Supraventricular Tachycardia FREE

Supraventricular tachycardia (SVT) is the most common rhythm disturbance in children.¹ It is estimated to occur in as many as 1 in 250 otherwise healthy children. Episodes are often recurrent and, although rarely life threatening, they may be life altering. Treatment of this disorder has undergone a remarkable transformation in the past quarter century. Although SVT accounts for a small proportion of children treated in an outpatient setting, the prevalence is high enough that most general pediatric practitioners will, at some point, care for a patient with this disorder. This review will cover the spectrum of this common rhythm disorder, from symptom recognition and epidemiology to management, with special attention to advancements in the available treatment options.

Before reviewing the mechanism underlying most forms of SVT, a review of how the electrical signals normally propagate through the heart is worthwhile. The cardiac conduction system extends from the atrium to the ventricular myocardium. In the normal heart, the atrial and ventricular myocardium are electrically insulated from one another except at the atrioventricular node (AVN) and bundle of His. Impulse generation typically originates in the sinus node, and the impulse is conducted through the atrial myocardium to the AVN. The major role of this structure is to allow conduction of the impulse to the ventricle; however, equally important is the inherent delay in the AVN that slows conduction from the atrium to the ventricle, allowing ventricular filling. From the AVN, there is rapid conduction via the specialized His-Purkinje system with associated right and left bundle branches that intercalate to the ventricular myocardium.

Supraventricular tachycardia is broadly defined as a narrow, complex tachycardia that requires atrial tissue or the AVN as an integral part of the arrhythmia substrate. The majority (>90%) of the clinically important SVT in otherwise healthy children is caused by the presence of an additional (or accessory) electrical connection between the atrium and ventricle (ie, the bundle of Kent) or within the AVN itself.² As such, SVT mediated by these accessory connections will be the focus of the remainder of this article. The mechanism underlying these forms of SVT is known as reentry, and the fundamental aspects of this arrhythmia mechanism were elucidated during the past century.³ Reentry requires the presence of 2 electrophysiologically distinct pathways around an insulated core (eg, the atrioventricular valve annulus) (Figure 1). In reentrant rhythms, the electrical impulse can cycle and recycle repetitively in a manner similar to a dog chasing its tail. Understanding the mechanism of reentry is critical to understanding the targets of short- and long-term therapeutic strategies for SVT (Figure 2).

As discussed in the preceding subsection, accessory connections provide the generic substrate for reentry SVT. However, a number of different types of accessory connections have been identified and subcategorized according to differing electrophysiological properties. These differences alter the clinical characteristics of the patient with SVT and have allowed us to define different subtypes of SVT as distinct clinical entities.

The Concealed Accessory Connection. In this case, the accessory connection between the atrium and ventricle does not disturb electrical signal transmission during normal sinus rhythm. Thus, on a routine electrocardiogram (ECG), this accessory connection is “concealed” or not visible, as described in the “Diagnostic Tools” subsection. This accessory connection provides abnormal electrical conduction backward or retrograde from the ventricle to the atrium. Because of this, the concealed accessory connection is also known as the unidirectional retrograde accessory pathway.

The Wolff-Parkinson-White Accessory Connection. Named after the 3 individuals credited with elucidating its electrical properties,⁴ the Wolff-Parkinson-White (WPW) accessory connection allows abnormal propagation of electrical signals forward (or antegrade) from the atrium to the ventricle and, in most cases, retrograde from the ventricle to the atrium. This not only creates a characteristic appearance on a baseline ECG (Figure 3), but also provides a substrate for typical reentry SVT and, importantly, may allow the rapid transmission of electrical signals generated in the atrium to the ventricle with potentially life-threatening consequences (described in the “Prognosis” section).

Permanent Junctional Reciprocating Tachycardia. The accessory connection in the permanent junctional reciprocating tachycardia subform of SVT behaves much the same way as a concealed accessory pathway, except that the rate of transmission of the electrical impulse through the accessory connection is slower than usual. This creates a unique ECG appearance during SVT and heart rates in SVT that can be remarkably slow (130-150 beats/min) by typical SVT standards. Patients with this form of SVT often have medically refractory arrhythmias that can be quite challenging to manage.

