include a bypass tract ( e.g. atrial fibrillation, AVNRT, ectopic atrial tachycardia, etc.). Infrequent presence of right or left bundle branch block during sinus rhythm indicates that tachycardia with wide QRS complexes represents aberrant conduction as long as the morphology of QRS complexes is the same during tachycardia and during sinus rhythm. Frequently aberrant ventricular conduction is rate dependent which means that right or left bundle branch block occurs only at rapid heart rates. Rate dependent aberrant conduction with right bundle branch block is more frequent than aberrant conduction with left bundle branch block.
Ventricular tachycardia (VT) is uncommon in children and adolescents. In general, idiopathic VT is not a life threatening condition although it may produce syncope. Life threatening VT may complicate different forms of cardiomyopathy or congenital heart defects (especially "repaired" or palliated heart disease). Ventricular tachycardia will always present with wide, abnormal looking QRS complexes. Only in infants, VT may present with relatively narrow QRS complexes. A-V dissociation is not always the feature of VT and at times it is not possible to distinguish VT from wide QRS complex PSVT in patients with ventricular preexcitation. Patients with wide QRS tachycardia should have an echocardiogram and should be evaluated by a pediatric cardiologist.
Acute treatment
Regardless of the type of PSVT there are certain general principles of therapy which apply to all patients. Physicians should teach their patients maneuvers which increase parasympathetic (vagal) tone, slow down conduction through the A-V node and break the reentry circuit responsible for PSVT in most cases. An ice-bag can be used at any age with good success.[7] It is important to place an ice-bag around the nose and mouth in order to make it effective. Immersion of the face in ice-cold water may also terminate PSVT. In the Valsalva maneuver patients are asked to take a deep breath and bear down. Valsalva maneuver may be combined with other measures such as an ice-bag or pressure on the abdomen. Carotid sinus massage, retching or gagging induced by a finger or a spoon, or blowing on a thumb may also be effective. Different measures may be successful in different children. Standing on the head (a rather tricky operation) seems to work well for some adolescents. Applying pressure on the eyes with closed eyelids must never be used because of a significant risk of injury to the eyes and retinal detachment.
Acute treatment in the emergency room depends on patient's symptoms. Children with unstable vital signs or signs of heart failure require electric cardioversion. Stable vital signs and normal blood pressure allow for less aggressive measures such as those described above. Intravenous administration of adenosine as a rapid bolus is safe in children of all ages Figure4. Adenosine is quickly cleared from circulation by cellular uptake, mostly by endothelial cells and erythrocytes. The half life in the blood stream is less than 10 seconds. The standard dose ranges from 50 to 200 micrograms per kilogram of body weight. The most commonly observed side effects are transient chest discomfort, dyspnea, facial flushing, sinus pauses and A-V block. Significant bradycardia may occur in patients with sinus node dysfunction, A-V conduction defects or after concurrent administration of other medications which affect the A-V node ( e.g. beta blockers, calcium channel blockers, amiodarone and other). Adenosine may produce bronchoconstriction in patients with asthma.
Following administration of adenosine, it is not unusual to see PSVT break for few seconds and resume with vengeance. It is very important to print a rhythm strip when adenosine is pushed in and review an ECG tracing after each injection. A rhythm strip may show sudden termination of tachycardia followed by merely one or several normal sinus beats and resumption of PSVT. In such cases a longer acting intravenous medication could be considered such as a beta blocker (propranolol), digoxin, procainamide, a calcium channel blocker (verapamil) or amiodarone. Calcium channel blockers should not be administered to children under 2 years of age because of a risk of cardiovascular collapse. One should remember that intravenous administration of many antiarrhythmic agents may produce significant arterial hypotension or induce life-threatening arrhythmia. It is important to follow guidelines regarding administration of each medication in regards to the correct dose and rate of intravenous infusion. Antiarrhythmic medications are grouped according to their electrophysiologic properties which allows for their proper use.[8]
A rhythm strip recorded during administration of adenosine may uncover atrial flutter or P waves of atrial tachycardia. Adenosine is not effective in converting atrial flutter or fibrillation into sinus rhythm but it is helpful in establishing correct diagnosis.
Chronic management
Chronic management of PSVT should be individualized. In general, the decision to treat a child with PSVT depends on the child's age, symptoms produced by paroxysmal tachycardia and the effect of PSVT on the quality of life. Prophylactic treatment with antiarrhythmic medications is prescribed for infants younger than one year of age because of nonspecific nature of symptoms and the risk of life-threatening heart failure from unrelenting rapid tachycardia.[9] Beta blockers are prescribed most frequently. Digoxin may be used in infants without ventricular preexcitation. One could also prescribe sotalol, flecainide or amiodarone in selected cases. At one year of age, prophylactic treatment could be withheld in order to see if tachycardia recurs. More than one half of toddlers with history of PSVT after birth will be free from tachycardia at least until school age when children are able to recognize symptoms of arrhythmia. Prophylactic treatment with medications may be extended into preschool years if tachycardia recurs or parents are unable to recognize recurrent arrhythmia. The dose of every medication should be increased over time in order to keep up with the child's weight gain. Patient's compliance improves if prescribed medication could be taken once or twice a day. Medications may become ineffective over time and may have to be replaced by other agents.
In school-aged children, the decision to treat PSVT usually depends on its effect on the quality of life. Infrequent PSVT accompanied by mild symptoms does not warrant any treatment especially if a child is able to interrupt tachycardia with appropriate maneuvers. Consumption of caffeine (coffee, tea, soft drinks containing caffeine) may increase the likelihood of recurrent tachycardia. Physicians may advise patients against taking such substances if it is clear that relapses of PSVT are associated with their intake or PSVT produces significant symptoms. Use of medications that augment sympathetic tone or those that suppress vagal tone may increase the likelihood of another attack of tachycardia. Patients report more problems with PSVT if they take beta adrenergic medications (salbutamol) or certain decongestants (pseudoephedrine, antihistamines). It is crucial to treat all patients with WPW syndrome who present with syncope or who participate in competitive sports. This recommendation also applies to isolated ventricular preexcitation with no history of paroxysmal tachycardia. It is important to remember that prophylactic treatment with medications does not cure the child from tachycardia but merely suppresses arrhythmia.
Symptomatic relief from PSVT can be achieved by implantation of a permanent anti-tachycardia pacemaker. Such device can recognize occurrence of paroxysmal tachycardia and interrupt it with overdrive atrial pacing. Anti-tachycardia pacemakers are considered only in selected symptomatic patients who failed medical treatment and ablation procedures.
Catheter Ablation Procedures
Elimination of the arrhythmic substrate and permanent cure from almost all forms of PSVT can be achieved through percutaneous catheter ablation or by means of surgery. [10],[11],[12] Catheter ablation procedures are performed in an electrophysiology laboratory and are similar to cardiac catheterization with the use of fluoroscopy. Radiofrequency current (RF) ablation is employed most frequently. During the procedure, a small metal tip of the catheter is heated to 50-60oC by alternating current at 350 kHz to 1 MHz.[13] This relatively low temperature produces a permanent small scar measuring approximately 4 mm in diameter and 4 mm in depth. An RF application directly to the arrhythmic substrate will destroy it permanently and prevent recurrences of paroxysmal tachycardia. A cardiologist has to apply from one to several "burns" before an RF application lands directly on the target. The initial success rate of RF ablation exceeds 90%. Infrequently, PSVT may recur if ablation lesions did not damage but only temporarily injured the arrhythmic focus. Large ablation lesions are produced by radiofrequency irrigation catheters and are frequently required for ablation procedures in patients with atrial flutter or postoperative atrial tachycardia. Cryoablation is employed in such cases when an arrhythmic substrate is located close to the A-V node. Cooling a metal tip of the catheter to minus 20oC freezes the tip to the target and allows the operator to make distinction between the arrhythmic focus and the A-V node without producing permanent damage. Cooling to minus 70oC produces a small permanent scar. Other forms of energy for ablation procedures are still under scientific investigation. Sophisticated three dimensional mapping systems allow for precise localization of arrhythmic foci and identification of waves of myocardial depolarization. Such mapping systems allow for successful ablation of complex arrhythmia, especially IART following heart surgery. Only very infrequently, a cardiologist may choose to proceed with ablation of the A-V node and implantation of a ventricular pacemaker for life threatening PSVT refractory to medical therapy or catheter ablation of the arrhythmic substrate.
Risks associated with ablation procedures depend on patient's age and location of the arrhythmic focus. Ablation of an A-V bypass tract situated next to the A-V node (para-hisian bypass tracts) carries a high risk of a complete heart block. A smaller risk of a complete A-V block is associated with ablation of microreentry circuits responsible for AVNRT and ablation of automatic foci located close to the A-V node. Generally, this risk is smaller with the use of cryoablation compared to RF current ablation. Ablation procedures in small children and transseptal puncture for ablation of left atrial arrhythmic foci carry a risk of atrial perforation and cardiac tamponade. Catheter manipulation in the left heart is associated with a risk of thrombosis and a stroke. Other risks may be present in individual cases. As a rule, ablation procedures are not recommended in children under 3 years unless other treatments fail to prevent recurrences of life threatening PSVT.
