Wedensky facilitation. Electrotonic potentiation during complete A-V block

The electrocardiogram recorded from a patient with third degree A-V block reflected almost regular A-V junctional escape rhythm. Some of the R-R cycles were slightly shorter than the basic escape cycle. A QRS complex ending such a relatively short R-R interval was always preceded by a sinus P wave, and had a QRS configuration which was minimally different from that of the escape complexes. The His bundle recording demonstrated that these minimally premature complexes were associated with an H-V interval which was shorter than that of the escape complexes. This indicates that the premature QRS complex could not be a capture beat. The relationship between the slightly premature QRS complex and the preceding sinus P-waves is explained on the basis of electrotonic potentiation or modulation to due Wedensky facilitation.     

Concealed intraventricular conduction in the His bundle electrogram.

Multiple areas of concealed intraventricular conduction are deduced on the basis of aftereffects observed in His bundle recordings. Electrocardiograms and His bundle recordings are presented from two patients with unstable bilateral bundle branch block, the instability of which depended on the interval at which ventricular depolarization was initiated by sinus or paced impulses. This circumstance allows postulation of 1) concealed transseptal retrograde penetration of the left bundle branch system; 2) concealed transseptal retrograde penetration of the right bundle branch system; 3) alternate beat Wenckebach phenomenon with two areas of block in the bundle branch system with concealed penetration of the proximal area; 4) concealed re-entry in the right bundle branch system during an H-V Wenckebach cycle with resetting of the sequence of 2:1 H-V block and return of the re-entry wave to the A-V node causing subsequent A-H block; 5) proximal 2:1 block and distal Wenckebach block producing only two consecutively blocked beats; and 6) infrahisian Wenckebach block with changes both in A-V conduction and QRS contour.     

The linking phenomenon in tachycardia-dependent intraventricular block

Seven cases of tachycardia-dependent, or phase-3, intraventricular block have been examined. Analysis revealed a constant overlap between the range of the R-R intervals ending in normal conduction and the range of the R-R intervals where the second beat is associated with intraventricular block. The block, indeed, may occur at the end of relatively long R-R intervals, whereas R-R intervals which are shorter (up to 0.11 sec) can unexpectedly result in normal intraventricular conduction. A relatively late QRS complex, however, can reflect an intraventricular block only when the preceding complex also manifests the block. This phenomenon has been interpreted as due to the so called "linking", namely the retrograde concealed penetration of an anterogradely blocked bundle branch by the impulse traversing the controlateral bundle branch. This delays the activation of the affected bundle branch, whose refractory period is, accordingly, "shifted to the right" within the cardiac cycle. A relatively late sinus impulse, thus, can result in intraventricular block since the refractory period of the affected bundle branch ends later, with respect to the beginning of the QRS complex, than it does after a normally conducted sinus impulse.     

Bradycardia-dependent bundle branch block Relation to supernormal conduction and phase 4 depolarization

An electrocardiogram is presented in which spontaneous conversion of 2:1 block with first degree block and incomplete left bundle branch block resulted in 3:2 block with normal conduction of the QRS complex after the short R-R interval. Alternation of rate and block with every second beat is a variation of previously described bradycardia-dependent bundle branch block.

Phase 4 depolarization and supernormal conduction are inseparable concepts, and both may play a causal role in bradycardia-dependent bundle branch block. Criteria for recognition of this type of block should not disqualify cases in which supernormal conduction may be present, particularly since proximity of the QRS interval to the preceding T wave may not be an adequate test for supernormal conduction in clinical situations.

A knowledge of phase 3 and phase 4 events allows one to relate different types of clinical aberrance, from premature aberrance through early beat “normalization,” as in bradycardia-dependent bundle branch block, to normal rate aberrance as in typical bundle branch block and, finally, to escape beat aberrance.     