Atrioventricular Nodal Reentry Tachycardia. Patients with the atrioventricular nodal reentry tachycardia (AVNRT) form of SVT have the functional equivalent of an extra connection within the AVN. Unlike an accessory pathway, these extra connections are not visible histologically. Nonetheless, they provide the necessary substrate for reentry arrhythmias in a manner similar to that seen with accessory connections.

The true incidence of SVT in children is unknown but has been estimated to be 1 in 250 to 1 in 1000 children.⁵ Approximately 50% of children with SVT will present with their first episode in the first year of life.^6- 7 After infancy, the incidence peaks in early childhood (ages 6-9 years) and then again in adolescence.^7- 8 In infants, spontaneous resolution occurs in more than 90% by 1 year of age. After a period of quiescence, up to one-third will have recurrence of SVT at a mean age of 8 years.⁷ Although spontaneous resolution is the norm in infants, only a small minority (15%) of patients who receive the diagnosis after 1 year of age will have spontaneous resolution.⁶ Supraventricular tachycardia due to concealed or WPW accessory pathways predominates throughout childhood and adolescence, whereas the relative proportion of patients with AVNRT tends to increase with age.² Most individuals with SVT have a structurally normal heart. However, the prevalence of structural congenital heart disease in patients with SVT has been estimated at 9% to 32%, which is substantially higher than in the general population. The most common association is noted between WPW syndrome and the Ebstein anomaly of the tricuspid valve, but a number of defects have been found, including ventricular or atrial septal defects, among others.^2,7- 9

Most cases of reentrant SVT are sporadic, with approximately 7% of patients having documented SVT in a first-degree relative.¹⁰ Most cases of WPW syndrome are also sporadic, although patients with WPW syndrome have a 3-fold higher risk than the general population of having an affected first-degree relative with WPW.¹¹

The clinical presentation of SVT is age and duration dependent. In infants with paroxysmal SVT, the heart rate is usually 220 to 320 beats/minute; in older children, it is 160 to 280 beats/minute.⁶ In infants, symptoms are usually nonspecific and include poor feeding, irritability, vomiting, cyanosis, and pallid spells. If the symptoms are unrecognized for hours to days, the infant can present with significant hemodynamic compromise or heart failure symptoms.¹² It is rare for infants who have SVT for less than 24 hours to develop signs of congestive heart failure at the time of presentation; however, congestive heart failure is present in 19% of infants who have SVT for 24 to 36 hours and in 50% who have SVT for more than 48 hours.⁶ Approximately 20% of infants receive a diagnosis during routine office visits and during asymptomatic episodes.^6,8 In verbal children with SVT, palpitations and fluttering in the chest are the usual presenting symptoms. Because reentrant arrhythmias are a circuit, they tend to be all or nothing, and the onset is frequently described as being abrupt, similar to a light switch being turned on. The offset may be less dramatic because the catecholamine level is typically elevated, with resultant sinus tachycardia at the termination of SVT and subsequent gradual slowing. Frequently, lightheadedness and dizziness due to transient hypotension can occur at the onset, but syncope is rare in SVT, and its presence should raise suspicion of something other than SVT. The frequency and duration of the episodes vary greatly from a few minutes to a few hours and occur as often as daily or as infrequently as once or twice per year. Although they are rare in verbal children, incessant SVT symptoms may go unrecognized until cardiac dysfunction develops.

Most patients presenting with episodic palpitations have a structurally normal heart and will have normal findings on the physical examination, particularly older children. Infants are more likely to present with signs of heart failure because the tachycardia may have gone unrecognized for longer periods.

