Catheter Ablation Of Accessory Pathways Research Articles

Body surface distribution of abnormally low QRST areas in patients with Wolff-Parkinson-White syndrome. Evidence for continuation of repolarization abnormalities before and after catheter ablation.

Whether the Wolff-Parkinson-White syndrome (WPW) is associated with repolarization abnormalities is controversial. The QRST isointegral map (I-map) is theoretically independent of the activation sequence and dependent on repolarization properties. There have been no reports concerning the effects of radiofrequency (RF) catheter ablation of accessory pathway (AP) on repolarization properties analyzed by I-mapping. I-maps were constructed from data recorded in 38 patients with WPW to investigate repolarization properties and their body surface distribution in a physiological state, without pharmacological influences, and in 13 ablated patients to elucidate the effects of RF ablation on repolarization properties. Patients were divided into three groups: group A, 15 patients with type A WPW (left-sided AP); group B, 10 patients with type B (right-sided AP); and group C, 13 patients who were successfully ablated. Group C consisted of three subgroups: subgroup CA, 7 patients with type A WPW; subgroup CB, 3 patients with type B WPW; and subgroup CC, 3 patients with concealed WPW. Controls consisted of 608 normals. Although I-maps of WPW were highly (r = .87) correlated with the mean normal I-map, the location of the minimum in groups A and B differed significantly from that in normals. The minimum was located over the upper right anterior chest in normal subjects, over the back in 82% of 22 patients with type A WPW including ablated patients (groups A+CA), and over the mid to lower right anterior chest in 62% of 13 patients with type B WPW including ablated patients (groups B+CB). Groups A+CA and B+CB had an abnormally low QRST area ("-2SD area") over the back and right anterior chest, respectively. The abnormally located minimum and the "-2SD area" were present in 7 of 10 ablated patients with manifest WPW (groups CA+CB). After RF ablation, the distribution of the minimum, initially abnormal, became normal over a period of days or weeks, and the "-2SD area" disappeared over 1 week in all 7 patients. Correlation coefficients between I-maps and the mean normal I-map increased after RF ablation. (1) WPW is often associated with abnormalities in repolarization properties. (2) Repolarization abnormalities were located over the back in type A WPW and over the right mid to lower chest in type B WPW: (3) The abnormalities remain immediately after RF ablation and gradually normalize. These findings support the concept that ST-T abnormalities in 12-lead ECGs following RF ablation are attributable to "cardiac memory."

Unmasking Accessory Pathway Conduction with Adenosine-induced Atrioventricular Nodal Block After Radiofrequency Catheter Ablation

Radiofrequency catheter ablation is very effective in eliminating conduction over accessory pathways in patients with Wolff-Parkinson-White syndrome. However, accessory pathway conduction recurs in approximately 5 to 9 percent of patients in the weeks to months following ablation. We describe two cases in which intravenous adenosine revealed persistent accessory pathway conduction after apparently successful ablation, thus providing an indication for the delivery of further ablative therapy. Adenosine may improve the long-term efficacy of radiofrequency catheter ablation of accessory pathways by manifesting latent accessory pathway conduction.

Low-energy ablation of accessory pathways in cases for which the radiofrequency approach had failed

The introduction of radiofrequency energy for catheter ablation of accessory pathways has offered certain advantages over high-energy direct-current shocks. With this approach, however, extremely long fluoroscopy time and procedure durations, up to 21 hours, 1 may be required for a successful outcome. On certain occasions, the small circumscribed lesion produced by radiofrequency current fails to interrupt the pathway, despite apparent adequate positioning of the ablating catheter as evidenced by satisfactory electrograms obtained during mapping. Poor catheter contact or location of the pathway relatively distant from the annulus may be responsible for such failures. Low-energy direct-current shocks, by producing a larger lesion, seem to offer a logical alternative without the risk of complications associated with conventional high-energy direct-current shocks. We report 3 cases in which the use of low-energy direct-current shocks resulted in successful ablation after repeated attempts with radiofrequency energy had failed.