Termination of circus movement tachycardia utilizing an accessory atrioventricular pathway by retrograde concealed penetration of the atrioventricular node through the bundle branch system: A mechanism of tachycardia termination in Wolff-Parkinson-White syndrome

A 30 year old woman with Wolff-Parkinson-White syndrome underwent electrophysiologic study for investigation of circus movement tachycardia utilizing the accessory pathway for retrograde conduction. The accessory pathway was located on the right side. Episodes of circus movement tachycardia with left and right bundle branch block were induced. Some episodes of circus movement tachycardia with left bundle branch block terminated spontaneously. Two episodes of spontaneous termination at the level of the atrioventricular (A-V) node were preceded by prolongation of the H-V interval causing delay in atrial activation. This delayed atrial cycle was then followed paradoxically by spontaneous termination of the tachycardia in the A-V node. A similar phenomenon could be demonstrated reproducibly with single echo beats induced by coronary sinus extrastimuli. It appears that retrograde concealed penetration of the A-V node through the bundle branch system during anterograde left bundle branch block is the most likely mechanism for this phenomenon.     

Influence of pacemaker-induced tachycardia with A-V block on left ventricular blood velocity in man.

Phasic instantaneous left ventricular blood velocity was measured by radiotelemetry in 28 subjects with a Doppler ultrasonic flowmeter catheter during atrial pacing and induced A-V block Type I Wenckebach A-V block with conduction ratios of 9:8 or lower generally produced a stepwise reduction of peak left ventricular blood velocity in relation to shortened R-R intervals. Longer Wenckebach periods resulted in little or no blood velocity alteration during 1:1 A-V conduction. Those beats following a blocked atrial depolarization were associated with augmented blood velocities. In three subjects, bigeminal periods of 3:2 A-V block resulted in larger left ventricular blood velocities when compared with 2:1 A-V block, despite identical R-R intervals following the blocked P wave. This latter phenomenon was attributed to diastolic augmentation of left ventricular contraction following the second and hemodynamically ineffective beat during 3:2 A-V block. Three patients manifested true blood velocity alternation during second-degree A-V block and changing R-R intervals. The variations in peak left ventricular blood velocity observed during atrial pacing and A-V block are related to changing inotropic state and cycle length dependent alterations of left ventricular diastolic filling.     

Wedensky facilitation and the wedensky effect during high grade A-V block in the human heart

Wedensky facilitation and the Wedensky effect—forms of temporary enhancement of depressed excitability and/or conductivity—are reported in a patient with high grade A-V block. Sinus impulses which occur within a well-defined, relatively prolonged period after a ventricular or A-V nodal escape beat are conducted; earlier or later impulses are blocked. Conduction is probably initiated by the mechanism of Wedensky facilitation and maintained by the Wedensky effect.     

Coexistent Wenckebach phenomenon in the distal branches of the specialized conduction system

A patient in whom Wenckebach phenomenon was observed to coexist in the three distal branches of the specialized ventricular conduction system, with an intermittent trifascicular block producing a second degree A-V block, is described. Surface electrocardiogram demonstrated the presence of this phenomenon in the right bundle branch (RBB) and the anterior division of the left bundle branch (LBB). Evidence of the presence of Wenckebach phenomenon in the remaining fascicle was provided by His bundle recording.     

Alternative mechanisms of apparent supernormal atrioventricular conduction

Alternative mechanisms were found to explain several different electrocardiographic examples of apparent supernormal atrioventricular (A-V) conduction in man using programmed premature atrial and ventricular stimulation and His bundle recordings. Sudden shortening of the P-R interval during A-V nodal Wenckebach phenomenon was due to manifest or concealed reentry within the A-V node. Gap phenomena in which late atrial premature depolarizations blocked while earlier atrial premature depolarizations conducted were shown to result from delay of earlier atrial premature depolarizations in the A-V node (type I gap) or in the His-Purkinje system (type II gap). Mechanisms analogous to the latter were found in cases of apparent supernormality of intraventricular conduction: Late atrial premature depolarizations resulted in aberration whereas earlier atrial premature depolarizations conducted normally because of delay within the A-V node or His-Purkinje system. Unexpected normalization of a bundle branch block pattern also resulted from Wenckebach phenomenon in the bundle branches. Atypical Wenckebach phenomenon with the first beat of the period demonstrated that aberration was due to phase 4 depolarization. Preexcitation of the ventricle before the delivery of a previously blocked atrial premature depolarization allowed conduction through the area of block (A-V node) because of earlier depolarization of the latter with earlier recovery. In the His-Purkinje system, 2:1 A-V block was converted to 1:1 conduction when a premature ventricular depolarization shortened the refractoriness of the His-Purkinje system.     