Surgical ablation of A-V bypass tracts and other arrhythmic substrates may be considered in such cases when arrhythmia is life threatening or severely incapacitating and it cannot be controlled by other means.
Mechanisms of PSVT
Most frequently PSVT has to be differentiated from accelerated sinus rhythm. Aside from physical activity, sinus tachycardia can be produced by a febrile illness, anemia, heart failure, hyperthyroidism, administration of beta-adrenergic medications, emotion, anxiety and other factors. Persistent inappropriate sinus tachycardia is encountered very infrequently. Children at school age, especially adolescents, may complain of a sensation of accelerated and pounding heart beat which may occur for no clear reason at rest and frequently at bed time. These children may complain that abnormal heart beat does not allow them to relax, makes them anxious and at times is associated with chest pain or lightheadedness. Accelerated heart rate usually does not exceed 120/min. In the majority of patients, sinus tachycardia has a functional background and does not require any therapy aside from reassurance.
Some children and adolescents may provide history of benign premature beats in such a way that a physician will get an impression of paroxysmal tachycardia. This is more likely to happen if a child presents with frequent premature beats, especially in a form of bigeminy or trigeminy. A biased physician may continue clinical interview in a mislead direction.
PSVT in childhood may have different mechanisms. The list below presents several types of PSVT in the decreasing order of their prevalence:
Atrio-ventricular reentry tachycardia (Wolff-Parkinson-White syndrome)
Atrio-ventricular node reentry tachycardia
Ectopic (automatic) atrial tachycardia
Atrial flutter and atrial fibrillation
Junctional tachycardia
Wolff-Parkinson-White syndrome
The prevalence of WPW syndrome in the general population is estimated at 0.15 to 0.3%. Boys are affected more frequently than girls. Occasionally, the syndrome may be inherited. WPW is more prevalent in children with the Ebstein anomaly of the tricuspid valve, A-V septal defects and ventricular septal defects. The syndrome is produced by the presence of an A-V bypass tract (accessory pathway) which is a thin and short muscular fiber connecting atria to the ventricles across the right or left A-V ring. Approximately 10% of patients may have two or more A-V bypass tracts. Accessory pathways behave like electric conduits between atria and ventricles but they do not carry the same specialized electrophysiologic properties as a normal A-V node. Location of an accessory bypass tract can be predicted from a 12-lead ECG.[14]
WPW syndrome is a congenital heart defect since accessory bypass tracts are already present at birth. An A-V bypass tract may be responsible for paroxysmal tachycardia in a fetus. Relatively infrequent and brief bouts of fetal tachycardia do not affect fetal well being, yet incessant tachycardia may result in heart failure, non-immune hydrops and fetal demise. A-V bypass tracts may produce tachycardia in the first weeks or months of life. It is important to treat PSVT at this age because it is difficult for parents to recognize tachycardia and the cardiac reserve in infants is small. PSVT resolves in more than 50% of infants by one year of age at which time a physician may choose to discontinue medications. In some preschool children it is necessary to continue medical treatment because PSVT recurs or because of social reasons. After "grace period" during preschool and early school years, PSVT may recur in adolescence when ablation procedures rather than medications become the treatment of choice. Majority of individuals born with A-V bypass tracts do not experience any tachycardia until adolescence or adulthood. Some may never present with any PSVT.
A typical ECG recorded during sinus rhythm in a patient with WPW syndrome shows ventricular preexcitation which is manifested by a short PR interval, a wide QRS complex and presence of a delta wave Figure5A. In many patients, ventricular preexcitation is present at all times and at all heart rates. In some patients, ventricular preexcitation may be intermittent. In such cases, preexcitation is usually present at lower heart rates and it may be documented with ambulatory Holter monitoring. Ventricular preexcitation during sinus rhythm is produced by a wave of depolarization which enters one of the ventricles through a bypass tract in the anterograde direction (from atria to ventricles).
The macroreentry circuit of PSVT is produced by the atrial muscle, the A-V node, the ventricular muscle and the accessory pathway. The reentry circuit allows for continuous alternate depolarization of atria and ventricles. During typical orthodromic reciprocating atrio-ventricular tachycardia (orthodromic AVRT), the macroreentry proceeds from the atria to the ventricles through the A-V node, and back up to the atria from the ventricles in the retrograde direction via the A-V bypass tract Figure5B. In most patients with WPW syndrome, an accessory pathway behaves like a two-way street for electric conduction. In orthodromic AVRT, QRS complexes are narrow and the retrograde P wave may be seen embedded into the early portion of the T wave. The retrograde P wave is usually best seen in leads II and V2. In orthodromic AVRT, QRS complexes may become wide because of aberrant conduction with a right or left bundle branch block. At times, one can see PSVT begin with wide QRS complexes for several beats followed by narrow QRS tachycardia Figure6.
Only infrequently, the reentry circuit may proceed in the opposite, antidromic direction allowing for ventricular depolarization through the A-V bypass tract and return of the depolarization wave back to the atria through the A-V node. Antidromic AVRT presents with very abnormal, wide QRS complexes which resemble ventricular tachycardia Figure3. Presence of ventricular preexcitation on a resting ECG points toward antidromic AVRT. Sudden death in patients with WPW syndrome is discussed in the section "Risk of Cardiac Arrest".
In many patients, a bypass tract behaves like a one way street only. A bypass tract which conducts exclusively in the anterograde direction from atria to ventricles manifests itself by ventricular preexcitation and may allow for antidromic reentry tachycardia. Even patients with isolated ventricular preexcitation and no history of PSVT are at risk of cardiac arrest from paroxysmal atrial fibrillation if the bypass tract is capable of very rapid conduction.
A concealed A-V bypass tract is an accessory pathway capable of conducting solely in the retrograde direction - from ventricles to atria. An electrocardiogram recorded during sinus rhythm is normal; there is no ventricular preexcitation. Concealed A-V bypass tracts participate in orthodromic AVRT. Concealed bypass tracts are very common.
Certain infrequently encountered accessory pathways have unusual conduction properties that may resemble those of an A-V node. Most of these unusual bypass tracts cross the tricuspid valve in the septal or inferior half of the valve ring. Permanent Junctional Reciprocating Tachycardia (PJRT) is produced by a slow-conducting A-V bypass tract which renders the A-V reciprocating tachycardia very stable and unremitting. In PJRT, an ECG shows inverted P waves in inferior limb leads that precede QRS complexes. PJRT may lead to heart failure in neonates and in older children. A Mahaim fiber is another example of an unusual A-V bypass tract which presents with slow conduction properties and connects to the right branch of the bundle of His. Mahaim fibers produce ventricular preexcitation that resembles left bundle branch block with a normal PR interval.
The general rules regarding prophylactic treatment of PSVT were already described in another section. Patients with WPW syndrome should not be treated with medications which enhance conduction along the bypass tract such as calcium channel blockers and digoxin. These medications increase the risk of cardiac arrest precipitated by atrial fibrillation. Class III antiarrhythmic agents such as amiodarone and sotalol are effective and safe. Class Ic agents (propafenone, flecainide) and beta adrenergic blockers are also effective. A physician may consider prophylactic treatment with digoxin in infants without ventricular preexcitation. Calcium channel blockers may be used for prevention of AVRT mediated by concealed A-V bypass tracts but must not be prescribed for children younger than 2 years of age. Ablation of the accessory pathway may be considered as the primary treatment option at school age and in small children who failed medical treatment for significant arrhythmia.
Atrio-ventricular node reentry tachycardia (AVNRT)
AVNRT is the most common form of paroxysmal tachycardia in adolescence and adulthood Figure7. AVNRT is very unusual in infants and toddlers and only school aged children may present with AVNRT for the first time. AVNRT is typically triggered by physical activity. This form of PSVT is not associated with a risk of cardiac arrest unless a patient has significant heart disease.
The anatomic substrate for AVNRT is a dual electric input from the right atrium into the A-V node.[15] This dual input is a normal, physiologic finding and consists of a "fast pathway" and a "slow pathway". The "fast pathway" is located in the low interatrial septum near the His bundle recording site. The "slow pathway" is found above the ostium of the coronary sinus and close to the ring of the tricuspid valve. Both pathways are connected by a broad area of tissue at the site of their input to the A-V node. In patients with AVNRT, physiologic conduction properties of both pathways are such that they allow for a microreentry circuit at the entrance to the A-V node. The most common direction of the reentry is such that the "slow pathway" conducts toward the A-V node and the "fast pathway" allows for retrograde activation of the atria. In an unusual atypical form of AVNRT, the direction of reentry is opposite.
An ECG recorded during sinus rhythm is normal. An ECG recorded during AVNRT shows normal, narrow QRS complexes unless tachycardia produces a functional bundle branch block. P waves are not clearly seen because activation of atria and ventricles occurs at the same time and P waves are embedded within QRS complexes.
