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Features Departments Information |
  Barbara J. Deal, MD |
Current concepts and treatment of BARBARA J. DEAL, MD aFall 1998 Fifteen-year-old L.C. is a competitive athlete for her high school teams, participating in basketball, soccer, and tennis. At the age of 12, she began experiencing episodes of a very rapid heart rate during peak exertion on the basketball court. She realized something was wrong when the rapid heart rate would continue even after sitting down and resting for 20 minutes. Physicians in the local emergency room found her in supraventricular tachycardia at 220 beats per minute and terminated the acute episode with intravenous adenosine. Her mother reports that it seemed they were back in the emergency room every week during the basketball season. Initial attempts to treat her with atenolol, a beta-blocking medication, were unsuccessful due to marked fatigue and inability to concentrate. She was subsequently treated with digoxin, which slowed the heart rate during tachycardia to 180 beats per minute. However, she continued to have one to three episodes of tachycardia monthly, which she was usually able to end by performing a valsalva maneuver. She was about to start her freshman year in high school and face an increased level of competition, and her parents were concerned that episodes of tachycardia would limit her team participation. Somewhat reluctantly, they agreed to consider a relatively new therapy, radiofrequency catheter ablation, as a potential cure for her tachycardia. On July 23, 1998, L.C. was admitted to Children's for a catheter ablation procedure performed under general anesthesia. It involved placing five catheters within her heart and, using pacing techniques, initiating supraventricular tachycardia. With all these catheters in place, the exact electrical activation of the heart could be closely mapped, identifying the circuits involved in her arrhythmia. In L.C.'s case, she had two types of tachycardia: one using an accessory connection on the left free wall of her heart and a second one involving reentry in her atrioventricular node. Following precise localization of the extra electrical connections, a catheter was placed at these sites and radiofrequency energy applied. This created a thermal lesion, effectively destroying the tissue at the tip of the catheter. Three hours later, both forms of tachycardia had been eliminated. L.C. went home the following morning and returned to sports three days later. She is now able to participate aggressively in her soccer and basketball games without fear of tachycardia and free of medications. What Is SVT? SUPRAVENTRICULAR TACHYCARDIA is a general term used to describe arrhythmias arising from or largely limited to the atria or atrioventricular node, although the ventricles may be part of the tachycardia circuit. Most clinical episodes of tachycardia are due to a reentry circuit, and the challenge is to identify the anatomic substrates involved in the reentrant circuit. In the past, SVT was referred to as "paroxysmal atrial tachycardia," or PAT. Currently, efforts are made to more accurately characterize the type of supraventricular tachycardia into three large groups: SVT that has an accessory connection, such as the Wolff-Parkinson-White syndrome; SVT involving reentry within the atrioventricular node; and SVT largely confined to atrial tissue, such as atrial flutter or an automatic atrial focus. Because each type of SVT has a distinct natural history and response to therapy, it is important to attempt to identify the exact mechanism of tachycardia.1  FIGURE 1. Mechanisms of common forms of SVT. (A) Orthodromic reciprocating tachycardia.
(B) AV node reentry. (C) Primary atrial tachycardia. Figure 1 illustrates the three major reentrant circuits in SVT. In SVT that has an accessory connection, conduction usually proceeds in a normal fashion from the atria via the atrioventricular node to the ventricles, returning to the atria via an accessory connection along the atrioventricular groove. This type of tachycardia is termed "orthodromic reciprocating tachycardia": ortho = same, dromic = direction, as in normal AV conduction. In atrioventricular nodal reentry tachycardia, there is functional and anatomic dissociation of conduction within the AV node, allowing electrical impulses to reenter the atria retrogradely as depolarization proceeds simultaneously to the ventricles. Primary atrial tachycardia includes all forms of SVT in which the electrical disturbance takes place within atrial tissue, and the AV node and ventricles are "bystanders" and are not necessary for the tachycardia to continue. A typical example of primary atrial tachycardia is atrial flutter, in which tachycardia persists in the setting of AV block. ECG Appearance SVT is typically recognized on the surface electrocardiogram by the rapid and regular rate with a normal QRS morphology. The average rate of SVT in infants is 280 beats per minute, and in older children it ranges from 160 to 300 beats per minute. The rapid rates of tachycardia in childhood make the ECG interpretation of the mechanism of tachycardia more difficult. However, an analysis of the QRS and P relationship may establish the type of SVT and help guide therapy.  