Recording a heart rhythm strip during symptoms remains the key to correct diagnosis and management. Options for this include 24-hour ambulatory monitoring, event recorders, and ECGs. Each of these means of obtaining a recording have associated advantages and disadvantages. The Holter monitor provides a continuous multichannel recording that usually allows the interpreter to see the whole episode, including onset and termination. Most patients, however, do not have daily symptoms, making the Holter monitor typically impractical in the evaluation of SVT. Event recorders are often the optimal solution for patients who have symptoms more than once per month. Patients can wear the monitor (loop recorder) or carry it with them (event monitor). In both cases, patients activate the recording device during symptoms. The advantage of the loop recorder is that the recording encompasses the time before, during, and after the monitor activation. Finally, for infrequent episodes lasting longer than 10 minutes, patients can often be referred to the local emergency department or fire station for acute monitoring.

Excellent reviews focus on the short-term management of SVT, including the pediatric advanced life support algorithm.¹³

Once the rhythm has been recorded and the diagnosis has been confirmed, patients are typically referred to a pediatric cardiologist. The management of SVT has many variables that need to be considered, including the age of the patient, the duration and frequency of the episodes, and the presence of ventricular dysfunction. There are also important social and geographical factors, including access to care, that play a role. For children with rare and mildly symptomatic episodes in whom SVT is easily terminated, the SVT may not merit treatment. For children with episodes that are difficult to terminate, occur frequently, or occur during athletic participation, it may be advisable to offer medical therapy or transcatheter ablation as therapeutic options.

Infants with SVT deserve special recognition in regard to treatment options. Most infants will undergo spontaneous resolution of SVT. When this is combined with the increased risk of transcatheter ablation in this age group, most electrophysiologists opt for medical management during the first year of life.

Medical Therapy. Optimal medical management (in terms of who to treat, with which medication, and for how long) of SVT in infants and children has not been well studied, and most current clinical practices are extrapolated from small studies of adults¹⁴ and noncontrolled pediatric studies.¹⁵ A multicenter prospective randomized controlled trial comparing digoxin and β-blockers for the treatment of SVT in children is currently in progress (clinicaltrials.gov Identifier: NCT00390546; Shubhayan Sanatani, MD, FRCPC, oral communication, May 14, 2008).

The intended effect of antiarrhythmic medications is to slow conduction, preferentially within 1 limb of the reentrant circuit, thereby terminating the tachycardia as the circulating wave front encounters refractory tissue. Nearly all classes of antiarrhythmic agents have been used to treat SVT successfully. The approach to antiarrhythmic therapy includes the daily prophylactic therapy and the single-dose “pill-in-the-pocket” approach whereby medication is taken only during an acute episode.¹⁶ The pill-in-the-pocket approach requires an immediate-release medication and is appropriate for patients who have infrequent episodes that are prolonged but well tolerated.

In most cases, first-line therapy is directed at modifying the conduction properties of the AVN and includes treatment with digoxin, β-blockers, and calcium channel blockers¹⁷ (Table 1 and Figure 2). Except in WPW syndrome, when use of calcium channel blockers and digoxin should be avoided, therapy can be initiated with limited regard to the underlying mechanism. As is typical with medical therapy, there is a significant reduction in the number of episodes, although complete suppression is rare.¹⁴ In general, a steady state is achieved after the drug has been administered continuously at the same dose and interval for at least 5 half-lives; therefore, caution is advised if considering recurrence of SVT a medication failure in the first few days of therapy. Supraventricular tachycardia refractory to first-line medications can often be controlled with more potent antiarrhythmic agents such as flecainide acetate, amiodarone, sotalol hydrochloride, or drug combinations. Sodium channel blockers such as flecainide are particularly effective in controlling SVT,¹⁶ but these agents are generally avoided in patients with structural or ischemic heart disease because of the risk of proarrhythmia. Sotalol, an agent with β-receptor and potassium channel blocking properties, is also quite effective¹⁸ but can lead to QT prolongation and proarrhythmia. When the potential for proarrhythmia and toxic effects from these agents is considered, their use should be overseen by an experienced electrophysiologist and reserved for patients in whom there is a higher risk of complications from an interventional electrophysiological procedure (eg, in infants and small children).