Mechanism and significance of widened QRS complexes during complete atrioventricular block in acute inferior myocardial infarction

Twelve of 35 consecutive patients admitted with complete, atrioventricular (A-V) block complicating acute inferior myocardial infarction manifested widened QRS complexes. The escape beats had the pattern of left bundle branch block in four patients, right bundle branch block in five patients and both left and right bundle branch block in three patients.

His bundle recordings in five patients with escape beats that had a left bundle branch block configuration revealed a His bundle potential preceding the widened QRS complex at His-V intervals of 45 to 60 msec. Bradycardia-dependent left bundle branch block was demonstrated in two patients by His bundle pacing. In three patients the conducted beats had a left bundle branch block configuration after critical lengthening of the R-R interval during second degree A-V block before or after the episode of complete A-V block. In six patients whose escape beats had a right bundle branch block configuration, His bundle recordings did not reveal a His bundle potential preceding these beats.

Our observations suggest that widened QRS complexes with a left bundle branch block configuration could be due to an A-V junctional escape rhythm with phase 4 left bundle branch block. Alternatively in association with a right bundle branch block configuration it is possible that the widened QRS complexes represent a ventricular or fascicular escape rhythm.

Two of 12 patients with widened QRS complexes died. There were no significant differences in immediate mortality, 6 month mortality or mean peak serum glutamic oxaloacetic transaminase (SGOT) values between patients with narrow and widened QRS complexes. This finding suggests that widened QRS complexes during complete A-V block in acute inferior myocardial infarction have no prognostic significance.     

A demonstration of differential refractoriness within a single fascicle of the human ventricular specialized conduction system

In 15 patients with left bundle branch block (LBBB), atrial (A), His bundle (H), and ventricular (V) electrograms were recorded. Successively more premature atrial depolarizations were introduced via a catheter in the right atrium. In eight patients, the ventricular specialized conducting system (VSCS) was the most refractory portion of the entire atrioventricular conducting system (AVCS) and A-V conduction, which had been occurring via the right bundle branch (RBB), failed below the His bundle as the effective refractory period (ERP) of the VSCS was reached. In two of these eight patients, after the ERP of the VSCS was exceeded, further shortening of the H₁-H₂ interval (by 40 to 50 msec.) resulted in an unexpected resumption of A-V conduction, but with markedly prolonged H-V intervals (160 to 230 msec.). This demonstrates that differential refractoriness exists within the RBB of these patients. A zone of maximal refractoriness was initially encountered within the RBB when the premature impulse first blocked below the His bundle. In relative terms, this zone was distal to a more proximal area of the RBB where, with further shortening of the H₁-H₂ interval, sufficient conduction delay occurred to permit recovery of excitability distally and the resumption of A-V conduction.     

Alternating right and left bundle branch block aberration during atrial tachycardia

In a patient with atrial tachycardia with a rate of 200 per minute, the A-V conduction ratio was at times 2:1, but often it was 4:3 or 3:2 with progressive P-R interval prolongation (Wenckebach mechanism, an expression of presumable A-V nodal block). In each episode of 4:3 conduction, the first QRS complex was narrow, and the 2 ensuing beats were wide due to aberrant conduction. Aberration did not occur with a constant configuration, but in consecutive episodes of 4:3 conduction ratio there was a regular alternation of left bundle branch block and right bundle branch block. The pattern was explained by concealed retrograde conduction into the anterogradely blocked bundle branch. This caused 2 distinct effects: (1) shifting “to the right” of the refractory period of the affected bundle branch, resulting in maintenance of aberration with the same configuration, if consecutive atrial impulses were conducted to the ventricles, and (2) shortening of the effective cycle of the affected bundle branch, resulting in aberration due to block of the controlateral bundle branch, whenever a pause occasioned by a nonconducted atrial impulse was followed by restoration of 1:1 conduction for 2 or more consecutive beats.     