Usefulness of serial follow-up electrophysiologic studies in predicting late outcome of radiofrequency ablation for accessory pathways and atrioventricular nodal reentrant tachycardia

A total of 408 patients received radiofrequency catheter ablation for paroxysmal supraventricular tachycardia, and 326 patients underwent serial follow-up electrophysiologic studies (early and late) after initially successful radiofrequency catheter ablation of accessory pathways (group 1, 186 patients with Wolff-Parkinson-White syndrome) and slow atrioventricular (AV) nodal pathways (group 2, 140 patients with AV nodal reentrant tachycardia). Among the patients in group 1, early (4 ± 1 days) and late (129 ± 14 days) studies found recurrent conduction through the accessory pathways in 12 and 16 patients, respectively. During a follow-up period of 21 ± 7 months, recurrence of accessory pathway-mediated tachyarrhythmias was noted in six patients. Of these six patients, all had tachycardia inducible in the late study but not in the early study. Among the patients in group 2, four had recurrence of AV nodal reentrant tachycardia during a follow-up of 16 ± 6 months. Of the four patients, one had tachycardia inducible in the early (4 ± 1 days) study and three in the late (130 ± 12 days) study. The results demonstrated that the early study was not as sensitive as the late follow-up electrophysiologic study in predicting late outcome of radiofrequency ablation, but both the early and late studies had a high total predictive accuracy (>90%) in groups 1 and 2. Furthermore, only 4 of the 326 patients had initial evidence of recurrent tachycardia activated by programmed electrical stimuli during follow-up studies, suggesting that follow-up electrophysiologic studies in asymptomatic patients are not warranted.

Radiofrequency catheter ablation of accessory pathway during entrainment of the atrioventricular reciprocating tachycardia

Radiofrequency catheter ablation of the concealed atrioventricular (AV) accessory pathway was performed during entrainment of the AV reciprocating tachycardia. Right ventricular pacing at a rate 5 to 15 beats/min faster than the tachycardia rate was performed during the tachycardia, which resulted in transient entrainment. In 2 patients with a right-sided accessory pathway, constant fusion of the QRS complex was observed during entrainment, whereas in 2 with a left-sided pathway, no fusion beat was noted (concealed entrainment). Radiofrequency energy was applied to the accessory pathway while entraining the tachycardia. One to 4.5 seconds after initiation of energy delivery, ventriculoatrial conduction Mock occurred (i.e., the accessory pathway was ablated). This was associated with a change in the left ventricular activation sequence from orthodromic capture through the normal AV conduction system to antidromic capture through the right ventricle and with a change in the QRS morphology in patients with a right-sided accessory pathway. The left ventricular activation sequence and QRS morphology remained unchanged in patients with concealed entrainment. Radiofrequency energy was effectively delivered for 30 seconds with a stable ablation catheter position, and termination of rapid pacing resulted in sinus rhythm. Thus, radiofrequency ablation of the reentry circuit component during manifest entrainment fulfilled the third entrainment criterion defined previously. The results suggest that tachycardia entrainment can be used for continuous application of radiofrequency energy during tachycardia in selected patients, because it provides a constant ventricular rate during the procedure and thus maintains the catheter in a stable position.

Design and clinical application of a low-pass input filter for the evaluation of intracardiac electrograms during radiofrequency catheter ablation

Successful ablation of an accessory pathway using radiofrequency current may be defined by the elimination of ventricular preexcitation or the loss of eccentric retrograde atrial activation. 1 Because of the low voltages of intracardiac electrograms (0.1 to 5.0 mV) compared with the high voltages applied during radiofrequency ablation (30 to 50 V), monitoring of local electrograms during ablation may not be feasible with conventional biopotential amplification and filtering systems 2. However, given the fundamental differences in signal frequencies between intracardiac electrograms (10 to 230 Hz) and radiofrequency current (550 to 750 KHz), selective attenuation of high-frequency radiofrequency signals and amplification of the lower frequency intracardiac electrograms may allow continuous monitoring of intracardiac electrograms during ablation. 3 The purpose of this study was to evaluate the feasibility and clinical use of an input bandpass filter, selective for frequencies < 1 KHz, which allowed continuous monitoring of intracardiac electrograms in patients undergoing radiofrequency catheter ablation of accessory pathways.

Effect of operator experience on outcome of radiofrequency catheter ablation of accessory pathways

Radiofrequency catheter ablation of accessory pathways has been shown to be safe and effective, and is rapidly becoming the treatment of choice for patients with the Wolff-Parkinson-White syndrome or paroxysmal supraventricular tachycardia involving a concealed accessory pathway. 1–6 Because radiofrequency catheter ablation requires skill in mapping and catheter manipulation, the outcome is at least in part operator-dependent. The purpose of this study was to determine the effect of operator experience on the outcome of radiofrequency catheter ablation of accessory pathways by analyzing the changes in efficacy and duration of the procedure as experience increased.

