Unmasking and conversion of gap phenomenon in the human heart

Two types of gap phenomena (types I and II ) have been described in human hearts and their electrophysiologic bases have been delineated. In both types of gap phenomena relatively early premature atrial impulses are blocked within some portion of the His-Purkinje system (HPS). By increasing the prematurity of the atrial depolarization, conduction to the ventricles resumes due to delay of the premature impulse with the atrioventricular node (A-VN) (type I gap) or within the proximal HPS (type II gap). Gap phenomena are not observed when the refractory period of the A-VN exceeds that of the HPS. Since atropine decreases refractoriness of the A-V node, its effect on the gap phenomena was studied in nine subjects. After administration of atropine (0.2-0.5 mg i.v.) type I gap was demonstrated in six subjects and type II gap in three subjects. When atropine shortened the functional and effective refractory period (ERP) of the A-V node, premature atrial impulses arrived at the HPS during its ERP. By a similar mechanism, type I gap was converted into type II gap in three subjects following atropine administration. Decreasing the basic atrial drive rate converted type II gap into type I (two subjects) and ultimately abolished both types of gap phenomena in all subjects. These results suggest that the gap penomenon may be functional in nature and may be readily manifested or abolished by varying the refractoriness of the A-V node relative to that of the HPS.

His bundle electrograms of dog. Correlation with intracellular recordings

Transmembrane potentials from the penetrating and branching portions of the bundle of His were simultaneously recorded along with bipolar extracellular electrograms. Recordings were obtained during periods of normal antegrade and retrograde conduction and also during various degrees of conduction delay and block within and distal to the bundle of His. The onset and termination of extracellular electrogram coincided with the upstrokes of the transmembrane action potentials recorded from the penetrating and branching portions respectively and the duration of the extracellular electrogram equaled the interelectrode conduction time. Thus, the extracellular electrogram accurately reflected the timing and duration of the electrical activity of the entire bundle of His (penetrating and branching portions). Intra-His bundle conduction delay as determined by an increase in interelectrode conduction time and a decrease in the upstroke velocity of phase O of the action potentials, resulted in a decrease in the amplitude and an increase in the duration of the extracellular recording. Significant intra-His bundle conduction delay resulted in two His bundle deflections (H and H') in the extracellular electrogram recordings. The onset of the two His deflections coincided with the upstrokes of the two transmembrane potential recordings. When intra-His bundle block occurred the action potential distal to the block and the corresponding deflection on the extracellular recording disappeared. The results of this study provide further evidence supporting the validity of clinical electrode-catheter recordings of His bundle activity.

Electrophysiologic properties of diphenylhydantoin

The electrophysiologic properties of diphenylhydantoin (DPH) (5-10 mg/kg) intravenously was studied in 14 subjects using His bundle recordings and correlated with blood levels. Conduction through the A-V conducting system (AVCS) was studied at various paced atrial rates and refractory periods determined using programmed atrial premature depolarization within 10 min of drug administration. In 11 of 14 subjects the sinus cycle length was shortened by an average of 110 msec, lengthened in three (average 116 msec). Conduction through the A-V node (AVN) was shortened in seven subjects (average 10 msec), lengthened in one (5 msec), and unchanged in the remaining six. Conductinged by 5 msec). Prior to DPH, the longest mained constant in all but two subjects (proloengthened in four, and was unchanged in the re-refractory period of the AVCS was in the AVN in nine subjects, the atrium in four and the HPS in one. After DPH, the following effects were noted: (1) the effective refractory period (ERP) of the atria shortened in four subjects, lengthened in four, and was unchanged in the remaining six; (2) the ERP of the AVN shortened in 6/9 subjects, lengthened in 3/9; (3) functional refractory period of AVN shortened in six, prolonged in three subjects, and remained unchanged in five subjects; (4) the relative refractory period (RRP) of the HPS shortened in 7/7 subjects; (5) ERP of HPS in 1/1 subject shortened. Thus, DPH showed varied effects on A-V nodal conduction, inconsistent effect in the atrium, and consistent shortening of the refractory period of the HPS. The data suggest DPH differs from other antiarrhythmic drugs such as quinidine and procaine amide.

Manifest and concealed reentry: a mechanism of A-V nodal Wenckebach in man

In five patients, bundle of His electrograms were recorded during right ventricular pacing at various cycle lengths. In all patients, as the cycle length of stimulation was decreased, the pattern of retrograde conduction proceeded from 1:1 retrograde conduction to retrograde Wenckebach with or without manifest reentry, to retrograde Wenckebach cycles with concealed reentry. The requisite condition for reentry was a critical retrograde A-V nodal delay. During retrograde Wenckebach cycles, reentry could be either concealed or manifest. Concealed reentry resulted in typical or ordinary Wenckebach cycles on the surface electrocardiogram and manifest reentry resulted in ventricular echo beats. Depending upon the cycle length of stimulation, reentry could be concealed on the surface electrocardiogram but manifest on the His bundle electrogram recording. Concealed reentry could be manifest by turning off the stimulator at the appropriate time in the cardiac cycle while manifest reentry could be concealed by changing the cycle length of stimulation. Reentry with collision of wavefronts within the A-V node or proximal His-Purkinje conduction system explains why the last ventricular impulse is not conducted to the atria during ordinary Wenckebach cycles.