FIGURE 2A. ECG tracings during SVT. Orthodromic reciprocating tachycardia: one-week-old infant with narrow QRS tachycardia at 210 beats per minute. Note retrograde P waves in leads 11, 111, AVF, and V1, with QRS-P interval of 80 msec.  FIGURE 2B. ECG tracings during SVT. AV nodal reentry: 11-year-old boy with regular narrow QRS tachycardia at 215 beats per minute. Note rsr' pattern in V1, and small deflection of retrograde P wave on ST segment in leads 11, 111.  FIGURE 2C. ECG tracings during SVT. Primary atrial tachycardia: 25-year-old man s/p Fontan repair of complex heart disease. Irregular tachycardia at 84 beats per minute. Note P waves in V1, at 260 beats per minute with variable AV conduction. Figure 2 illustrates the three most common forms of SVT as seen on electrocardiogram. In orthodromic reciprocating tachycardia, the P wave follows the QRS and is best seen as a small deflection in the ST segment in leads II, III, AVF, and especially in lead V1. The QRS-P interval is at least 70 milliseconds, representing the time necessary to travel through the ventricular muscle to the accessory connection. In atrioventricular nodal reentry tachycardia, the reentry loop is very small, and the retrograde P wave is inscribed almost simultaneously with the QRS: the P wave is then buried in the QRS or on the tail of the S wave. A typical appearance for AV nodal reentry tachycardia is an RSR' in V1, with a QRS-P interval less than 70 milliseconds. In primary atrial tachycardia, a distinct P wave is usually seen preceding the QRS, with an appearance distinct from a sinus P wave. Conduction through the atrioventricular node may be variable, and first- or second-degree block is often seen. Symptoms of SVT It took several episodes of tachycardia for the diagnosis of SVT to become apparent in L.C.'s case. This is a frequent problem in older children or adolescents with symptoms of palpitations, sometimes because of their inability to describe the racing heart beat. At other times, the symptoms are nonspecific, such as chest pain, stomach ache, or difficulty in breathing. Because initial episodes may be brief or self limited, they are often dismissed by family members or physicians. In infants, SVT may be noticed incidentally on a well-child examination, or the babies may present with upper respiratiory symptoms of congestion and poor feeding. Almost 50% of infants with SVT present with evidence of congestive heart failure, a condition that is quite rare in older children, probably because the symptoms are recognized earlier. The exception to this rule is in children with an automatic atrial tachycardia: because of incessant tachycardia at relatively slower rates, the condition is not recognized until heart failure develops, which may be severe. In a review of patients listed for heart transplantation with a diagnosis of "idiopathic dilated cardiomyopathy" at Children's, atrial tachycardia—a potentially curable diagnosis—was identified as the cause in 27% of patients. Risk of Sudden Death In a small number of patients—those with a condition known as Wolff-Parkinson-White syndrome, in which preexcitation is evident on the electrocardiogram—there is a small but significant risk of cardiac arrest during their tachycardia. Cardiac arrest is due to rapid conduction of atrial fibrillation over the accessory connection to the ventricle, precipitating ventricular fibrillation. Because most episodes of cardiac arrest due to Wolff-Parkinson-White syndrome occur in the first three decades of life, pediatric patients appear to be at the highest risk for this complication. Of most concern is the fact that in almost half of patients suffering cardiac arrest, the initial sustained arrhythmia is the cardiac arrest. Therefore, young patients with Wolff-Parkinson-White identified on ECG at Children's typically undergo screening using Holter monitoring and trans-esophageal pacing techniques to identify patients potentially at increased risk for cardiac arrest. Age-Related Incidence of SVT Studies performed at Children's have helped characterize the distinct age-related incidence in these different types of SVT. SVT using an accessory connection is the most common form of SVT in childhood and accounts for at least 80% of SVT in infancy, decreasing to about 45% of SVT in adulthood. Atrioventricular nodal reentry tachycardia occurs rarely in infancy, initially occurring in five- to ten-year-old children, and accounting for at least half of SVT in adults. Primary atrial tachycardias account for 10 to 15% of clinical SVT at all ages.
