Stafford I. Cohen

Study of atrioventricular conduction in man using electrode catheter recordings of His bundle activity.

The technique of recording electrograms of the His bundle, using a tripolar electrode catheter positioned across the tricuspid valve, was applied in a physiological study of atrioventricular conduction in man. Increasing the heart rate to 160 beats/min by right atrial pacing produced progressive prolongation of the P-H interval while the H-Q interval remained constant. At any given paced heart rate, digitalis caused a prolongation of the P-H interval. Isoproterenol and atropine markedly shortened the P-H interval at any given heart rate. Neither drug had any significant effect on the H-Q interval. Coupled premature atrial stimulation produced prolongation of the P-H interval alone when the resultant ventricular depolarization was normal, and prolongation of both the P-H and H-Q intervals when the resultant ventricular depolarization was aberrant.

Variations of aberrant ventricular conduction in man: evidence of isolated and combined block within the specialized conduction system. An electrocardiographic and vectorcardiographic study.

Aberrant ventricular conduction (AbC) was produced in 52 subjects, including 14 normal volunteers, by the introduction of atrial premature beats through a transvenous catheter electrode. Simultaneous standard electrocardiograms and vectorcardiogams permitted detailed analysis of configuration. Multiple patterns of AbC were produced in 29 of 52 subjects. The general categories and their frequency were as follows: right bundle-branch block without significant axis shift from control, 31; right bundle-branch block with left axis deviation, 27; left axis deviation, 14; inferior axis deviation, six; right bundle-branch block with inferior axis deviation, seven; and complete left bundle-branch block, six. These patterns were believed to depend on the location of isolated or combined functional block in the major pathways of the specialized conduction system, which include the right bundle branch, common left bundle branch, anterior division of the left bundle branch, and posterior division of the left bundle branch. The present study has documented the general varieties of AbC, their relevance to clinical cardiology, and the varieties most likely to cause diagnostic difficulty.

Experimental Production of Aberrant Ventricular Conduction in Man

Aberrant ventricular conduction (AbC) was produced in 17 of 18 subjects, including six normal persons, by the introduction of electric stimuli via a transvenous catheter situated in the right atrium.Analysis of electrophysiological determinants indicated that AbC was favored by a long cycle preceding the short cycle which terminated in the aberrant complex, a short R to stimulus coupling interval, and fast atrioventricular conduction time.Electrocardiographic analysis of AbC revealed that all patients had right bundle-branch block (RBBB). In addition, incomplete right bundle-branch block was present in eight patients and was transformed to RBBB in all cases by shortening the coupling interval. Ten patients had slight but distinct variations of their RBBB pattern. There were two instances each of left bundle-branch block and intraventricular conduction disturbance. Hence, any patient may have the potential to produce a variety of AbC patterns. AbC was produced in six normal subjects and must be considered a physiological event which does not require covert pathology of the specialized conduction system.

Controlled Heart Rate by Atrial Pacing in Angina Pectoris A Determinant of Electrocardiographic S-T Depression

This study was undertaken with a consideration of the physiological relationship between myocardial oxygen consumption and heart rate. Atrial pacing was performed in 63 resting subjects, 36 of whom had been referred for evaluation of angina pectoris. Pacing at critical controlled heart rates evoked angina pectoris in six patients of this group with chest pain and, consistently and more frequently, produced significant S-T segmental depression in 28 patients as a manifestation of myocardial ischemia. The functional test employing controlled heart rates has several advantages over other stress tests. The use of the sole stress of controlled atrial tachycardia is a new technique which has provided relatively safe, reproducible results in the objective evaluation and investigation of myocardial ischemia.

Alternate Patterns of Premature Ventricular Excitation During Induced Atrial Bigeminy

Alternate patterns of premature ventricular excitation have been observed during induced atrial bigeminy in 18 subjects, including five normal volunteers. Each study was performed in the cardiac catheterization suite where a transvenous catheter electrode was positioned in the right atrium. Coupled or paired stimuli were delivered to the atrium by an isolated battery-powered source at an adjusted interval which resulted in alternate patterns of ventricular excitation from alternate premature beats (“alternating premature ventricular excitation”).In most instances “alternating premature ventricular excitation” occurred when parameters of preceding cycle length, premature coupling interval, and atrioventricular conduction time were constant.In man also some observations were made of His bundle excitation during premature atrial stimulation, and in the intact dog heart some observations were made of alternating premature ventricular excitation during His bundle stimulation.A tenable explanation for alternating premature ventricular excitation is advanced which rests on three postulates: (1) A long cycle length is followed by a long refractory period. (2) The refractory period of each branch of the specialized conduction system is dependent on its preceding cycle length or recovery period; and (3) the diastolic recovery period of a blocked segment of the specialized conduction system is shorter than the recovery period when blockade does not occur.

P waves and P loops in coronary sinus and left atrial rhythms.