All medications that slow conduction through the A-V node are effective in preventing recurrences of AVNRT. Beta adrenergic blockers and calcium channel blockers are frequently used. Digoxin usually does not control PSVT. Ablation procedures for AVNRT are associated with a 1-2% risk of a heart block which requires implantation of a permanent pacemaker. At the time of an ablation procedure, an operator usually disrupts the slow pathway leading into the A-V node.
Ectopic (automatic) atrial tachycardia
Atrial tachycardia is an uncommon variety of PSVT in children and adolescents.[16] In infants, atrial tachycardia is more prevalent and may account for 15% of all cases of supraventricular tachycardia. Atrial tachycardia may be paroxysmal or incessant. The tachycardia usually originates from a small ectopic focus or from an area of abnormal atrial muscle in the right or in the left atrium. The electrophysiologic basis of atrial tachycardia is either increased automaticity of myocardial cells in the atrium or reentry within the atrial myocardium. Atrial tachycardia may produce rapid heart rates and may prove refractory to medical treatment. Incessant atrial tachycardia may lead to life-threatening heart failure.
As opposed to the AVRT and AVNRT, atrial tachycardia may have a relatively slow onset and a gradual offset. Patients may report that their tachycardia begins suddenly or the heart rate increases rather gradually. Many patients are asymptomatic. An ECG usually shows regular tachycardia with narrow QRS complexes unless the tachycardia produces a functional bundle branch block in which case QRS complexes are wide. Ventricular rhythm is irregular in such cases when atrial rate is so rapid that not every atrial beat is conducted to ventricles Figure8. P waves are usually best seen in leads II, V1 and V2. When every atrial beat is conducted to ventricles, the PR interval is usually shorter than the RP interval and the morphology of P waves is abnormal. P waves produced by atrial tachycardia may resemble normal P waves of sinus rhythm when the ectopic focus is located in close proximity to the sinus node.
Chaotic (multifocal) atrial tachycardia is very unusual in children. Atrial activation from multiple foci in the atrial myocardium produces P waves of different morphologies and atrial rate may exceed 400/min. An echocardiogram frequently shows normal structure and function of both atria and ventricles.
Vagal maneuvers do not interrupt atrial tachycardia. Adenosine usually does not terminate tachycardia, however it will produce a higher degree A-V block allowing for clear visualization of rapid atrial P waves. Infrequently, adenosine may terminate atrial tachycardia which makes correct diagnosis more difficult.
Short, asymptomatic and even relatively frequent bouts of atrial tachycardia might not require any antiarrhythmic therapy since brief spells of rapid heart rate do not impair ventricular function. Digoxin, beta adrenergic blockers and calcium channel blockers may not prevent recurrences of tachycardia. Medications that belong to class Ia, Ic and III are more effective. One may consider therapy with propafenone or flecainide alone or in combination with digoxin or a beta blocker. Flecainide must not be used in patients with heart disease other than PSVT. Sotalol is used as monotherapy, in combination with digoxin or cautiously in combination with class Ic agents. Amiodarone may be effective but long term therapy is associated with several side effects. Medications that belong to class I and III should be prescribed by an experienced physician since their administration is associated with a risk of significant side effects and proarrhythmia. An ablation procedure should be considered in children at school age when it allows for permanent cure in more than 80% of cases. An ablation procedure may be effective in infants with rapid heart rates or ventricular dysfunction who failed medical therapy.
Intermittent ectopic atrial rhythms at rates close to the normal heart rate for age are common in children of school age and in adolescence. Ectopic atrial rhythms produce abnormal P wave morphology but they do not represent any heart disease and they do not lead to any ventricular dysfunction. As a rule, even very frequent premature atrial beats and ectopic atrial rhythms do not predict occurrence of PSVT and do not degenerate into ectopic atrial tachycardia. An exercise stress test will show normalization of P waves as the sinus rhythm takes over even with mild physical activity. Ectopic atrial beats and intermittent ectopic atrial rhythms do not require any treatment.
Atrial flutter, IART and atrial fibrillation
Atrial flutter is an unusual arrhythmia in children. It may occur in a fetus when it presents with ventricular rates of 150 - 220/min. Atrial flutter may be present at birth or it may occur in the first days of life. Intravenous administration of adenosine does not terminate this tachycardia but it allows for clear visualization of "saw tooth" flutter waves Figure9. Electric cardioversion should be done in a neonate with heart failure. Treatment with digoxin is frequently effective in an infant with preserved heart function but it may take several days before atrial flutter terminates. Class Ia (procainamide), Ic (propafenone, flecainide) or class III (sotalol, amiodarone) antiarrhythmic medications can be used if digoxin fails to restore sinus rhythm. All antiarrhythmic agents with the exception of digoxin exert negative inotropic effect on ventricular myocardium and may precipitate life threatening hypotension. Electromechanical dissociation has been reported after intravenous administration of amiodarone. Once neonatal flutter terminates it usually does not recur. It is advisable to continue treatment with digoxin for six months.
Atrial flutter is very unusual in older children and adolescents with normal hearts. It is more common in children with congenital heart disease. In typical atrial flutter, repetitive atrial activation results from continuous flow of a depolarization wave around the tricuspid valve (macroreentry). This reentry circuit becomes very stable in patients with a dilated right atrium. Antiarrhythmic medications are only infrequently successful in converting atrial flutter back to sinus rhythm. Ablation procedures for typical atrial flutter target the isthmus between inferior vena cava and the ring of the tricuspid valve.
Patients with heart disease and especially those after open heart surgery often present with atrial tachycardia resulting from several different reentry circuits in the right or left atrium.[17] The right astronomy scar, scaring from repair of septal defects as well as other suture lines in atrial walls produce obstruction to uniform propagation of atrial activation during sinus rhythm and create complex pathways for intra-atrial reentry tachycardia (IART). Electric cardioversion is generally effective in restoring sinus rhythm although IART may recur within a short period of time. Atrial surgery predisposes to sinus node dysfunction which further increases risk of recurrent IART. Incisional atrial tachycardia is consistently resistant to medical therapy and should be treated in specialized cardiac centers. At times IART can be suppressed with sotalol or amiodarone. Infrequently, a patient with atrial scarring may present with automatic atrial tachycardia from an ectopic focus rather than with IART.
Paroxysmal atrial fibrillation is even less prevalent than atrial flutter. An ECG recorded during atrial fibrillation shows an irregularly irregular rapid heart rhythm. QRS complexes are narrow although some may become wide because of intermittent aberrant ventricular conduction. Infrequent bouts of atrial fibrillation in patients with normal heart function do not require any treatment except for administration of aspirin to prevent thromboembolic complications. Children with congenital or acquired (rheumatic) heart disease are more likely to develop atrial fibrillation. Treatment should address the cause of hemodynamic dysfunction before targeting arrhythmia. The efficacy, safety and long term outcomes of catheter ablation procedures for atrial fibrillation in children are unknown. Electric cardioversion for sustained atrial flutter, fibrillation or IART must be preceded with several weeks of anticoagulation.
Junctional tachycardia
Congenital junctional ectopic tachycardia (JET) is also very uncommon. It generally presents in the first 6 months of life. An ECG shows regular tachycardia at heart rates ranging from 150 to 200 per minute and narrow QRS complexes. Retrograde P waves are found just behind QRS complexes or there may be complete A-V dissociation with the atrial rate slower than the ventricular rate. Incessant JET at a rapid heart rate may lead to heart failure. Electric cardioversion does not terminate this tachyarrhythmia. JET is difficult to treat although it may yield to therapy with amiodarone at a dose as high as 250-500 mg/m2 per day orally. Patients with severe ventricular dysfunction who fail medical treatment should be considered for catheter ablation therapy. Transient postoperative JET is commonly seen in small children during the first week following open heart surgery for complex heart disease.
Slow escape junctional rhythms during sinus bradycardia as well as transient accelerated junctional rhythms are common in healthy children and adolescents. These rhythms are a normal finding and do not require any therapy.
Conclusion
PSVT in children may have several different underlying mechanisms. General rules regarding acute and chronic therapy are similar for most forms of paroxysmal tachycardia, however clear understanding of pathophysiology of different tachyarrhythmias will assist a physician in selecting the best possible treatment. PSVT is usually benign and patients with asymptomatic and infrequent tachycardia do not require any intervention. Conversely, frequent and symptomatic tachycardia or PSVT presenting with syncope should be treated. Neonates, patients with ventricular preexcitation and those with impaired ventricular function from heart disease are at risk of cardiovascular collapse or sudden death. Patients with ventricular preexcitation must not be allowed to participate in competitive sports until a bypass tract is interrupted with an ablation procedure. Medical treatment is the preferred option for infants and toddlers while ablation procedures are very effective and safe in older children and adolescents. Patients with less common forms of PSVT as well as those with heart disease should be referred to a pediatric cardiology clinic. Rapid administration of adenosine not only terminates most common forms of PSVT but also allows establishing correct diagnosis in refractory cases; it is essential to record a rhythm strip within seconds after adenosine push.
References
1. Rodriguez LM, De Chillou C, Schlapfer J, Metzger J, Baiyan X, Van den Dool A, Smeets JLRM, Wellens HJJ. Age at onset and gender of patients with different types of supraventricular tachycardias. Am J Cardiol 1992; 70: 1213-1215.