Effect on coronary artery anatomy of radiofrequency catheter ablation of atrial insertion sites of accessory pathways

Objectives. The purpose of this study was to analyze the effects of radiofrequency catheter ablation of the atrial insertion site of accessory pathways on the angiographie appearance of coronary arteries.Background. Radiofrequency catheter ablation of accessory pathways requires the application of energy to the endocardial surface of the atrioventricular groove adjacent to the major epicardial coronary arteries. A systematic analysis of the effect of radiofrequency ablation on coronary arteries has not previously been demonstrated.Methods. Seventy consecutive patients with 76 accessory pathways (7 right free wall, 44 left free wall, 12 posteroseptal, 8 anteroseptal and 5 midseptal) were studied. Quantitative coronary angiography was performed before, immediately after and a mean of 69 ± 42 days after radiofrequency catheter ablation.Results. Coronary artery diameter adjacent to the ablating electrode was 2.6 ± 0.9 mm before ablation, 2.7 ± 0.9 mm immediately after ablation and 2.7 ±1.0 mm at the time of follow-up study. Angiographie findings were unchanged from baseline in 69 of 70 patients immediately after ablation and in all 70 patients at the time of follow-up Study.Conclusions. Radiofrequency catheter ablation of the atrial insertion site of accessory pathways does not result in short-term angiographie changes in coronary artery anatomy.

Dressler's Syndrome Complicating Radiofrequency Ablation of an Accessory Atrioventricular Pathway

Radiofrequency energy is being used frequently in catheter ablation of accessory pathways. This case reports a patient who underwent radiofrequency ablation that was complicated by the development of Dressler's syndrome.

Pace mapping using body surface potential maps to guide catheter ablation of accessory pathways in patients with Wolff-Parkinson-White syndrome.

BACKGROUND. A pace mapping technique using body surface potential maps (BSPMs) was developed to guide the positioning of an ablation catheter at the ventricular insertion point of accessory pathways (AP) in patients with the Wolff-Parkinson-White syndrome (WPW). METHODS AND RESULTS. The study was performed on 30 WPW patients. BSPMs were recorded with 63 leads distributed over the entire torso surface. The catheter used for radiofrequency ablation was first placed in the vicinity of the ventricular preexcitation site predicted by BSPMs recorded during the delta wave. BSPMs were then recorded during pacing with this catheter, the comparison between the preexcited and paced BSPMs indicated whether the pacing site was too anterior or posterior with respect to the preexcitation site, and the catheter was moved accordingly. This process was repeated until the preexcited and paced BSPMs were highly correlated (r > or = 0.8), and ablation then was attempted. It was possible to successfully ablate the AP in 28 patients after an investigation that lasted 54 +/- 44 minutes between the recording of the first paced BSPM and that of the BSPM paced at the successful ablation site. Patients with left free wall pathways needed less investigation time compared with patients with pathways of other locations (46 +/- 9 versus 100 +/- 25 minutes, p = 0.031). The sensitivity of BSPM pace mapping was assessed using pacing with a multipolar catheter, and significant changes were observed on the BSPMs for beats with pacing sites that were only 5 mm apart. CONCLUSIONS, BSPM pace mapping allowed us to achieve a 93% success rate with short investigation durations, provides significant information that cannot be obtained with the standard 12-lead ECG, is a self-correcting procedure that reduces the importance of BSPM alterations due to individual differences in the shape of the torso or heart, and is applicable only to patients with AP showing antegrade conduction.