Abstract P-wave and P-loop changes during transvenous pacing of specific locations in the coronary sinus and left atrium were studied in dogs and in man. During coronary sinus rhythm, the diagnostic criteria for P-wave changes were always met. In addition, inversion of the P wave in Lead V 6 was consistently obtained. The P loop was always directed cephalad. Dome-dart P waves in V 1 were present in one of 14 patient studies. Pacing the left atrial appendage produced less consistent P-wave changes; domedart P waves in V 1 were present in 3 of 7 dog studies and in one of 3 patients. The P vector was directed caudad, rightward, and ventrad. Pacing the inferior left atrium in close proximity to the coronary sinus produced P-wave and P-loop changes which were identical with and indistinguishable from those of coronary sinus rhythm. It is suggested that a common site of impulse formation or a common intra-atrial conduction pathway is utilized. Confirmatory evidence was obtained from intra-atrial electrograms. Additional evidence was obtained from Bachman bundle potential recordings, which indicated a reversal of atrial activation during left atrial rhythm.

Effects of Diphenylhydantoin on Atrioventricular Conduction in Man

The effects of intravenous diphenylhydantoin (5 mg/kg) on atrioventricular conduction in man at controlled heart rates in the digitalized and undigitalized state were studied. In undigitalized subjects, diphenylhydantoin administration consistently decreased A-V conduction time in each individual subject when identical heart rates were compared. Ouabain infusion produced an increase in A-V conduction time and this prolongation was always reversed by diphenylhydantoin. In two patients with atrial flutter and in two with atrial fibrillation, diphenylhydantoin caused a decrease in ventricular rate without affecting the atrial arrhythmias.These findings are of clinical importance, especially in the therapy of digitalis toxicity. When digitalis excess is manifested by both ectopic beats and incomplete A-V block, diphenylhydantoin would have special utility since, in contrast with the commonly used antiarrhythmic agents, it decreases A-V conduction time in addition to suppressing ectopic beats. Diphenylhydantoin-induced depression of the ventricular rate in atrial flutter or fibrillation may also be useful clinically. However, since diphenylhydantoin markedly decreases ventricular automaticity, its use in patients with complete heart block is contraindicated.

Effects of reserpine therapy on cardiac output and atrioventricular conduction during rest and controlled heart rates in patients with essential hypertension.

The effects of long-term reserpine therapy (1) on cardiac output during rest and induced atrial tachycardia and (2) on atrioventricular (A-V) conduction were examined in a group of eight men with previously untreated essential hypertension. The patients were studied before and after 20 to 205 days of reserpine therapy. All studies were performed in the cardiopulmonary laboratory with the patients supine. Heart rate was controlled with a transvenous pacing catheter and a battery-powered pacemaker. After therapy six of six patients had a fall in cardiac output with a statistically significant difference (P<0.001) between the means of paired pre-reserpine and reserpine cardiac outputs during both rest and paced tachycardia. At comparable heart rates, atrioventricular conduction increased in six of eight patients following reserpine, and a significant difference (P<0.05) was evident when the means of pooled paired pre-and post-reserpine A-V conductions were compared.This study indicates that therapeutically administered reserpine in a hypertensive population (1) may significantly lower cardiac output at rest with no further decrease during induced atrial tachycardia, and (2) may increase A-V conduction and enhance second degree heart block during induced atrial tachycardia.

Concealed conduction during atrial fibrillation

Abstract Incomplete penetration of an atrial impulse into the atrioventricular node is termed “concealed conduction” and results in a prolongation of the refractory period, which may block or delay the passage of subsequent atrial impulses. The influence of concealed conduction on ventricular rate during atrial fibrillation was determined from data collected on 5 patients who experienced atrial fibrillation concurrent with rapid atrial stimulation by a catheter electrode during the course of atrial pacing studies. One patient who had atrial fibrillation after the occurrence of atrial tachycardia during diagnostic cardiac catheterization is also included. The observations fortuitously noted in man were duplicated in a planned experiment in 10 anesthetized dogs by means of right atrial stimulation with a bipolar pervenous catheter electrode at the fastest rate that would result in 1:1 atrioventricular response. This was immediately followed by induced atrial fibrillation. The ventricular response in both man and dog was greater during atrial tachycardia than during atrial fibrillation. Data analysis revealed a significant difference between the means of paired ventricular rates during induced atrial tachycardia and atrial fibrillation ( P Our observations indicate that concealed conduction within the atrioventricular conduction system influences ventricular rate during atrial fibrillation.

Cardiovascular response to acute thermal stress (hot dry environment) in unacclimatized normal subjects

Abstract The effects of acute heat stress (a hot dry environment) on cardiac hemodynamics were studied in 16 normal unacclimatized male subjects. The results indicate that the cardiac output obtained at a comfortable environment (78° F.) remains essentially unchanged until an ambient temperature of 115° F. is exceeded. At 125° F. the cardiac output both at rest and during exercise is significantly increased over that obtained at 78° F. These increases in cardiac output are associated with inverse changes in A-V oxygen differences. A decrease in the mean pulmonary and brachial artery pressures occurred over the full range of temperatures studied.