Ablation Therapy. The management of SVT has been revolutionized with the development of catheter-based ablative techniques that serve to eliminate the abnormal electrical connections that allow SVT to occur. Although once reserved for the sickest patients, catheter-based ablative therapy is now considered the standard of care for older children and adolescents with symptomatic SVT owing to its outstanding efficacy and safety profile.

Overview. Ablation therapy begins with a diagnostic electrophysiological study. This is performed in a cardiac catheterization laboratory. For children, the procedure may be performed under general anesthesia to minimize patient movement. Access is obtained to the venous or the arterial circulation through established techniques,¹⁹ and catheters with electrode sensors are advanced into specific areas of the heart, usually the right atrium, the crux of the heart near the normal atrioventricular conduction system, in the coronary sinus and the right ventricle. Using various pacing maneuvers, one can identify the abnormal electrical connection that serves as the substrate for SVT by its alteration of the normal electrical signals. A specialized ablation catheter is then used to perform millimeter-by-millimeter mapping of the abnormal electrical signals until the precise location of the extra electrical connection is identified. The tip of the mapping catheter is then used to disrupt (or ablate) the discrete region of tissue beneath the catheter, thus eliminating the abnormal electrical connection and the substrate for SVT. Because it is a catheter-based procedure, recovery is minimal. Patients are usually sent home the same day as the procedure and return to full activity within a week.

Ablation Energy Source. Some of the greatest advances in catheter-based ablation therapy since its inception in the early 1980s have been centered on the ablation energy source. Early catheter-based ablation efforts used energy from a direct current,²⁰ making them relatively crude by today's standards.^21- 22

Radiofrequency (RF) ablation followed, with the first use of RF catheter ablation in children reported in the early 1990s.²³ Radiofrequency energy allows for much greater control of the energy delivery and therefore greatly enhanced the safety of catheter-based ablation. Ultimately, RF catheter ablation supplanted ablation that used direct current. The safety and efficacy of RF ablation in children was established through the development of an ablation registry.²⁴ Based on findings in this cohort, the early results of ablation procedures are quite promising, with a short-term success rate of 93% and a low incidence of complications (1%).^25- 26 Follow-up analysis of the cohort demonstrated that, overall, 90% of patients who underwent an acutely successful ablation remained free of recurrence at the 1-year follow-up.²⁷

In the early part of this century, the armamentarium of the interventional electrophysiologist was expanded further with the addition of cryoablation technology. Rather than heat disruption, cryoablation technology causes tissue disruption by freezing the tissue under the ablation catheter tip. Cryoablation has several advantages compared with RF ablation, particularly for children. Most notably, cryoablation has essentially eliminated the risk of unintentional heart block during ablation procedures. Unfortunately, the enhanced safety afforded by this technology may be offset by higher SVT recurrence rates.²⁸ For this reason, in most centers, cryoablation has not replaced RF ablation, but rather serves as an important adjunct technology in situations where RF energy would be deemed higher risk (eg, near the normal conduction system, in the coronary sinus). The interested reader is referred to an outstanding review on this topic.²⁹

In the absence of structural heart disease or cardiomyopathy, the prognosis of SVT is believed to be excellent.

Wolff-Parkinson-White syndrome deserves special consideration with respect to prognosis owing to the small but real risk of sudden cardiac death associated with this condition. The principal indicator of risk in adults with WPW syndrome is the presence of symptoms (eg, palpitations and syncope). Symptomatic patients with WPW syndrome have an estimated 3% to 4% lifetime risk (0.25% per year) of sudden cardiac death.³⁰ In children and adolescents, however, asymptomatic may be better termed presymptomatic. It has been estimated that 55% of asymptomatic children and adolescents will become symptomatic by 40 years of age.³¹ For that reason, all children or adolescents in whom WPW syndrome is identified on an ECG, regardless of the presence of symptoms, should be referred to a pediatric electrophysiologist for evaluation. Patients with WPW accessory pathways who are defined as low risk on the basis of electrophysiological study findings appear to be at no increased risk of sudden cardiac death compared with the general population.