A prototype coronary electrode catheter for intracoronary electrogram recording

Successful catheter ablation of accessory pathways is critically dependent on precise pathway localization. 1 Rapid and precise pathway localization can also dramatically shorten procedure times. Mapping of left-sided accessory pathways is facilitated by use of the coronary sinus which provides a stable conduit for catheter mapping along the left atrioventricular groove. Mapping of right-sided accessory pathways is technically more difficult and requires an appreciation of the radiographic anatomy of the right atrioventricular groove. 2 The right coronary artery courses through the right atrioventricular groove and can provide a means to record right-sided epicardial electrograms. Localization of right-sided accessory pathways using the right coronary artery was first reported in 1990 by Swartz et al. 3 In 1991 Lesh et al 4 reported their experience with this technique. In both of these investigations, a 2.2Fr wire manufactured for pacing purposes was modified to record unipolar electrograms. The initial experience in a combined total of 10 patients suggested that this technique was safe and effective. The particular technology used in these cases, however, has several disadvantages. First, the recording wire is stiff and subjects patients to the risks of intimal dissection, perforation or coronary artery spasm. Second, repeated mapping requires that the wire traverse the coronary artery each time. Finally, only unipolar electrogram recordings are possible.

Variable location of accessory pathways associated with the permanent form of junctional reciprocating tachycardia and confirmation with radiofrequency ablation

Permanent Junctional reciprocating tachycardia (PJRT) occurs primarily in young patients and causes nearly incessant tachycardia that is frequently refractory to pharmacologic treatment. Previous nonpharmacologic therapy has included surgical or direct-current catheter ablation of either the His bundle or the accessory pathway. The accessory pathway in PJRT has been described as having retrograde and anterograde decremental conduction properties, and is typically identified in the posteroseptal location. This report describes radiofrequency catheter ablation of accessory pathways in 8 patients with PJRT. All ablations were successful and without adverse effects. Accessory pathway potentials were detected just before atrial activation in 6 of 8 patients. A new finding was that 5 of the 8 pathway locations, as identified by the site of successful ablation, were not in the typical posteroseptal region. In 1 patient it was located in the right posteroseptal region, 2 were in the right atrial freewall, 1 was in the right anterior septum and 1 was in the left posterior region just outside of the septal region. In conclusion, radiofrequency catheter ablation can be a highly effective and safe method for treatment of young patients with PJRT. Because the accessory pathways can be located outside of the posteroseptal region, careful mapping of both the right and left atrioventricular groove may be necessary for successful ablation.

Temperature monitoring during radiofrequency catheter ablation of accessory pathways.

Animal studies have suggested that the temperature of the electrode-tissue interface during radiofrequency catheter ablation accurately predicts lesion size. The purpose of the current study was to evaluate the utility of continuous temperature monitoring during radiofrequency catheter ablation in patients with Wolff-Parkinson-White syndrome. Twenty patients with manifest preexcitation were included in the study. The ablation catheter was positioned on the ventricular side of the mitral annulus for left-sided accessory pathways and on the atrial side of the tricuspid annulus for right-sided and septal accessory pathways. A thermistor imbedded in the distal electrode of the ablation catheter allowed continuous temperature monitoring during each energy application. To define the relation between power and temperature, radiofrequency current was applied several times at each site using outputs of 20, 30, 40, and 50 W. The accessory pathways were successfully ablated in each of the 20 patients. Because of marked variability in the efficiency of heating between sites, power output did not predict temperature. However, at any given site, there was a positive dose-response relation between power and temperature. Radiofrequency energy applications on the atrial side of the tricuspid annulus produced lower temperatures than did applications on the ventricular side of the mitral annulus (49 +/- 7 versus 60 +/- 16 degrees C, p = 0.0001). Transient block in the accessory pathways occurred at a mean of 50 +/- 8 degrees C, whereas permanent block was seen at a mean of 62 +/- 15 degrees C (p = 0.0001). Less than half of the applications at outputs < or = 40 W produced temperatures adequate to interrupt accessory pathway conduction. An abrupt rise in impedance caused by coagulum formation occurred only at temperatures between 95 and 100 degrees C. Temperature monitoring may facilitate radiofrequency catheter ablation of accessory pathways. By adjusting power output to ensure that adequate but not excessive temperatures have been achieved, a rise in impedance can be avoided and the total number of energy applications and procedure duration may be reduced.

Electrogram patterns predictive of successful radiofrequency catheter ablation of accessory pathways.