2. Boldt T, Eronen M, Andersson S. Long-term outcome in fetuses with cardiac arrhythmias. Obstet Gynecol 2003; 102: 1372-1379.
3. Juneja R, Shah S, Naik N, Kothari SS, Saxena A, Talwar KK. Management of cardiomyopathy resulting from incessant supraventricular tachycardia in infants and children. Indian Heart J 2002; 54: 176-180.
4. Bromberg BI, Lindsay BD, Cain ME, Cox JL. Impact of clinical history and electrophysiologic characterization of accessory pathways on management strategies to reduce sudden death among children with Wolff-Parkinson-White syndrome. J Am Coll Cardiol 1996; 27: 690-695.
5. Dubin AM, Collins KK, Chiesa N, Hanisch D, Van Hare GF. Use of electrophysiologic testing to assess risk in children with Wolff-Parkinson-White syndrome. Cardiol Young 2002; 12: 248-252.
6. Pappone C, Manguso F, Santinelli R, Vicedomini G, Sala S, Paglino G, Mazzone P, Lang CC, Gulletta S, Augello G, Santinelli O, Santinelli V. Radiofrequency ablation in children with asymptomatic Wolff-Parkinson-White syndrome. N Engl J Med 2004; 351: 1197-1205.
7. Bisset GS, Gaum W, Kaplan S. The ice bag: a new technique for interruption of supraventricular tachycardia. J Pediatr 1880; 97: 593-595.
8. Vaughan Williams EM. A classification of antiarrhythmic actions reassessed after a decade of new drugs. J Clin Pharmacol 1984; 24: 129-147.
9. Weindling SN, Saul JP, Walsh EP. Efficacy and risks of medical therapy for supraventricular tachycardia in neonates and infants. Am Heart J 1996; 131: 66-72.
10. Kugler JD, Danford DA, Deal BJ, Gillette PC, Perry JC, Silka MJ, Van Hare GF, Walsh EP. Radiofrequency catheter ablation for tachyarrhythmias in children and adolescents. N Engl J Med 1994; 330: 1481-1487.
11. Rao PS, Gupta ML, Balaji S. Recent advances in pediatric cardiology - electrophysiology, transcatheter and surgical advances. Indian J Pediatr 2003; 70: 557-564.
12. Kantoch MJ, Frost GF, Robertson MA. Use of Transesophageal Echocardiography in Radiofrequency Catheter Ablation in Children and Adolescents. Can J Cardiol 1998; 14: 519-523.
13. Haines DE. The biophysics of radiofrequency catheter ablation in the heart: the importance of temperature monitoring. PACE 1993; 16: 586-591.
14. Fitzpatrick AP, Gonzales RP, Lesh MD, Modin GW, Lee RJ, Scheinman MM. New algorithm for the localization of accessory atrioventricular connections using a baseline electrocardiogram. J Am Coll Cardiol 1994; 23: 107-116.
15. Sung RJ, Laurer MR, Chun H. Atrioventricular node reentry: Current concepts and new perspectives. PACE 1994; 17: 1413-1430.
16. Salerno JC, Kertesz NJ, Friedman RA, Fenrich AL. Clinical course of atrial ectopic tachycardia is age-dependent: results and treatment in children <3 or >3 years of age. J Am Coll Cardiol 2004; 43: 438-444.
17. Triedman JK, Jenkins KJ, Colan SD, Saul JP, Walsh EP. Intra-atrial reentrant tachycardia after palliation of congenital heart disease: characterization of multiple macroreentrant circuits using fluoroscopically based three-dimensional endocardial mapping. J Cardiovasc Electrophysiol 1997; 8: 259-270.(Kantoch Michal J)
Abstract
Several different mechanisms are responsible for paroxysmal supraventricular tachycardia in children. Different forms of tachycardia occur at different age. Atrio-ventricular reentry tachycardia results from the presence of congenital atrio-ventricular bypass tracts and is frequently encountered at all ages. Infants may present with ectopic atrial tachycardia or atrial flutter. Atrio-ventricular node reentry tachycardia becomes more frequent in adolescence. Atrial scarring resulting from open heart surgery predisposes to complex intra-atrial reentry. Certain forms of congenital and acquired heart disease are associated with specific types of arrhythmia. Many children with paroxysmal supraventricular tachycardia do not require any therapy. The decision to proceed with treatment should be based on the frequency and severity of symptoms and on the effect of arrhythmia on the quality of life. Infants require medical treatment because of the difficulty to recognize symptoms of tachycardia and a risk of heart failure. Patients with Wolff-Parkinson-White syndrome as well as those with significant heart disease are at risk of sudden death. Syncope in children with paroxysmal tachycardia may indicate a severe fall in cardiac output from extremely rapid heart rate. Patients with potentially life-threatening arrhythmia should not participate in competitive physical activities. Treatment options have undergone significant evolution over the past decade. Indications for the use of specific antiarrhythmic medications have been refined. Contemporary catheter ablation procedures employ different forms of energy allowing for safe and effective procedures. Catheter ablation is the treatment of choice for symptomatic paroxysmal tachycardia in school children and in some infants who failed medical treatment. Surgery is the preferred treatment in few selected cases. The goal of this review is to present the state of the art approach to the diagnosis and management of paroxysmal supraventricular tachycardia in infants, children and adolescents.
Keywords: Supraventricular tachycardia; Arrhythmia; Heart failure; Children
Paroxysmal supraventricular tachycardia (PSVT) is the most common symptomatic arrhythmia in young patients. It affects children of all ages. Its prevalence is estimated at more than one in 500 children. A pediatrician may see several new patients with PSVT in a year. A child with new onset PSVT is likely to present with the same paroxysmal tachycardia for at least 10 subsequent years. Different forms of PSVT present at different patient age.[1]
The goal of this review is to provide practical guidelines regarding diagnosis and management of supraventricular tachycardia in infants, children and adolescents. The review explains pathophysiology of different forms of PSVT and risks of heart failure or sudden death.
Presentation of PSVT in infants
Symptoms of PSVT in infancy differ from those in childhood and adolescence. A newborn may present with history of fetal tachycardia or with signs of left ventricular dysfunction from tachycardia in fetal life. Hydrops fetalis represents severe heart failure from persistent rapid tachycardia. Frequently, PSVT becomes less frequent and easier to manage after birth.[2] Conversely, neonates with no history of fetal tachycardia may present with incessant and difficult to treat pathological tachycardia after birth. PSVT may occur in the first days of life or somewhat later in the first year of life. Most infants with PSVT have structurally normal hearts. In 15% of infants, paroxysmal tachycardia is associated with heart disease, drug administration or a febrile illness. Incessant tachycardia at a rate of more than 200/min leads to progressive left ventricular dysfunction. Persistent very rapid tachycardia at a rate of 250/min or more may produce life threatening heart failure within few days. Symptoms of PSVT in infants are inconspicuous and include irritability, poor feeding, tachypnea, diaphoresis and poor color. When counseling parents about risks of recurrent PSVT, it is important to stress that symptoms of tachycardia masquerade those of many other common illnesses in infancy. It is prudent to advise parents to check infant's heart rate when the child is quiet or asleep. Parents may verify heart rate at the time of feeding by placing hand on the child's chest or with an inexpensive stethoscope. In infants, atrio-ventricular reentry tachycardia (AVRT) resulting from the presence of an accessory atrio-ventricular (A-V) bypass tract accounts for approximately 80% of all cases of PSVT. Permanent junctional reciprocating tachycardia (PJRT) and automatic atrial tachycardia are much less common but they frequently pose a therapeutic challenge. Atrial flutter may occur in newborns and usually resolves within few days or with administration of digoxin. When tachycardia is recognized, it is important to obtain an echocardiogram in order to rule out structural heart disease and to assess ventricular function.
Presentation of PSVT in children and adolescents
While obtaining clinical history from children, physicians have to inquire about the sensation produced by arrhythmia, how frequently symptoms occur, how long they persist each time and what other symptoms accompany arrhythmia. It is important to learn in detail about each episode of rhythm disturbance. Physicians may choose to inquire about the most recent bout of arrhythmia first and then ask about each previous episode. Frequently physicians have to put much effort into prompting their patients to report everything they can remember regarding presentation and circumstances surrounding each attack of arrhythmia. It is essential to inquire if there is any family history of arrhythmia or any family history of sudden death or a convulsive disorder which may represent life threatening arrhythmia.
Typically, PSVT will produce impression of a very rapid and regular heart beat that is very difficult or outright impossible to count. Patients may describe that the heart is fluttering in the throat. A sensation of somewhat accelerated pounding (painful) heart beat in the chest usually indicates sinus tachycardia. In general, PSVT presents with heart rates well in excess of 150/min while sinus tachycardia at rest does not exceed such rate even in a stressful situation.