Functional 2:1 A-V block within the His-Purkinje system. Simulation of type II second-degree A-V block

In eight subjects in whom the effective refractory period of the His-Purkinje system was determined to be longer than that of the A-V node or atrium, abrupt acceleration of the atrial rate resulted in either 2:1 block within the His-Purkinje system or 1:1 A-V conduction. The former conduction pattern simulated a type II second-degree A-V block. The occurrence of either 2:1 block within the His-Purkinje system or 1:1 A-V conduction was determined primarily by (1) an effective refractory period of the His-Purkinje system which exceeded that of the A-V node, (2) the coupling interval of the first atrial capture beat and its effect or lack thereof on the refractory period of His-Purkinje system as it relates to changes in ventricular cycle length, and (3) the relative speed of A-V nodal conduction time. The distinction between this functional 2:1 block within the His-Purkinje system and a true type II second-degree block is discussed from both the clinical and electrophysiologic points of view. Functional 2:1 block within the His-Purkinje can also result when sudden acceleration of atrial rate occurs spontaneously such as in atrial flutter or A-V nodal reentrant atrial tachycardias.

Demonstration of entrance block into the atrioventricular node of man

Entrance block of an atrial premature beat (APB) into the atrioventricular node is demonstrated by its lack of effect on the A-V nodal conduction of a subsequent beat which is introduced before the full recovery time of the A-V node. This phenomenon was demonstrated in five patients by using programmed atrial stimulation and His bundle recordings. Entrance block into the A-V node occurred in a narrow range (28-33%) of the basic cycle length and concealed conduction occurred at coupling intervals longer than this range. It appears that entrance block of these early APBs is due to a "functional block" between the atrium and the A-V node. The concept of an entrance block into the A-V node is useful in explaining some forms of the supernormal phase of A-V conduction as well as in the interpretation of complex arrhythmias.

Manifest and concealed reentry. A mechanism of AV nodal Wenckebach phenomenon

The mechanism of the AV nodal Wenckebach phenomenon was studied in 25 dogs by multiple atrial and His bundle electrogram (HBE) recordings. During ventricular stimulation with 1:1 retrograde conduction, the interval from the stimulus artifact to the retrograde His deflection (S-H) interval) remained constant. Decreasing the cycle length of stimulation (CLS) resulted in retrograde AV nodal Wenckebeach cycles. When retrograde AV nodal delay reached a critical value, the Wenckebach cycles were terminated by a reciprocal beat (manifest reentry). The His bundle and ventricles were antegradely depolarized by the reentrant impulse. A further decrease in CLS produced what electrocardiographically looked like ordinary Wenckebach cycles. However, the HBE tracing revealed that reentry was occurring but was concealed by the CLS. This was confirmed by noting that the His deflection of the last ventricular paced beat of the Wenckebach cycle was antegradely depolarized and had a shorter S-H interval then all other beats of that cycle. The reentrant and retrograde impulses collided in the bundle-branch system. Further decreases in CLS masked the reentrant phenomenon on both standard ECG and HBE tracings. The S-H interval remained constant for all ventricular paced beats of the Wenckebach cycle. Under these circumstances, reentry could still be uncovered by turning off the stimulator at an appropriate time during the Wenckebach cycle. This maneuver exposed the reciprocal beat. Thus collision of the reentrant and retrograde impulses occurred within the AV node. These findings provide a satisfactory explanation for why the last beat of the Wenckebach cycle is not conducted retrogradely to the atria.

Electrophysiologic studies during accelerated idioventricular rhythms

Accelerated idioventricular rhythms (AIVR) are ectopic ventricular rhythms with rates intermediate between idioventricular escape rhythms (30 to 40/min) and ventricular tachycardia (120 to 180/min). Differentiation of AIVR from supraventricular arrhythmias rests primarily on demonstration of their ventricular origin. His bundle electrograms (HBE) were recorded in four patients during AIVR. HBE verified the idioventricular nature of the ectopic rhythm and excluded supraventricular rhythm with aberration as a cause. In addition, they permitted the recognition of normally conducted sinus beats, fusion beats, and idioventricular beats. The pacemaker site for the AIVR was below the bundle of His. AIVR became manifest when the heart rate was slowed by increasing vagal tone, premature atrial stimulation, and high degree atrioventricular (A-V) block. AIVR could be suppressed and 1:1 A-V conduction established by increasing the atrial rate with atropine or by atrial pacing.

Use of His bundle recordings in understanding A-V conduction disturbances

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