We assessed anterograde conduction electrogram parameters at successful ablation sites according to accessory pathway (AP) location and compared them to the most favorable electrograms at unsuccessful sites. A median of three radiofrequency energy impulses was applied to ablate 97% of 136 APs versus four impulses to ablate 90% of 65 concealed APs. Electrograms at successful sites showed variable A/V ratio (0.04-7), and a QS pattern of unipolar ventricular wave in 90%. Electrograms were different in right versus left AP: AV time 29 +/- 7 versus 38 +/- 10; and timing of ventricular deflection: 17 +/- 9 versus 2 +/- 9 msec, respectively. Analysis in each patient of the mapping parameters at successful versus "most favorable" unsuccessful sites showed an improvement in at least one parameter in 55%, no apparent change in all parameter values in 30%, and even less favorable parameters in 15% of patients. In patients with manifest AP, overall comparison of electrograms at successful versus unsuccessful sites showed no difference in A/V ratio (1.3 +/- 1.5 vs 1.2 +/- 1.6), unipolar pattern distribution, and AV time (34 +/- 10 vs 35 +/- 9), but earlier bipolar main ventricular potential (-4 +/- 12 vs -1.5 +/- 10 msec) and unipolar intrinsic deflection timings (-5.2 +/- 11 vs -1.8 +/- 10 msec). In patients with concealed AP, a retrograde continuous electrical activity was recorded in 72% of successful versus 38% of unsuccessful sites (P = 0.03). Electrogram characteristics at successful radiofrequency ablation are different in right and left manifest AP.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors predicting success in DC catheter ablation of accessory pathways.

In a series of 33 patients with accessory pathways, 26 had successful catheter ablation (fulguration [23 patients] or modification [3 patients]) of their accessory pathway conduction, and could be considered as a clinical success. One hundred thirteen single discharge or double discharge shocks were delivered, and each shock was studied to reveal which parameters were important to predict the success or failure of catheter ablation. Double discharge shocks resulted in successful accessory pathway modification or ablation twice as often as single discharge shocks (32% vs 16%). This effect was more pronounced in left lateral accessory pathways (48% vs 4%). Shocks in the electrophysiologically defined ventricular zone were more likely to be successful (33%) than shocks delivered in the atrial zone (14%), irrespective of accessory pathway location. The presence of a probable Kent potential was the parameter most strongly associated with success. The parameter most strongly associated with failure, with a 100% negative predictive value, was the absence of earliest activation recorded on the ablating catheter prior to shock delivery. An AV interval of < 60 msec significantly divided the successful from the unsuccessful shocks (P = 0.01). The VA interval during orthodromic reciprocating tachycardia or right ventricular stimulation did not allow for significant division into successful and unsuccessful attempts in this relatively short series. VA intervals, when longer, were predictive of failure but, when shorter, had low positive predictive value. Mean follow-up in 25 successful patients was 15 +/- 6 months. All patients did well in the follow-up period. Neither those patients with ablation nor modification of the accessory pathway had recurrent episodes of tachycardia or required pharmacological treatment for control of arrhythmias.

Does delayed loss of preexcitation after unsuccessful radiofrequency catheter ablation of accessory pathways result in permanent cure?

Unsuccessful radiofrequency catheter ablation of accessory atrioventricular (AV) pathways may be followed by subsequent spontaneous loss of preexcitation with apparent “delayed cure.” The mechanism of this phenomenon is uncertain but may result from inflammation at the ablation site, adjacent to the accessory pathway. The aim of this study was to determine if delayed loss of accessory pathway conduction after unsuccessful catheter ablation resulted in permanent loss of accessory pathway conduction.

Frequency of recurrent atrial fibrillation after catheter ablation of overt accessory pathways

The effect of successful catheter ablation of overt accessory pathways on the incidence of atrial fibrillation (AF) was studied in 129 symptomatic patients with (n = 75) or without (n = 54) previous documented AF. Fourteen had had ventricular fibrillation. Factors predictive of recurrence were examined, including electrophysiologic parameters. Atrial vulnerability was defined as induction of sustained AF (>1 minute) using single, then double, atrial extrastimuli at 2 basic pacing cycle lengths. When compared to patients with only reciprocating tachycardia, patients with clinical AF included more men (77 vs 54%, p = 0.008) and were older (35 ± 12 vs 29 ± 12 years, p = 0.01). They had a significantly shorter cycle length leading to anterograde accessory pathway block (252 ± 42 vs 298 ± 83 ms, p < 0.001), greater incidences of atrial vulnerability (89 vs 24%, p < 0.001) and subsequent need for cardioversion (51 vs 15%, p < 0.001). After discharge, the follow-up period was 35 ± 12 months (range 18 to 76): 7 patients with previous spontaneous AF (9%) had recurrence at a mean of 10 months after ablation. Age, presence of structural heart disease, accessory pathway location, atrial refractory periods and accessory pathway anterograde conduction parameters were not predictive of AF recurrence. Persistence of atrial vulnerability after ablation was the only factor associated with further recurrence of AF. Atrial vulnerability was observed after ablation in only 56% of patients with previous AF versus 89% before ablation. It is concluded that successful catheter ablation of accessory pathways prevents further recurrence of AF in 91% of patients. This result favors the use of ablative procedures in the Wolff-Parkinson-White syndrome.