As a rule, atrio-ventricular node reentry tachycardia (AVNRT) is triggered by vigorous physical activity while AVRT mediated by an A-V bypass may also occur at rest. Patients may recognize a sudden onset of PSVT even during sinus tachycardia. They report that the heart rate suddenly accelerates and does not slow down for several minutes or longer even though they stop physical activity and lie down. Typical PSVT also terminates suddenly as if turned off with a switch. Gradual acceleration and deceleration of tachycardia usually indicate sinus tachycardia or ectopic (automatic) atrial tachycardia. Infrequently, children affected by paroxysmal tachycardia of truly sudden onset and offset may still report that the tachycardia starts suddenly but terminates gradually or it begins and wanes gradually.
The majority of older children and adolescents provide very precise description of their symptoms allowing for a secure diagnosis of PSVT (although it is always preferred to record a rhythm strip with arrhythmia). Even children as young as 3 years of age may recognize symptoms of PSVT. "My heart is racing, beeping, skipping, hurting, jumping up and down" - these are only few of many descriptions that young children may use to explain their sensation of tachycardia. Sometimes, children are able to give merely nebulous history of vague palpitations especially if they cannot recognize when and how their arrhythmia starts or what it feels like. Preschool children may only say that their heart hurts. At times, children may complain of general malaise or indistinct pressure or discomfort in the throat. Some children may complain of isolated headaches. Few patients might deny any symptoms.
Tachycardia may be accompanied by fatigue, chest discomfort, shortness of breath or lightheadedness. Severe cardiovascular compromise produces more pronounced dyspnea and lightheadedness. Syncope is unusual and may indicate life threatening arrhythmia which necessitates rapid diagnosis and treatment. Older children and adolescents may present with two or more forms of arrhythmia and it is important to distinguish what symptoms represent each heart rhythm disturbance such as paroxysmal tachycardia, accelerated sinus rhythm or premature contractions. It is very important to remember that clinical symptoms of relatively benign PSVT are not different from those produced by life threatening ventricular tachycardia.
Risk of Cardiac Arrest
The risk of cardiac arrest from PSVT in children and adolescents is very low. Three general groups of children at risk of life threatening heart failure or sudden death are neonates with PSVT, patients with Wolff-Parkinson-White (WPW) syndrome and children with heart disease.
Nonspecific presentation of PSVT in infants makes it difficult to recognize. It is essential to remember that cardiac reserve in neonates is very small and typical supraventricular tachycardia with heart rate exceeding 200/min may lead to life threatening myocardial dysfunction within several days.[3] Neonates may already present with subclinical heart failure from tachycardia before birth. Every neonate with PSVT should be evaluated with an echocardiogram in order to rule out structural heart disease and to assess ventricular function. Administration of intravenous adenosine in order to terminate PSVT or to make precise diagnosis of tachycardia is safe. It is important to remember that all antiarrhythmic medications with the exception of digoxin exert a negative inotropic effect on the myocardium and therefore may produce significant hypotension and lead to cardiac arrest. Even oral administration of a beta blocker in an infant with impaired ventricular function may lead to life threatening arterial hypotension. All newborns should be hospitalized and their vital signs should be closely monitored during initial therapy with all antiarrhythmic agents.
The risk of sudden death in adult patients with WPW syndrome is well recognized. Cardiac arrest is usually precipitated by atrial fibrillation which is transmitted into ventricles through a rapid conducting A-V bypass tract. Atrial fibrillation is uncommon in children, nevertheless sudden death is a well known complication of WPW syndrome in young patients as well Figure1A, B and Figure2. Patients at high risk are those in whom the shortest interval between two subsequent preexcited ventricular beats during atrial fibrillation is less than 220 milliseconds.[4], [5] This measurement could be taken from an ECG or a rhythm strip recorded at the time of atrial fibrillation or during electrophysiologic testing of the A-V bypass tract. Syncope in patients with WPW syndrome may indicate high risk of sudden death although sudden death may be the very first presentation of life-threatening arrhythmia. Even a single attack of syncope in a child with WPW syndrome should lead to electrophysiology studies and ablation of the bypass tract. Successful ablation of the A-V bypass tract eliminates the risk of sudden death from WPW syndrome.[6] Strenuous physical activities shorten the refractory period of the bypass tract and may precipitate atrial fibrillation or flutter. Sometimes, ventricular preexcitation resolves during physical activity, for instance during an exercise stress test. It is not known, however, if these patients are at low risk of cardiac arrest. Competitive sports should be discouraged in children with ventricular preexcitation on a surface ECG.
Many forms of congenital and acquired heart disease may impair ventricular function or disturb coronary circulation and thus increase risks associated with PSVT. Rapid tachycardia shortens filling time of ventricles and decreases stroke volume and cardiac output. Coronary blood flow is reduced. Patients with borderline left ventricular function are at particularly high risk of life threatening arterial hypotension from PSVT. Few examples of heart disease leading to ventricular dysfunction in children include myocarditis, dilated cardiomyopathy, congenital defects producing large shunts (in infants and small children) and complex heart anomalies. Stenotic and regurgitant lesions of heart valves significantly decrease stroke volume which is already compromised during tachycardia because of abnormally short ventricular filling and ejection times. PSVT in patients with pulmonary hypertension is life threatening. Physicians have to keep in mind that many patients with operated heart disease are left with considerable ventricular dysfunction even though parents may inform them that the heart was "fixed".
Certain forms of heart disease predispose to the occurrence of certain types of PSVT. Patients with hypertrophic cardiomyopathy are at risk of atrial fibrillation which produces a major reduction in the left ventricular stroke volume in a setting of a thick, non-compliant myocardium and leads to myocardial ischemia. Rheumatic disease of the mitral valve is also associated with paroxysmal atrial fibrillation and a risk of thromboembolic events. Children with the Ebstein anomaly of the tricuspid valve present with AVRT which may produce significant hemodynamic derangement in a setting of severe tricuspid insufficiency. Children with Kawasaki disease and coronary artery abnormalities are at risk of myocardial ischemia and infarction. Children and adults with operated congenital or acquired heart disease may present with incisional atrial reentry tachycardia (IART). This rapid tachycardia at atrial rates of 180 - 250 resembles atrial flutter although it is more difficult to manage. Detailed discussion of risks associated with PSVT in patients with heart disease falls outside of the scope of this review.
Presence of severe non-cardiac disease may augment symptoms associated with PSVT.
Diagnosis
The diagnosis of PSVT is based on thorough clinical history and electrocardiograms (ECG's) or rhythm strips recorded at the time of tachycardia and during sinus rhythm. It is very important to encourage patients and their care givers to go to the emergency room or to their clinic in order to document prolonged attacks of arrhythmia so that their physician can make a proper diagnosis. An ECG has to be repeated after tachycardia resolves. Resting ECGs have to be recorded on several occasions since ventricular preexcitation may be intermittent and may not be obvious on each tracing. Different types of PSVT present differently on a 12 lead ECG. An ECG recording of the tachycardia allows a cardiologist to design the best possible treatment. A single lead tracing should be recorded only if a complete 12 lead ECG cannot be done. Clinicians should attempt to identify P waves representing atrial activation during tachycardia. Short PR tachycardia is such PSVT when the PR interval is shorter than the RP interval. Short PR tachycardia includes sinus tachycardia, ectopic atrial tachycardia, PJRT and an atypical AVNRT. Short RP tachycardia is such PSVT when the RP interval is short and the PR interval is long. The P wave is usually superimposed on the T wave and very close to the preceding QRS complex. The P wave may be recognizable in few leads only and usually in limb lead II and in precordial leads V1 and V2. Short RP tachycardia includes AVRT mediated by an A-V bypass tract (WPW syndrome) and typical AVNRT.
An exercise stress test may induce tachycardia and allow for its documentation. Ambulatory Holter monitoring may be successful in recording paroxysmal tachycardia in patients with frequent recurrences of PSVT. Holter is not likely to document PSVT in patients with very infrequent bouts of tachycardia although it may show intermittent ventricular preexcitation pointing to WPW syndrome. Clinicians are more likely to document heart rhythm responsible for sporadic palpitations and other symptoms suggestive arrhythmia if they choose ambulatory event recorders which patients may keep at home for several weeks.
Brief and infrequent attacks of palpitations even accompanied by significant symptoms are very difficult to record. Symptoms usually resolve by the time patients arrive at the emergency room or in an ambulatory clinic. Clinicians may choose to teach patients or their parents how to take pulse rate or how to count heart rate with a stethoscope. Patients may want to start a diary indicating the date of their symptoms, their activity when palpitations or tachycardia occurred, the pulse rate or the heart rate as well as accompanying symptoms and duration of symptoms.
PSVT with wide QRS complexes
Infrequently, an electrocardiogram recorded during PSVT may show tachycardia with wide QRS complexes. This may be the case in the following situations:
Antidromic reciprocating tachycardia in patients with WPW syndrome Figure3.
PSVT with right or left bundle branch block (aberrant ventricular conduction).