Mobitz II atrioventricular block due to infra-Hisian block and bundle branch extrasystoles.

An asymptomatic elderly male presented with complete right bundle branch block, left anterior fascicular block and Mobitz Type II second degree atrioventricular block. In addition, he was noted to have ventricular premature systoles. Electrophysiologic studies demonstrated two apparently different mechanisms for the second degree A-V block--infra-Hisian block and A-V block due to bundle branch extrasystoles arising in the affected right bundle branch. Concealed conduction of the bundle branch extrasystoles as the proximate cause of infra-Hisian block could not be excluded. However, both forms of A-V block were evidently dependent upon significant distal conduction system disease; this consideration was felt to warrant permanent pacemaker implantation.     

Asymmetry of retrograde conduction and reentry within the His-Purkinje system: A comparative analysis of left and right ventricular stimulation

Objectives. The purpose of this study was to delineate retrograde His-Purkinje system conduction and reentry (V₃phenomenon) during left ventricular extrastimulation and compare them with right ventricular extrastimulation.Background. The V₃phenomenon has been well described in the past during right ventricular extrastimulation; however, it has not been studied systematically during left ventricular extrastimulation.Methods. Left and right ventricular pacing were performed in 13 patients. Retrograde and anterograde routes of impulse propagation were determined on the basis of the sequence of His (H) and right bundle (RB) potentials, H-RB intervals, as well as the QRS configuration and axis of V₃beats.Results. During right ventricular pacing, retrograde conduction of V₂, when discernible, occurred exclusively through the left bundle at all coupling intervals equal to or shorter than the His-Purkinje relative refractory period, with the exception of two isolated beats. During left ventricular extrastimulation, His bundle activation was through the left bundle in nine patients and through the right or left bundle in three other patients. In one patient, the route could not be determined. The V₃phenomena occurred in eight patients during right ventricular pacing. Seven patients had a left bundle branch block pattern QRS configuration, and one had a right bundle branch block pattern configuration. V₃beats occurred in five patients during left ventricular apex pacing: left bundle branch block pattern configuration in one patient and right bundle branch block pattern configuration in four. In three of these four patients, the reentry was interfascicular and limited to the left bundle branch system.Conclusions. The left-sided His-Purkinje system is the preferred retrograde route of impulse propagation during both left and right ventricular extrastimulation. Reentry within the His-Purkinje system elicited by right ventricular extrastimulation involves both bundle branches, whereas this reentry tends to occur within the left-sided His-Purkinje system during left ventricular pacing.     

Coexistence of left branch block and ectopic rhythm simulating alternating block

During the acute phase of diaphragmatic myocardial infarction with septal extension, the ECG of a patient with a chronic left bundle branch block changed in a period of seconds from complete left bundle branch block to incomplete right bundle branch block then to narrow QRS complexes followed by incomplete and then complete left bundle branch block: the same QRS changes then occurred in reverse order; the atrial rhythm was absolutely stable during the recording. These appearances are explained by fusion of sinus and of an ectopic rhythm arising distal to the zone of block, the rate of which (sometimes faster and sometimes slower than the sinus rhythm) could have been influenced by an electrotonic effect after retrograde activation of the right bundle and concealed conduction in the left bundle. Appearances of bundle branch block may be recorded when the ventricle is partially activated from the point of breakthrough of the blocked branch.     

Congenital and surgical atrioventricular block within the His bundle

In three patients with congenital heart disease the site of atrioventricular (A-V) block was localized within the His bundle with the aid of His bundle electrograms. In one patient with first degree A-V block and normal QRS configuration, electrophysiologic studies revealed “split” His potentials. The other two patients had complete A-V block, and their His bundle electrograms revealed His spikes both proximal and distal to the site of block. One of the two patients, who had a pattern of left bundle branch block in the electrocardiogram, had surgically induced complete A-V block after repair of an ostium primum atrial septal defect. The other patient with congenital A-V block had a narrow QRS complex and, in addition to complete block within the His bundle, prolonged A-V nodal conduction time but no associated cardiac anomaly. Both patients with complete heart block required pacemaker insertion.