Empowering families should be one of the priorities of physicians who treat children with SVT. Physicians should reiterate that SVT is typically not life threatening but can be life altering. Education should focus on the diagnosis, prognosis, and treatment options. Emphasis should be placed on decreasing anxiety and alleviating the fear of the unknown. The family should understand what to expect, whom to contact when an episode occurs, and when to contact emergency service personnel. School emergency plans are also important and should include similar information (Table 2).

Most neighborhood and elementary school activities for young children involve lesser degrees of physical intensity and should be an allowable component of the school curriculum for patients with a diagnosis of SVT.³² It is generally believed that these recreational athletes may be more likely to report symptoms and willfully stop participation.

In comparison, competitive athletics require systematic training and regular competition against others and place a high premium on athletic excellence and achievement.³³ There is a strong inclination among competitive athletes to push beyond their physical limits. Therefore, the competitive athlete may incorrectly perceive an important symptom as a normal physiologic response to extreme exertion or may ignore the symptoms entirely to prevent early extrication from competition. In the case of high-speed or contact sports, this lack of recognition may place the athlete at risk of injury, particularly if there is a transient loss of mental function with a resultant loss of physical control.³⁴ As an example, a downhill skier or platform diver with dizziness from episodic SVT may be at greater risk of injury than a basketball player.

The American Heart Association has published guidelines for sports eligibility in patients with SVT.³⁴ In general, asymptomatic athletes without structural heart disease who have reproducible exercise-induced SVT prevented by medical therapy can participate in all competitive sports.³⁴ Athletes who have undergone successful transcatheter ablation, are asymptomatic, and have no inducible arrhythmia on follow-up testing can return to full competition several days after the procedure.³⁴

Because of the small risk of sudden cardiac death, symptomatic athletes with WPW syndrome should undergo a formal electrophysiological study, risk stratification, and possible ablation of their accessory pathway before participating in competitive sports. In asymptomatic athletes, the risks are less clear. Because younger patients may become symptomatic as they get older, the Bethesda Guidelines for athletic participation state, “in younger age groups, a more in-depth evaluation including an electrophysiology study may be recommended before allowing participation in moderate to high-intensity competitive sports.”^34(p1359) At a minimum, asymptomatic athletes with a WPW pattern on their ECG should undergo exercise testing or ambulatory monitoring during exercise. Those deemed to be at low risk by demonstration of abrupt loss of preexcitation at faster sinus rates may not require further testing. Those who remain in a state of preexcitation at maximal sinus rates should probably undergo a formal electrophysiological study and risk stratification.

In conclusion, SVT is a common condition in the pediatric population. In most cases, it represents a benign heart rhythm disorder, but infants, athletes, and patients with WPW syndrome deserve special consideration because of greater associated risks in these populations. Treatment with suppressive medications remains the first-line therapy for infants and small children. For symptomatic older children and adolescents, catheter ablation appears to be a safe, effective form of treatment. Education of patients and caregivers about the implications of SVT and development of age-appropriate care plans in conjunction with a pediatric cardiologist is critical to achieve optimal outcomes.

Correspondence: Jack C. Salerno, MD, Department of Pediatrics, Seattle Children's Hospital, 4800 Sand Point Way NE, Pediatric Cardiology, M/S G-0035, Seattle, WA 98105 (jack.salerno@seattlechildrens.org).

Accepted for Publication: August 28, 2008.

Author Contributions:Study concept and design: Salerno and Seslar. Acquisition of data: Seslar. Analysis and interpretation of data: Salerno and Seslar. Drafting of the manuscript: Salerno and Seslar. Critical revision of the manuscript for important intellectual content: Salerno and Seslar. Administrative, technical, and material support: Seslar. Study supervision: Salerno.

Financial Disclosure: None reported.