PSVT with bystander participation of an accessory A-V bypass tract
Electrocardiograms recorded during tachycardia and in sinus rhythm frequently allow a clinician to establish proper diagnosis. Presence of ventricular preexcitation during sinus rhythm may point to antidromic reciprocating tachycardia or bystander participation of an accessory A-V bypass tract in patients with WPW syndrome. The term "bystander participation of a bypass tract" implies that a bypass tract transmits rapid atrial activation to the ventricles but the mechanism of tachycardia does not include a bypass tract ( e.g. atrial fibrillation, AVNRT, ectopic atrial tachycardia, etc.). Infrequent presence of right or left bundle branch block during sinus rhythm indicates that tachycardia with wide QRS complexes represents aberrant conduction as long as the morphology of QRS complexes is the same during tachycardia and during sinus rhythm. Frequently aberrant ventricular conduction is rate dependent which means that right or left bundle branch block occurs only at rapid heart rates. Rate dependent aberrant conduction with right bundle branch block is more frequent than aberrant conduction with left bundle branch block.
Ventricular tachycardia (VT) is uncommon in children and adolescents. In general, idiopathic VT is not a life threatening condition although it may produce syncope. Life threatening VT may complicate different forms of cardiomyopathy or congenital heart defects (especially "repaired" or palliated heart disease). Ventricular tachycardia will always present with wide, abnormal looking QRS complexes. Only in infants, VT may present with relatively narrow QRS complexes. A-V dissociation is not always the feature of VT and at times it is not possible to distinguish VT from wide QRS complex PSVT in patients with ventricular preexcitation. Patients with wide QRS tachycardia should have an echocardiogram and should be evaluated by a pediatric cardiologist.
Acute treatment
Regardless of the type of PSVT there are certain general principles of therapy which apply to all patients. Physicians should teach their patients maneuvers which increase parasympathetic (vagal) tone, slow down conduction through the A-V node and break the reentry circuit responsible for PSVT in most cases. An ice-bag can be used at any age with good success.[7] It is important to place an ice-bag around the nose and mouth in order to make it effective. Immersion of the face in ice-cold water may also terminate PSVT. In the Valsalva maneuver patients are asked to take a deep breath and bear down. Valsalva maneuver may be combined with other measures such as an ice-bag or pressure on the abdomen. Carotid sinus massage, retching or gagging induced by a finger or a spoon, or blowing on a thumb may also be effective. Different measures may be successful in different children. Standing on the head (a rather tricky operation) seems to work well for some adolescents. Applying pressure on the eyes with closed eyelids must never be used because of a significant risk of injury to the eyes and retinal detachment.
Acute treatment in the emergency room depends on patient's symptoms. Children with unstable vital signs or signs of heart failure require electric cardioversion. Stable vital signs and normal blood pressure allow for less aggressive measures such as those described above. Intravenous administration of adenosine as a rapid bolus is safe in children of all ages Figure4. Adenosine is quickly cleared from circulation by cellular uptake, mostly by endothelial cells and erythrocytes. The half life in the blood stream is less than 10 seconds. The standard dose ranges from 50 to 200 micrograms per kilogram of body weight. The most commonly observed side effects are transient chest discomfort, dyspnea, facial flushing, sinus pauses and A-V block. Significant bradycardia may occur in patients with sinus node dysfunction, A-V conduction defects or after concurrent administration of other medications which affect the A-V node ( e.g. beta blockers, calcium channel blockers, amiodarone and other). Adenosine may produce bronchoconstriction in patients with asthma.
Following administration of adenosine, it is not unusual to see PSVT break for few seconds and resume with vengeance. It is very important to print a rhythm strip when adenosine is pushed in and review an ECG tracing after each injection. A rhythm strip may show sudden termination of tachycardia followed by merely one or several normal sinus beats and resumption of PSVT. In such cases a longer acting intravenous medication could be considered such as a beta blocker (propranolol), digoxin, procainamide, a calcium channel blocker (verapamil) or amiodarone. Calcium channel blockers should not be administered to children under 2 years of age because of a risk of cardiovascular collapse. One should remember that intravenous administration of many antiarrhythmic agents may produce significant arterial hypotension or induce life-threatening arrhythmia. It is important to follow guidelines regarding administration of each medication in regards to the correct dose and rate of intravenous infusion. Antiarrhythmic medications are grouped according to their electrophysiologic properties which allows for their proper use.[8]
A rhythm strip recorded during administration of adenosine may uncover atrial flutter or P waves of atrial tachycardia. Adenosine is not effective in converting atrial flutter or fibrillation into sinus rhythm but it is helpful in establishing correct diagnosis.
Chronic management
Chronic management of PSVT should be individualized. In general, the decision to treat a child with PSVT depends on the child's age, symptoms produced by paroxysmal tachycardia and the effect of PSVT on the quality of life. Prophylactic treatment with antiarrhythmic medications is prescribed for infants younger than one year of age because of nonspecific nature of symptoms and the risk of life-threatening heart failure from unrelenting rapid tachycardia.[9] Beta blockers are prescribed most frequently. Digoxin may be used in infants without ventricular preexcitation. One could also prescribe sotalol, flecainide or amiodarone in selected cases. At one year of age, prophylactic treatment could be withheld in order to see if tachycardia recurs. More than one half of toddlers with history of PSVT after birth will be free from tachycardia at least until school age when children are able to recognize symptoms of arrhythmia. Prophylactic treatment with medications may be extended into preschool years if tachycardia recurs or parents are unable to recognize recurrent arrhythmia. The dose of every medication should be increased over time in order to keep up with the child's weight gain. Patient's compliance improves if prescribed medication could be taken once or twice a day. Medications may become ineffective over time and may have to be replaced by other agents.
In school-aged children, the decision to treat PSVT usually depends on its effect on the quality of life. Infrequent PSVT accompanied by mild symptoms does not warrant any treatment especially if a child is able to interrupt tachycardia with appropriate maneuvers. Consumption of caffeine (coffee, tea, soft drinks containing caffeine) may increase the likelihood of recurrent tachycardia. Physicians may advise patients against taking such substances if it is clear that relapses of PSVT are associated with their intake or PSVT produces significant symptoms. Use of medications that augment sympathetic tone or those that suppress vagal tone may increase the likelihood of another attack of tachycardia. Patients report more problems with PSVT if they take beta adrenergic medications (salbutamol) or certain decongestants (pseudoephedrine, antihistamines). It is crucial to treat all patients with WPW syndrome who present with syncope or who participate in competitive sports. This recommendation also applies to isolated ventricular preexcitation with no history of paroxysmal tachycardia. It is important to remember that prophylactic treatment with medications does not cure the child from tachycardia but merely suppresses arrhythmia.
Symptomatic relief from PSVT can be achieved by implantation of a permanent anti-tachycardia pacemaker. Such device can recognize occurrence of paroxysmal tachycardia and interrupt it with overdrive atrial pacing. Anti-tachycardia pacemakers are considered only in selected symptomatic patients who failed medical treatment and ablation procedures.
Catheter Ablation Procedures
Elimination of the arrhythmic substrate and permanent cure from almost all forms of PSVT can be achieved through percutaneous catheter ablation or by means of surgery. [10],[11],[12] Catheter ablation procedures are performed in an electrophysiology laboratory and are similar to cardiac catheterization with the use of fluoroscopy. Radiofrequency current (RF) ablation is employed most frequently. During the procedure, a small metal tip of the catheter is heated to 50-60oC by alternating current at 350 kHz to 1 MHz.[13] This relatively low temperature produces a permanent small scar measuring approximately 4 mm in diameter and 4 mm in depth. An RF application directly to the arrhythmic substrate will destroy it permanently and prevent recurrences of paroxysmal tachycardia. A cardiologist has to apply from one to several "burns" before an RF application lands directly on the target. The initial success rate of RF ablation exceeds 90%. Infrequently, PSVT may recur if ablation lesions did not damage but only temporarily injured the arrhythmic focus. Large ablation lesions are produced by radiofrequency irrigation catheters and are frequently required for ablation procedures in patients with atrial flutter or postoperative atrial tachycardia. Cryoablation is employed in such cases when an arrhythmic substrate is located close to the A-V node. Cooling a metal tip of the catheter to minus 20oC freezes the tip to the target and allows the operator to make distinction between the arrhythmic focus and the A-V node without producing permanent damage. Cooling to minus 70oC produces a small permanent scar. Other forms of energy for ablation procedures are still under scientific investigation. Sophisticated three dimensional mapping systems allow for precise localization of arrhythmic foci and identification of waves of myocardial depolarization. Such mapping systems allow for successful ablation of complex arrhythmia, especially IART following heart surgery. Only very infrequently, a cardiologist may choose to proceed with ablation of the A-V node and implantation of a ventricular pacemaker for life threatening PSVT refractory to medical therapy or catheter ablation of the arrhythmic substrate.
Risks associated with ablation procedures depend on patient's age and location of the arrhythmic focus. Ablation of an A-V bypass tract situated next to the A-V node (para-hisian bypass tracts) carries a high risk of a complete heart block. A smaller risk of a complete A-V block is associated with ablation of microreentry circuits responsible for AVNRT and ablation of automatic foci located close to the A-V node. Generally, this risk is smaller with the use of cryoablation compared to RF current ablation. Ablation procedures in small children and transseptal puncture for ablation of left atrial arrhythmic foci carry a risk of atrial perforation and cardiac tamponade. Catheter manipulation in the left heart is associated with a risk of thrombosis and a stroke. Other risks may be present in individual cases. As a rule, ablation procedures are not recommended in children under 3 years unless other treatments fail to prevent recurrences of life threatening PSVT.