How to Treat SVT The approach to treatment of SVT is two-fold: the acute termination of an episode of SVT, and how to manage or prevent recurrences.2Table 1 (above) summarizes this approach. Acute treatment of SVT There are three general options for acute termination of tachycardia: vagal maneuvers, pharmacological conversion, and electrical conversion. Vagal maneuvers and adenosine produce transient atrioventricular block and result in abrupt termination of SVT in tachycardia involving the AV node but not in most forms of primary atrial tachycardia. The vagal maneuver most often used in infancy is placing a bag of ice water over the affected infant's eyes and face for ten to 20 seconds, which is effective in about 25% of cases. Ocular pressure is not recommended in children; older children are taught to perform a Valsalva maneuver. Adenosine, an endogenous nucleoside producing acute AV node blockade, is now the first drug of choice for terminating SVT and is effective in about 90% of cases. Digoxin is no longer used for the acute termination of SVT and should be avoided in older children with Wolff-Parkinson-White syndrome, due to the risk of accelerating conduction over the accessory connection. Electrical cardioversion with 0.25–1 joule/kg is always the treatment of choice for SVT with critical hypotension. Long-Term Therapy The age of the patient and the type of SVT are major determinants for choosing optimal chronic therapy. Among infants presenting with SVT, approximately one third will outgrow their tachycardia, and at least one half of patients with automatic atrial tachycardia may have resolution of tachycardia. The presence of preexcitation on electrocardiogram prompts an evaluation for stratification of risk of cardiac arrest plus the avoidance of digoxin or verapamil therapy. The symptoms during tachycardia and frequency of episodes, as well as the sophistication of the patient's caretaker, also are important guides to treatment. In many patients, no chronic therapy may be necessary: these are patients with infrequent, well-tolerated episodes of tachycardia not associated with preexcitation. Infants with recurrent or symptomatic tachycardia will require daily medications such as propranolol, sotalol, or amiodarone, for most of their first year of life. Certain older children are able to end acute episodes of SVT with a Valsalva maneuver or may prefer an infrequent trip to the emergency room over chronic medications. In symptomatic patients with recurrent tachycardia over the age of five years, radiofrequency catheter ablation has become the treatment of choice. Patients presenting with tachycardia in this age group are not likely to "outgrow" their tachycardia and are not likely to tolerate or remain compliant with daily medications. In addition, some asymptomatic patients with Wolff-Parkinson-White syndrome who are thought to be at increased risk for cardiac arrest, may be referred for ablation. Ablation is performed in patients between two to five years of age when SVT is refractory to anti-arrhythmic medications or when there is concern over potential drug side-effects during long-term administration. In the past, the only options for patients with refractory or life-threatening arrhythmias were anti-tachycardia pacemakers or surgical ablation. Radiofrequency Catheter Ablation Since 1988, radiofrequency catheter ablation has been used as definitive therapy for most forms of supraventricular tachycardia and some forms of ventricular tachycardia. Using intracardiac catheters, radiofrequency energy is used to dessicate a small, well-circumscribed area of cardiac tissue thought to be essential to the arrhythmia circuit, such as the accessory connection (Figure 3).  FIGURE 3. Catheter positions used for electrophysiology study and radiofrequency catheter ablation of SVT using a left laternal accessory connection. Destruction of cardiac tissue using this thermal lesion results in interruption of the electrical impulses essential for tachycardia, generally while preserving normal conduction. Between 1991 and 1997, more than 5,200 pediatric patients have undergone this procedure in the United States. The major pediatric centers performing this procedure, including Children's Memorial Hospital, have formed a voluntary registry for reporting and following the acute results, complications, and long-term follow-up of radiofrequency catheter ablation, which were reported in The New England Journal of Medicine.3 Acute success rates for curing SVT using the catheter ablation technique vary with the type of tachycardia, including the location of an accessory connection, the presence of structural heart disease, and the size of the patient. For left-sided accessory connections, acute success is reported in about 95% of patients, decreasing to 85% for right-sided connections. For atrioventricular nodal reentry tachycardia, the success rate is 96%. Automatic ectopic atrial tachycardia foci are cured about 87% of the time, and significantly lower success rates are encountered with other forms of primary atrial tachycardia. For atrial "flutter" or reentry tachycardia following repair of congenital heart disease, acute success rates are at most 50%. Recurrence of tachycardia following the ablation procedure is often disappointing and may be due to resolution of tissue edema or trauma. For accessory connections and AV nodal reentry tachycardia, the recurrence of tachycardia ranges from 10% to 40%, although recurrent tachycardia may be infrequent or reappear only after two to three years of follow-up. The recurrence risk is highest in patients with atrial reentry tachycardia following surgery for structural heart disease; in these cases it approaches 50 to 75%. Complications occur in 3 to 8% of catheter ablation procedures, with major complications occurring in 3% of cases. Significant risks include radiation exposure, cardiac perforation, tamponade, AV node