Wolff-Parkinson-White syndrome

Concealed conduction into accessory atrioventricular pathways has been postulated to explain variability of R-R intervals during atrial fibrillation in patients with Wolff-Parkinson-White syndrome. We examined the occurrence of concealed conduction into atrioventricular pathways using extrastimulus techniques in 26 consecutive patients undergoing electrophysiologic studies for the Wolff-Parkinson-White syndrome. Anterograde pathway concealment was demonstrated (10 patients) by introducing a second atrial extrastimulus (A3) after block in the accessory pathway occurred following the first extrastimulus (A2). The apparent effective refractory period (ERP) of the atrioventricular pathway with A3 (after A, blocked in the pathway), or ERPB' was always greater than the ERP of the atrioventricular pathway (505 + 100 vs 323 ± 105 msec, mean + SD; p < .001), a finding explained by concealment into the pathway by the blocked A2. A measure of the apparent prolongation of refractoriness due to anterograde concealment (A ERPB), defined as the difference between ERP and ERPB at a given cycle length, was derived. The average R-R interval in atrial fibrillation correlated better with A ERPB (r = .8, p < .01) than with the ERP (r = .6, p = NS), supporting the influence of anterograde atrioventricular pathway concealment in modulating the ventricular response during atrial fibrillation. By similar techniques, concealed retrograde conduction in the atrioventricular pathway could be demonstrated in 16 of 26 patients. In two of these patients "bystander" atrioventricular pathway conduction during orthodromic reciprocating tachycardia that did not involve the atrioventricular pathway did not occur, even though the ERP of the pathway should have permitted it, a finding readily explained by repetitive retrograde concealment into the atrioventricular pathway during tachycardia. Concealed conduction can be demonstrated in most patients with Wolff-Parkinson-White syndrome and is an important factor in the clinical expression of their arrhythmias. Circulation 70, No. 3, 402-411, 1984. CONCEALED CONDUCTION has been defined as the conduction of excitation through certain parts of the cardiac tissue that does not produce an identifiable waveform on the electrocardiogram but can be indirectly implicated by its effects on subsequent impulse formation or conduction."! 2 Concealed conduction has been described in most cardiac tissues, including the atrium, sinus node, atrioventricular node, intraventricFrom the Clinical Electrophysiology Laboratory. University Hospital, London, Ontario. Dr. Klein is a Senior Research Fellow of the Ontario Heart Foundation. This study was supported by the Ontario Heart Foundation, Toronto. Address for correspondence: George J. Klein, M.D., Clinical Electrophysiology Laboratory, Cardiac Investigation Unit, University Hospital, London, Ontario, Canada N6A 5A5. Received Nov. 21, 1983; revision accepted April 26, 1984. 402 ular conduction tissue, and ventricular myocardium.' In patients with Wolff-Parkinson-White (WPW) syndrome, concealed conduction into accessory pathways has been suggested as an explanation for the variability of R-R intervals during atrial fibrillation3-' and the presence or absence of "bystander" accessory pathway participation in atrioventricular nodal reentrant tachycardia.' Individual examples of retrograde concealment into accessory pathways, as demonstrated by the extrastimulus technique, have been described.' 0 We used the extrastimulus technique to demonstrate changes in the refractoriness of the accessory pathway that could only be accounted for by concealed conduction in a consecutive series of patients with WPW syndrome and related this phenomenon to the manifestation of their clinical arrhythmias. CIRCULATION by gest on N ovem er 7, 2017 http://ciajournals.org/ D ow nladed from PATHOPHYSIOLOGY AND NATURAL HISTORY-PREEXCITATION Methods Patients. The study population consisted of 26 consecutive patients with WPW syndrome who underwent electrophysiologic studies in the clinical electrophysiology laboratory between February 1983 and September 1983 (table 1). No patient had coexistent heart disease. Written and verbal informed consent was obtained before the study. Electrophysiologic evaluation. The method of study of patients with preexcitation in our laboratory has been described." All patients were studied while in the nonsedated, postabsorptive state after all antiarrhythmic medications had been discontinued for at least five drug half-lives. The study included incremental atrial and ventricular pacing and atrial (right atrium) and ventricular extrastimulus testing at multiple cycle lengths. Standard criteria for determining the participation of the accessory pathway in reentrant circuits and for localizing the accessory pathway were used.'2 Atrial fibrillation was induced by rapid atrial pacing if it did not occur during the course of the study. In some patients, atrial fibrillation could only be maintained by rapid atrial pacing. Intervals were measured over a 1 min sample of stable atrial fibrillation and included the shortest R-R interval between preexcited beats (SRR), the average R-R interval (ARR), and the longest R-R interval (LRR) between any 2 beats. 1 Demonstration of concealment. We used the occurrence of an "apparent" prolongation of refractoriness of the accessory pathway in the cycle after block in the pathway to demonstrate partial (or concealed) conduction into the accessory pathway. Anterograde concealment in an accessory pathway was demonstrated as follows. After a drive of eight atrial stimuli (A1), a single atrial extrastimulus (A2) was made progressively more premature until complete block in both the accessory pathway and atrioventricular node was observed. At this point, a second atrial extrastimulus (A3) was introduced and made progressively more premature until loss of preexcitation in the QRS after A3 was observed. Anterograde concealment into the accessory pathway was considered to have occurred if subsequent omission of A2 resulted in return of preexcitation after A3 (A2A3 technique; figure 1). The atrial electrogram closest to the atrioventricular pathway was used for measurement. Retrograde concealment into the accessory pathway was demonstrated in one of three ways. (1) A ventricular extrastimulus (V2) after a ventricular drive (V,) was made progressively more premature until complete retrograde block was observed. At this point, an atrial extrastimulus (A3) was introduced and made progressively more premature until loss of preexcitation was observed after A3. Retrograde concealment into the accessory pathway was considered to have occurred if subsequent TABLE 1 Summary of data