Impedance monitoring during radiofrequency catheter ablation in humans.

Radiofrequency catheter ablation of accessory pathways and the atrioventricular junction often requires multiple applications of energy. The inability to determine the effects of any given application on the underlying tissue may contribute to this problem. In the present study, impedance was monitored in 20 patients undergoing radiofrequency catheter ablation, and the relationship between an initial decrease in impedance and subsequent effects were examined. An initial fall in impedance of more than 10 omega was 78% sensitive and 88% specific for predicting subsequent evidence of tissue heating (interruption of conduction or an abrupt rise in impedance due to coagulum formation). In contrast, initial values of voltage, current, or impedance did not distinguish between effective and ineffective applications of radiofrequency energy. Continuous monitoring of impedance may facilitate radiofrequency catheter ablation.

Radiofrequency catheter ablation of accessory pathways: A learning experience

Success rates of approximately 90% have recently been reported with radiofrequency catheter ablation of accessory pathways. This study determined whether this success could be repeated using a conservative approach generally limiting fluoroscopy time to 1 hour. Consecutive patients referred for management of arrhythmias associated with accessory atrioventricular pathways were included over a 9-month period. Ablation was attempted in 75 patients with 84 pathways. Overall success rate (including second attempts in 9 patients) was 60 of 84 accessory pathways (71%). Success rates for the first 3 months (n = 23) were 52%, the second 3 months (n = 23) 60% and the last 3 months (n = 38) 90%. Success rate varied with pathway location, with left lateral pathways having the best early success rates. Mean fluoroscopy time for successful procedures of 33 ± 21 minutes was shorter than the time for unsuccessful procedures of 63 ± 24 minutes (p = 0.001). There were no major complications and no patients with successful procedures (n = 53) have had recurrence of accessory pathway conduction or reciprocating tachycardia (follow-up 1 to 10 months). A conservative approach can yield success rates approaching 90% in a short time. The absence of major complications supports earlier reports suggesting that radiofrequency catheter ablation of accessory pathways is a reasonable first-line therapy in the Wolfff-Parkinson-White syndrome.

Reduction in medical care cost associated with radiofrequency catheter ablation of accessory pathways

The cost of definitive therapy was compared in 25 patients who underwent radiofrequency catheter ablation of accessory pathways in 1990 and 25 patients who underwent surgical ablation of accessory pathways in 1989. In the radiofrequency group, 23 of 25 patients had a single accessory pathway and the remaining 2 patients each had 2 accessory pathways. In the surgical group, 20 patients had a single accessory pathway and 5 patients each had 2 accessory pathways. The success rate was 96% in each group. The mean duration of hospitalization was 3 ± 1 days in the radiofrequency group and 9 ± 4 days in the surgical group (p < 0.0001). All the cost data are expressed in fiscal year 1990/1991 dollar values. The total cost of therapy in the radiofrequency group was $14,919 ± $6,740 compared with $53,265 ± $12,755 in the surgical group (p < 0.0001). The cost of radiofrequency ablation consisted of a hospital charge of $7,753 ± $3,472 and physician fees of $7,166 ± $3,439. The hospital charge included charges for use of the electrophysiology laboratory, hospital stay, electrocardiograms, echocardiograms and blood studies. The cost of surgery consisted of a hospital charge of $37,708 ± $10,179 and physician fees of $15,557 ± $3,149. The hospital charge in the surgical group included the costs of a baseline electrophysiology study, in-hospital care and a follow-up office visit. In conclusion, radiofrequency catheter ablation of accessory pathways results in a dramatic reduction in the cost of definitive therapy in patients with the Wolff-Parkinson-White syndrome.