Bundle branch block after ventricular tachycardia: a manifestation of "fatigue" or "overdrive suppression"

Change in left bundle branch potential-ventricular interval during lead implantation: what is the mechanism?

A 75-year-old male with sick sinus syndrome (SSS) underwent left bundle branch (LBB) pacing (LBBP) implantation. Intraoperative recordings showed that the LBB potential-ventricular (LBB-V) interval gradually decreased from 47 to 19 ms (ms). Two-year follow-up indicated stable pacing parameters and consistent electrocardiogram (ECG) results. Despite potential conduction delays caused by lead implantation, a watchful waiting strategy demonstrated recovery potential without impacting long-term prognosis or pacing parameters.

Overdrive suppression of antegrade conduction over the accessory pathway

In a patient with Wolff-Parkinson-White syndrome whose accessory pathway was primarily capable of bidirectional conduction, antegrade conduction over the accessory pathway was transiently inhibited after rapid atrial or ventricular pacing or after spontaneous termination of atrioventricular reentrant tachycardia. Pacing rate and duration of tachycardia were related to the duration of the suppression of preexcitation, while the coupling interval of the first sinus beat to the last driven or tachycardia beat was irrelevant to the phenomenon. Thus, overdrive suppression of conduction may be the most likely mechanism of this phenomenon.

Fatigue phenomenon in accessory pathways

Fatigue phenomenon is transient failure of conduction following a period of repetitive excitation. Fatigue in accessory pathways is uncommon, and its electrophysiologic characteristics and clinical implications are unknown. Among the 215 patients who underwent electrophysiology studies from July 1992 to December 1993, 4 (2%) were found to exhibit fatigue over accessory pathways. The accessory pathway was posteroseptal in three patients and right free wall in one patient. The mean anterograde effective refractory period of the accessory pathway was 295 +/- 26 msec (range 270 to 330, basic drive cycle length 600 msec). Three patients had neither retrograde accessory pathway conduction nor inducible tachycardia even with infusion of isoproterenol. The fatigue phenomenon was observed after both atrial and ventricular stimulation in three patients and only after ventricular stimulation in one patient. Fatigue was dependent on duration more than rate of stimulation. We conclude that pathways exhibiting fatigue have a low margin of safety for conduction and are unlikely to be clinically problematic.

Overdrive prolongation of refractoriness and fatigue in the early stages of human bundle branch disease

OBJECTIVES

The aim of this study was to assess the response of refractoriness in normal and diseased human bundle branches to changes in cycle length, as well as during a long period of continuous overdrive pacing.

BACKGROUND

The anterograde refractory period of the bundle branches in patients with functional bundle branch block shortens as the rate is increased. The rate-dependent response of refractoriness in diseased bundle branches is quite different. However, this difference has not been precisely delineated, and its physiologic meaning is uncertain.

METHODS

Refractoriness of the bundle branches was measured by the extrastimulus technique in 16 patients with tachycardia-dependent bundle branch block and 10 patients with functional bundle branch block, both after basic trains of 8 atrial-paced impulses at different cycle lengths and during a 10-min period of continuous overdrive pacing.

RESULTS

The baseline refractory period in the bundle branches of patients with functional bundle branch block measured 430 +/- 32 ms (mean +/- SD) and shortened to 368 +/- 30 ms at the shortest cycle length. The maximal effect was reached within the 1st min of overdrive pacing. The baseline refractory period of the bundle branches was significantly longer in patients with tachycardia-dependent bundle branch block (611 +/- 184 ms) and demonstrated a cumulative overdrive prolongation in 15 (83%) of 18 studies with typical manifestations of fatigue. In two other studies, this occurred only after ajmaline administration.

CONCLUSIONS

A rate- and time-dependent prolongation of refractoriness frequently occurs in diseased human bundle branches. When absent, this response may be induced under the effects of sodium channel blockers. This would suggest that an abnormality in the recovery from inactivation of the sodium channel might underlie the early stages of bundle branch disease.

Differentiation between aberrant ventricular conduction and ventricular ectopy in atrial fibrillation using RR interval scattergram

BACKGROUND

Differentiation between aberrant ventricular conduction and ventricular ectopy during atrial fibrillation (AF) is of etiologic, prognostic, and therapeutic importance. We developed a noninvasive technique to diagnose aberrant ventricular conduction and ventricular ectopy in AF.