Surgical ablation of A-V bypass tracts and other arrhythmic substrates may be considered in such cases when arrhythmia is life threatening or severely incapacitating and it cannot be controlled by other means.
Mechanisms of PSVT
Most frequently PSVT has to be differentiated from accelerated sinus rhythm. Aside from physical activity, sinus tachycardia can be produced by a febrile illness, anemia, heart failure, hyperthyroidism, administration of beta-adrenergic medications, emotion, anxiety and other factors. Persistent inappropriate sinus tachycardia is encountered very infrequently. Children at school age, especially adolescents, may complain of a sensation of accelerated and pounding heart beat which may occur for no clear reason at rest and frequently at bed time. These children may complain that abnormal heart beat does not allow them to relax, makes them anxious and at times is associated with chest pain or lightheadedness. Accelerated heart rate usually does not exceed 120/min. In the majority of patients, sinus tachycardia has a functional background and does not require any therapy aside from reassurance.
Some children and adolescents may provide history of benign premature beats in such a way that a physician will get an impression of paroxysmal tachycardia. This is more likely to happen if a child presents with frequent premature beats, especially in a form of bigeminy or trigeminy. A biased physician may continue clinical interview in a mislead direction.
PSVT in childhood may have different mechanisms. The list below presents several types of PSVT in the decreasing order of their prevalence:
Atrio-ventricular reentry tachycardia (Wolff-Parkinson-White syndrome)
Atrio-ventricular node reentry tachycardia
Ectopic (automatic) atrial tachycardia
Atrial flutter and atrial fibrillation
Junctional tachycardia
Wolff-Parkinson-White syndrome
The prevalence of WPW syndrome in the general population is estimated at 0.15 to 0.3%. Boys are affected more frequently than girls. Occasionally, the syndrome may be inherited. WPW is more prevalent in children with the Ebstein anomaly of the tricuspid valve, A-V septal defects and ventricular septal defects. The syndrome is produced by the presence of an A-V bypass tract (accessory pathway) which is a thin and short muscular fiber connecting atria to the ventricles across the right or left A-V ring. Approximately 10% of patients may have two or more A-V bypass tracts. Accessory pathways behave like electric conduits between atria and ventricles but they do not carry the same specialized electrophysiologic properties as a normal A-V node. Location of an accessory bypass tract can be predicted from a 12-lead ECG.[14]
WPW syndrome is a congenital heart defect since accessory bypass tracts are already present at birth. An A-V bypass tract may be responsible for paroxysmal tachycardia in a fetus. Relatively infrequent and brief bouts of fetal tachycardia do not affect fetal well being, yet incessant tachycardia may result in heart failure, non-immune hydrops and fetal demise. A-V bypass tracts may produce tachycardia in the first weeks or months of life. It is important to treat PSVT at this age because it is difficult for parents to recognize tachycardia and the cardiac reserve in infants is small. PSVT resolves in more than 50% of infants by one year of age at which time a physician may choose to discontinue medications. In some preschool children it is necessary to continue medical treatment because PSVT recurs or because of social reasons. After "grace period" during preschool and early school years, PSVT may recur in adolescence when ablation procedures rather than medications become the treatment of choice. Majority of individuals born with A-V bypass tracts do not experience any tachycardia until adolescence or adulthood. Some may never present with any PSVT.
A typical ECG recorded during sinus rhythm in a patient with WPW syndrome shows ventricular preexcitation which is manifested by a short PR interval, a wide QRS complex and presence of a delta wave Figure5A. In many patients, ventricular preexcitation is present at all times and at all heart rates. In some patients, ventricular preexcitation may be intermittent. In such cases, preexcitation is usually present at lower heart rates and it may be documented with ambulatory Holter monitoring. Ventricular preexcitation during sinus rhythm is produced by a wave of depolarization which enters one of the ventricles through a bypass tract in the anterograde direction (from atria to ventricles).
The macroreentry circuit of PSVT is produced by the atrial muscle, the A-V node, the ventricular muscle and the accessory pathway. The reentry circuit allows for continuous alternate depolarization of atria and ventricles. During typical orthodromic reciprocating atrio-ventricular tachycardia (orthodromic AVRT), the macroreentry proceeds from the atria to the ventricles through the A-V node, and back up to the atria from the ventricles in the retrograde direction via the A-V bypass tract Figure5B. In most patients with WPW syndrome, an accessory pathway behaves like a two-way street for electric conduction. In orthodromic AVRT, QRS complexes are narrow and the retrograde P wave may be seen embedded into the early portion of the T wave. The retrograde P wave is usually best seen in leads II and V2. In orthodromic AVRT, QRS complexes may become wide because of aberrant conduction with a right or left bundle branch block. At times, one can see PSVT begin with wide QRS complexes for several beats followed by narrow QRS tachycardia Figure6.
Only infrequently, the reentry circuit may proceed in the opposite, antidromic direction allowing for ventricular depolarization through the A-V bypass tract and return of the depolarization wave back to the atria through the A-V node. Antidromic AVRT presents with very abnormal, wide QRS complexes which resemble ventricular tachycardia Figure3. Presence of ventricular preexcitation on a resting ECG points toward antidromic AVRT. Sudden death in patients with WPW syndrome is discussed in the section "Risk of Cardiac Arrest".
In many patients, a bypass tract behaves like a one way street only. A bypass tract which conducts exclusively in the anterograde direction from atria to ventricles manifests itself by ventricular preexcitation and may allow for antidromic reentry tachycardia. Even patients with isolated ventricular preexcitation and no history of PSVT are at risk of cardiac arrest from paroxysmal atrial fibrillation if the bypass tract is capable of very rapid conduction.
A concealed A-V bypass tract is an accessory pathway capable of conducting solely in the retrograde direction - from ventricles to atria. An electrocardiogram recorded during sinus rhythm is normal; there is no ventricular preexcitation. Concealed A-V bypass tracts participate in orthodromic AVRT. Concealed bypass tracts are very common.
Certain infrequently encountered accessory pathways have unusual conduction properties that may resemble those of an A-V node. Most of these unusual bypass tracts cross the tricuspid valve in the septal or inferior half of the valve ring. Permanent Junctional Reciprocating Tachycardia (PJRT) is produced by a slow-conducting A-V bypass tract which renders the A-V reciprocating tachycardia very stable and unremitting. In PJRT, an ECG shows inverted P waves in inferior limb leads that precede QRS complexes. PJRT may lead to heart failure in neonates and in older children. A Mahaim fiber is another example of an unusual A-V bypass tract which presents with slow conduction properties and connects to the right branch of the bundle of His. Mahaim fibers produce ventricular preexcitation that resembles left bundle branch block with a normal PR interval.
The general rules regarding prophylactic treatment of PSVT were already described in another section. Patients with WPW syndrome should not be treated with medications which enhance conduction along the bypass tract such as calcium channel blockers and digoxin. These medications increase the risk of cardiac arrest precipitated by atrial fibrillation. Class III antiarrhythmic agents such as amiodarone and sotalol are effective and safe. Class Ic agents (propafenone, flecainide) and beta adrenergic blockers are also effective. A physician may consider prophylactic treatment with digoxin in infants without ventricular preexcitation. Calcium channel blockers may be used for prevention of AVRT mediated by concealed A-V bypass tracts but must not be prescribed for children younger than 2 years of age. Ablation of the accessory pathway may be considered as the primary treatment option at school age and in small children who failed medical treatment for significant arrhythmia.
Atrio-ventricular node reentry tachycardia (AVNRT)
AVNRT is the most common form of paroxysmal tachycardia in adolescence and adulthood Figure7. AVNRT is very unusual in infants and toddlers and only school aged children may present with AVNRT for the first time. AVNRT is typically triggered by physical activity. This form of PSVT is not associated with a risk of cardiac arrest unless a patient has significant heart disease.
The anatomic substrate for AVNRT is a dual electric input from the right atrium into the A-V node.[15] This dual input is a normal, physiologic finding and consists of a "fast pathway" and a "slow pathway". The "fast pathway" is located in the low interatrial septum near the His bundle recording site. The "slow pathway" is found above the ostium of the coronary sinus and close to the ring of the tricuspid valve. Both pathways are connected by a broad area of tissue at the site of their input to the A-V node. In patients with AVNRT, physiologic conduction properties of both pathways are such that they allow for a microreentry circuit at the entrance to the A-V node. The most common direction of the reentry is such that the "slow pathway" conducts toward the A-V node and the "fast pathway" allows for retrograde activation of the atria. In an unusual atypical form of AVNRT, the direction of reentry is opposite.
An ECG recorded during sinus rhythm is normal. An ECG recorded during AVNRT shows normal, narrow QRS complexes unless tachycardia produces a functional bundle branch block. P waves are not clearly seen because activation of atria and ventricles occurs at the same time and P waves are embedded within QRS complexes.