The natural history of intra-His bundle block is not known, and it is difficult to recommend appropriate therapy. More electrophysiologic studies are needed in patients with A-V block to determine the prognostic significance of such block or conduction delay in the His bundle.     

Apparent bradycardia-dependent right bundle branch block associated with atrial fibrillation: concealed electrotonic conduction as a possible mechanism

A 79-year-old woman with atrial fibrillation was reported in whom apparent bradycardia-dependent right bundle branch block was suggested. When a conducted supraventricular impulse occurred within a critical period after the preceding conducted impulse, the impulse was blocked in the right bundle branch except when it fell in the supernormal period of the right bundle branch. When the conducted impulse occurred between the critical period and another longer period, it was conducted without bundle branch block. When the impulse occurred beyond that longer period, it was usually blocked in the bundle branch again. However, when the impulse occurred beyond a still longer period, it was conducted without bundle branch block again. These findings suggest that when impulses fell in the right bundle branch shortly after the preceding conducted impulses, they were blocked in both bundle branches; however, it seemed that concealed electrotonic conduction of the blocked impulses affected conduction of the subsequent impulses.     

His bundle electrocardiography in manifest and concealed right bundle branch extrasystoles.

His bundle electrocardiography was helpful in the diagnosis of impulse formation in the right bundle branch. Ten patients with narrow QRS complexes had ectopic beats with an "incomplete" left bundle branch pattern and almost simultaneous activation of His bundle and ventricles. Both QRS morphology and H- - V intervals depended on the more proximal or distal location of the ectopic focus. In four patients with "complete" right bundle branch block the morphology of ectopic ventricular complexes and H- - V intervals also depeneded on the presence or absence of retrograde block and differential degrees of forward and/or retrograde conduction delays. Nine patients with "complete" right bundle branch block and four with "complete" left bundle branch block had premature beats which could have originated in the proximal right bundle branch, proximal left bundle branch, or distal His bundle. In one patient with "complete" left bundle branch block, "concealed" His bundle depolarizations (probably originating in an ectopic focus located in the right bundle branch) produced pseudo Type II (Mobitz) A-V block. Although lidocaine appeared to have been more effective in patients with bundle branch block than in those with narrow QRS complexes, further studies are necessary to corroborate this impression.     

Combined intra-Hisian block and A-V nodal bypass

A patient with first and second degree atrio-ventricular (A-V) block and left bundle branch block was shown on electrophysiological study to have an intra and infra-His bundle branch block. In addition, this patient also had an A-V nodal bypass tract. The intra-Hisian block was concealed on the resting His bundle electrogram and became evident only during atrial pacing. The A-V nodal bypass was masked on the surface electrocardiogram (ECG) by the associated A-V conduction defect. The clinical significance of A-V bypass and combined block below the A-V node is discussed. The diagnostic value of His bundle electrocardiography in a patient with complex electrophysiological abnormalities is stressed.     

Observations during clinical 2:1 and 3:1 A-V block below the A-V node. Evidence of partial penetration of atrial impulses into the His bundle

A clinical His bundle recording during 2:1 A-V block below the A-V node displayed RBBB, a prolonged H-V interval, and alternating amplitude and duration of the His potentials. The reduced amplitude of the non-conducted His potential suggests a lesser depth of penetration into the His tissue with subsequent block. The reduced His potential amplitude may be due to decremental conduction within the His bundle and/or prolonged refractoriness of the His tissue following atrioventricular conduction of the preceding atrial impulse.During 3:1 A-V block progressively deeper penetration of the atrial impulses into the His-Purkinje system occurred. Progressive penetration into the more proximal His-Purkinje system may have permitted recovery of a more distal area of refractoriness with subsequent atrioventricular conduction. This mechanisms appears similar to one of the mechanisms of 3:1 A-V block demonstrated experimentally, except that in this clinical record the major site of impaired conduction and progressive penetration is within the His-Purkinje system rather than within the A-V node.