block requiring a permanent pacemaker, brachial plexus injury, pneumothorax, peripheral embolization, and vascular injury, including acute coronary injury. At least six deaths have been reported as complications of ablation. Experimental data in lambs has shown growth of the ablation lesion in myocardial tissue with growth of the animal, which may then serve as an arrhythmogenic focus; this phenomenon is thought not to occur in mature animals, where the lesions contract over time. These findings have prompted most centers to avoid this procedure in young infants, except in the rare setting of life-threatening arrhythmias. Clearly, the risks of the ablation procedure need to be examined carefully in comparison with the risks of the tachycardia and drug therapy. Surgical Therapy of Arrhythmias The advent of catheter ablation techniques largely eliminated the traditional role of a surgical therapy for accessory-connection mediated tachycardia. In contrast, catheter ablation procedures have poor success rates in patients with atrial reentry tachycardia following surgical repair of congenital heart disease; mortality from tachycardia in these patients is about 20%. Reasons for these poor results include distorted anatomy, extensive scarring, and the inability of current catheter techniques to deliver lesions of sufficient depth in thickened and hypertrophied atrial tissue. At Children's Memorial Hospital, we speculated that many of the problems limiting the transcatheter technique could be overcome with a direct surgical approach employing a combination of incisions and cryoablation lesions. At the same time, residual hemodynamic abnormalities creating atrial or ventricular hypertension could be corrected. At Children's Memorial, we have embarked on a unique surgical program to treat refractory arrhythmias in youngsters who were not good candidates for catheter ablation techniques. Together with Drs. Constantine Mavroudis and Carl Backer from the Division of Cardiovascular-Thoracic Surgery and Christopher Johnsrude from the Division of Cardiology, we have developed a coordinated program of combining surgical repair or revision of complex congenital heart disease with direct cryoablation of the arrhythmia circuit. Using this approach, we have successfully operated on 16 patients with refractory atrial tachycardia following prior Fontan surgeries. These patients had failed multiple antiarrhythmic medications and underwent extensive preoperative, intraoperative, and postoperative electrophysiologic studies. The surgical technique has included right atrial resection, cryoablation of critical areas of the atrial arrhythmia circuit, conversion of the Fontan to a total cavopulmonary connection, and transmural insertion of an atrial anti-tachycardia pacemaker. Tachycardia has been successfully eliminated in more than 90% of patients, with no mortality; all patients have experienced a significiant improvement in functional class and exercise capacity. These results have generated widespread interest from other centers in the United States, Europe and Australia in duplicating this approach in a problematic patient population, who would otherwise be considered for heart transplantation. Encouraged by the results in the Fontan population, the surgical arrhythmia program has expanded to other patient populations. We are now performing ablations in patients undergoing their initial Fontan procedure, in the expectation that this will eliminate a significant potential arrhythmia circuit. In addition, we have combined the cryoablation technique in lieu of a transcatheter approach in patients with an arrhythmia who require intracardiac surgery. By localizing the tachycardia circuit preoperatively, we eliminate the extensive catheter manipulation and fluoroscopy exposure necessary to perform the radiofrequency ablation and instead perform direct surgical cryoablation. Perhaps most dramatically, there are a few patients with structurally normal hearts and debilitating arrhythmias who have failed multiple attempts at catheter ablations. One teenage boy with life-threatening ventricular tachycardia underwent successful resection of the tachycardia focus from the left-ventricular outflow tract and has been free of tachycardia, without medications, for more than a year. Another adolescent girl underwent atrial resection of the lateral right atrial wall and incisional isolation of the pulmonary veins to eliminate a wide area of reentry within the atria. Most recently, a modified Maze procedure has been performed to eliminate atrial fibrillation in patients undergoing concurrent repair of congenital heart disease; to our knowledge, this type of surgery has not been performed previously in this setting. Summary Current understanding of tachycardia mechanisms allows a more precise definition of the type of supraventricular tachycardia experienced by patients. Armed with this knowledge, cardiologists can more accurately guide both the acute therapy and the expectations for recurrent arrhythmia and long-term outcome. Using a wide array of techniques including transesophageal pacing, newer antiarrhythmic medications, catheter ablation techniques, anti-tachycardia pacemakers, and surgical intervention, we can offer most patients the expectation of life free of tachycardia. REFERENCES 1. Deal BJ: Supraventricular tachycardia: Mechanisms and natural history. In Current Concepts in Diagnosis and Management of Arrhythmias in Infants and Children, Deal, Wolff, Gelband (eds.), Armonk, NY: Future Publishing Co., Inc., 1998. 2. Benson DW, Deal BJ: Primary treatment of supraventricular tachycardia in infants and children. Prog Pediatr Cardiol 1995;4:209–214. 3. 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;330:1481–87. 4. Mavroudis C, Backer CL, Deal BJ, Johnsrude CL: Fontan conversion to cavopulmonary connection and arrhythmia circuit cryoablation. J Thorac Cardiovasc Surg 1998;115:547–556. |