METHODS AND RESULTS

We studied the Holter ECGs of 34 patients with paroxysmal AF and 62 patients with chronic AF. In all the patients, frequent wide QRS complexes were observed, and 32 patients were shown by electrophysiological examination to have ventricular ectopies or aberrant ventricular conductions. We obtained the RR interval scattergrams by plotting sequential pairs of RR intervals. Each point has the (n)th RR interval as its x value and the (n + 1)th RR interval as its y value. The irregularity of the RR intervals in AF resulted in widely scattered points delineated by the envelope along the axes. The y value of the envelope along the x axis indicates the shortest coupling interval to the preceding RR interval. Therefore, this curve defines the functional refractory period of atrioventricular conduction. The scattergram of the RR interval pairs immediately preceding the aberrant conduction (coupling points of aberrant conduction) specifically distributed along the envelope. In contrast, the coupling points of ventricular ectopies showed different distributions that had no relation to the envelope. That is, it included three typical patterns, ie, linear distribution below the envelope, linear distribution partially overlapped in the area of normal AF conduction, and chaotic distribution in the AF area. None of the scattergrams of ventricular ectopies showed curvilinear distribution along the envelope as aberrant conduction did. The specific distribution of the aberrant conduction on the RR interval scattergram suggested that aberrant conduction in AF could result from the difference of refractory periods between the AV node and bundle branch block.

CONCLUSIONS

We conclude that the RR interval scattergram makes it possible to differentiate between aberrant ventricular conduction and ventricular ectopy in atrial fibrillation, and thus, it is a useful noninvasive clinical tool.

Effect of verapamil on an accessory pathway manifesting as "fatigue phenomenon" in Wolff-Parkinson-White syndrome

Electrophysiologic study was performed in a 52-year-old man with type A ventricular preexcitation. An accessory atrioventricular pathway with no ventriculoatrial conduction was localized to the posteroseptal region. "Fatigue phenomenon," defined as suppression of atrioventricular conduction following rapid pacing, was observed to be provoked by atrial pacing in a rate- and duration-dependent manner. Administration of 5 mg of intravenous verapamil during sinus rhythm abolished the delta waves. These observations may indicate that pathologic changes in the accessory pathway are responsible for these phenomena.

Overdrive suppression of conduction at the canine Purkinje-muscle junction

We have shown previously that overdrive suppression of conduction in depolarized His-Purkinje tissue requires conduction asymmetry. In this study we examined whether overdrive suppression of conduction can occur at the Purkinje-muscle junction, where natural asymmetry of conduction exists. Canine Purkinje-muscle preparations were superfused with hyperkalemic Tyrode's solution (KCl 8 to 12 mM), and action potentials were recorded from Purkinje, junctional, and muscle cells. Initially, the Purkinje fiber was paced at the shortest cycle length at which 1:1 anterograde Purkinje-muscle conduction occurred. The papillary muscle then was paced for 10 to 50 beats at shorter cycle lengths during which, because of conduction asymmetry at the Purkinje-muscle junction, 1:1 retrograde muscle-Purkinje conduction also occurred. After overdrive papillary muscle pacing, Purkinje fiber pacing at the same cycle length that previously resulted in 1:1 conduction now produced transient Purkinje-muscle conduction block (overdrive suppression of conduction). The degree and duration of overdrive suppression of conduction were proportional to the rate and duration of overdrive pacing. After overdrive pacing, Purkinje cell action potential amplitude and Vmax recovered within 300 msec, yet conduction block persisted for up to 7 sec. In contrast, excitability in papillary muscle cells near the Purkinje-muscle junction increased continuously after overdrive pacing. These data suggest that rapid activation of Purkinje cells during overdrive pacing was not required for overdrive suppression of conduction and that restoration of conduction after overdrive pacing was determined primarily by recovery of excitability in papillary muscle cells. Transient Purkinje-muscle conduction block after periods of rapid ventricular rates might account for overdrive-induced conduction disturbances normally attributed to bundle branch block.

Rate-related suppression and facilitation of conduction in isolated canine cardiac Purkinje fibers

Previous studies have shown that antegrade conduction through damaged His Purkinje tissue may be suppressed following rapid ventricular pacing (overdrive suppression of conduction). We studied this phenomenon using isolated Purkinje fibers placed in a three-chamber bath. Superfusates for the left, middle, and right segments of the fiber were altered to produce action potentials that resembled those of normal bundle branch, damaged His bundle, and normal His bundle, respectively. To produce anisotropic conduction, the left segment of the fiber was adjusted to be three to four times longer than the right segment. Pacing the right segment at intermediate rates produced maximal action potential amplitude in the middle segment and 1:1 right-to-left conduction, whereas pacing at faster or slower rates reduced action potential amplitude and produced block. Pacing the left segment at fast or slow rates also reduced action potential amplitude in the middle segment, but conduction was maintained (anisotropy). After rapid or slow left segment pacing, action potential amplitude in the middle segment remained low during subsequent right segment pacing at intermediate rates, and transient block occurred (overdrive or underdrive suppression of conduction). With time, action potential amplitude normalized and conduction resumed. In other more severely depressed preparations, conduction block occurred even at intermediate right segment pacing rates prior to left segment pacing. Under these conditions, pacing the left segment at intermediate rates increased action potential amplitude in the middle segment and temporarily permitted 1:1 conduction at intermediate right segment pacing rates (overdrive facilitation of conduction).(ABSTRACT TRUNCATED AT 250 WORDS)