All medications that slow conduction through the A-V node are effective in preventing recurrences of AVNRT. Beta adrenergic blockers and calcium channel blockers are frequently used. Digoxin usually does not control PSVT. Ablation procedures for AVNRT are associated with a 1-2% risk of a heart block which requires implantation of a permanent pacemaker. At the time of an ablation procedure, an operator usually disrupts the slow pathway leading into the A-V node.
Ectopic (automatic) atrial tachycardia
Atrial tachycardia is an uncommon variety of PSVT in children and adolescents.[16] In infants, atrial tachycardia is more prevalent and may account for 15% of all cases of supraventricular tachycardia. Atrial tachycardia may be paroxysmal or incessant. The tachycardia usually originates from a small ectopic focus or from an area of abnormal atrial muscle in the right or in the left atrium. The electrophysiologic basis of atrial tachycardia is either increased automaticity of myocardial cells in the atrium or reentry within the atrial myocardium. Atrial tachycardia may produce rapid heart rates and may prove refractory to medical treatment. Incessant atrial tachycardia may lead to life-threatening heart failure.
As opposed to the AVRT and AVNRT, atrial tachycardia may have a relatively slow onset and a gradual offset. Patients may report that their tachycardia begins suddenly or the heart rate increases rather gradually. Many patients are asymptomatic. An ECG usually shows regular tachycardia with narrow QRS complexes unless the tachycardia produces a functional bundle branch block in which case QRS complexes are wide. Ventricular rhythm is irregular in such cases when atrial rate is so rapid that not every atrial beat is conducted to ventricles Figure8. P waves are usually best seen in leads II, V1 and V2. When every atrial beat is conducted to ventricles, the PR interval is usually shorter than the RP interval and the morphology of P waves is abnormal. P waves produced by atrial tachycardia may resemble normal P waves of sinus rhythm when the ectopic focus is located in close proximity to the sinus node.
Chaotic (multifocal) atrial tachycardia is very unusual in children. Atrial activation from multiple foci in the atrial myocardium produces P waves of different morphologies and atrial rate may exceed 400/min. An echocardiogram frequently shows normal structure and function of both atria and ventricles.
Vagal maneuvers do not interrupt atrial tachycardia. Adenosine usually does not terminate tachycardia, however it will produce a higher degree A-V block allowing for clear visualization of rapid atrial P waves. Infrequently, adenosine may terminate atrial tachycardia which makes correct diagnosis more difficult.
Short, asymptomatic and even relatively frequent bouts of atrial tachycardia might not require any antiarrhythmic therapy since brief spells of rapid heart rate do not impair ventricular function. Digoxin, beta adrenergic blockers and calcium channel blockers may not prevent recurrences of tachycardia. Medications that belong to class Ia, Ic and III are more effective. One may consider therapy with propafenone or flecainide alone or in combination with digoxin or a beta blocker. Flecainide must not be used in patients with heart disease other than PSVT. Sotalol is used as monotherapy, in combination with digoxin or cautiously in combination with class Ic agents. Amiodarone may be effective but long term therapy is associated with several side effects. Medications that belong to class I and III should be prescribed by an experienced physician since their administration is associated with a risk of significant side effects and proarrhythmia. An ablation procedure should be considered in children at school age when it allows for permanent cure in more than 80% of cases. An ablation procedure may be effective in infants with rapid heart rates or ventricular dysfunction who failed medical therapy.
Intermittent ectopic atrial rhythms at rates close to the normal heart rate for age are common in children of school age and in adolescence. Ectopic atrial rhythms produce abnormal P wave morphology but they do not represent any heart disease and they do not lead to any ventricular dysfunction. As a rule, even very frequent premature atrial beats and ectopic atrial rhythms do not predict occurrence of PSVT and do not degenerate into ectopic atrial tachycardia. An exercise stress test will show normalization of P waves as the sinus rhythm takes over even with mild physical activity. Ectopic atrial beats and intermittent ectopic atrial rhythms do not require any treatment.
Atrial flutter, IART and atrial fibrillation
Atrial flutter is an unusual arrhythmia in children. It may occur in a fetus when it presents with ventricular rates of 150 - 220/min. Atrial flutter may be present at birth or it may occur in the first days of life. Intravenous administration of adenosine does not terminate this tachycardia but it allows for clear visualization of "saw tooth" flutter waves Figure9. Electric cardioversion should be done in a neonate with heart failure. Treatment with digoxin is frequently effective in an infant with preserved heart function but it may take several days before atrial flutter terminates. Class Ia (procainamide), Ic (propafenone, flecainide) or class III (sotalol, amiodarone) antiarrhythmic medications can be used if digoxin fails to restore sinus rhythm. All antiarrhythmic agents with the exception of digoxin exert negative inotropic effect on ventricular myocardium and may precipitate life threatening hypotension. Electromechanical dissociation has been reported after intravenous administration of amiodarone. Once neonatal flutter terminates it usually does not recur. It is advisable to continue treatment with digoxin for six months.
Atrial flutter is very unusual in older children and adolescents with normal hearts. It is more common in children with congenital heart disease. In typical atrial flutter, repetitive atrial activation results from continuous flow of a depolarization wave around the tricuspid valve (macroreentry). This reentry circuit becomes very stable in patients with a dilated right atrium. Antiarrhythmic medications are only infrequently successful in converting atrial flutter back to sinus rhythm. Ablation procedures for typical atrial flutter target the isthmus between inferior vena cava and the ring of the tricuspid valve.
Patients with heart disease and especially those after open heart surgery often present with atrial tachycardia resulting from several different reentry circuits in the right or left atrium.[17] The right astronomy scar, scaring from repair of septal defects as well as other suture lines in atrial walls produce obstruction to uniform propagation of atrial activation during sinus rhythm and create complex pathways for intra-atrial reentry tachycardia (IART). Electric cardioversion is generally effective in restoring sinus rhythm although IART may recur within a short period of time. Atrial surgery predisposes to sinus node dysfunction which further increases risk of recurrent IART. Incisional atrial tachycardia is consistently resistant to medical therapy and should be treated in specialized cardiac centers. At times IART can be suppressed with sotalol or amiodarone. Infrequently, a patient with atrial scarring may present with automatic atrial tachycardia from an ectopic focus rather than with IART.
Paroxysmal atrial fibrillation is even less prevalent than atrial flutter. An ECG recorded during atrial fibrillation shows an irregularly irregular rapid heart rhythm. QRS complexes are narrow although some may become wide because of intermittent aberrant ventricular conduction. Infrequent bouts of atrial fibrillation in patients with normal heart function do not require any treatment except for administration of aspirin to prevent thromboembolic complications. Children with congenital or acquired (rheumatic) heart disease are more likely to develop atrial fibrillation. Treatment should address the cause of hemodynamic dysfunction before targeting arrhythmia. The efficacy, safety and long term outcomes of catheter ablation procedures for atrial fibrillation in children are unknown. Electric cardioversion for sustained atrial flutter, fibrillation or IART must be preceded with several weeks of anticoagulation.
Junctional tachycardia
Congenital junctional ectopic tachycardia (JET) is also very uncommon. It generally presents in the first 6 months of life. An ECG shows regular tachycardia at heart rates ranging from 150 to 200 per minute and narrow QRS complexes. Retrograde P waves are found just behind QRS complexes or there may be complete A-V dissociation with the atrial rate slower than the ventricular rate. Incessant JET at a rapid heart rate may lead to heart failure. Electric cardioversion does not terminate this tachyarrhythmia. JET is difficult to treat although it may yield to therapy with amiodarone at a dose as high as 250-500 mg/m2 per day orally. Patients with severe ventricular dysfunction who fail medical treatment should be considered for catheter ablation therapy. Transient postoperative JET is commonly seen in small children during the first week following open heart surgery for complex heart disease.
Slow escape junctional rhythms during sinus bradycardia as well as transient accelerated junctional rhythms are common in healthy children and adolescents. These rhythms are a normal finding and do not require any therapy.
Conclusion
PSVT in children may have several different underlying mechanisms. General rules regarding acute and chronic therapy are similar for most forms of paroxysmal tachycardia, however clear understanding of pathophysiology of different tachyarrhythmias will assist a physician in selecting the best possible treatment. PSVT is usually benign and patients with asymptomatic and infrequent tachycardia do not require any intervention. Conversely, frequent and symptomatic tachycardia or PSVT presenting with syncope should be treated. Neonates, patients with ventricular preexcitation and those with impaired ventricular function from heart disease are at risk of cardiovascular collapse or sudden death. Patients with ventricular preexcitation must not be allowed to participate in competitive sports until a bypass tract is interrupted with an ablation procedure. Medical treatment is the preferred option for infants and toddlers while ablation procedures are very effective and safe in older children and adolescents. Patients with less common forms of PSVT as well as those with heart disease should be referred to a pediatric cardiology clinic. Rapid administration of adenosine not only terminates most common forms of PSVT but also allows establishing correct diagnosis in refractory cases; it is essential to record a rhythm strip within seconds after adenosine push.
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