SUMMARYTo obtain information conceming the time course and instantaneous distribution of the excitatory process of the normal human healt, studies were made on isolated human hearts from seven individuals who died from various cerebral conditions, but who had no history of cardiac disease. Measurements were made from as many as 870 intramural terminals.In the isolated human hearts three endocardial areas were synchronously excited o to 5 msec after the start of the left ventricular activity potential. These areas increased rapidly in si ze dUl'ing the next 5 to 10 msec and became confluent in 15 to 20 msec. The left ventricular areas Rrst excited were (1) high on the anterior paraseptal wall just below the attachment of the mitral valve; (2) central on the left surface of the interventricular septum and (3) posterior paraseptal about one third of the distance from apex to base. The last part of the left ventricle to be activated usually was the posterobasal area. Endocardial activation of the right ventricle was found to start ne ar the insertion of the anterior papillary muscle 5 to 10 msec af ter onset of the left ventricular cavity potential. Septal activation started in the micldle third of the left side of the interventricular septurn, somewhat anteriorly, and at the lower third at the junction of the septum and posterior wall. The epicardial excitation pattem reflected the movements of the intramural excitation wave. Conduction velo city was determined in one heart by an appropriate stimulation technic. Atrial excitation, explored in two hearts, spread more or less according to concentric isochronic lines. Control studies, carried out on Rve canine hearts, disclosed that the pattem of ventricular excitation did not change af ter isolation and perfusion. However, total excitation was completed earlier in the isolated heart, and conduction velo city increased.Careful mapping illustrations are presented.Additional Indexing W ords:Ventricular excitation Intramural conduction velo city Epicardial excitation Atrial excitation K NOWLEDGE of the time course and instantaneous distribution of the excitatory pro ce ss of the normal human heart would be of value for an understanding of the QRS complex.Studies of this process during surgical intervention for heart disease or pulmonary

Supraventricular tachycardia in an isolated rabbit heart preparation was repeatedly initiated and terminated by carefully timed atrial premature beats. Transmembrane action potentials of AV nodal cells were recorded simultaneously by a "brush electrode" consisting of 10 microelectrodes. Surface electrograms of atrium and His bundle were also recorded. The moments of activation of 54 different AV nodal cells, both during regular driving of the atrium and during tachycardia were ascertained. Premature atrial beats introduced during tachycardia would either "reset" the tachycardia or terminate it. The sequence of activation of the AV nodal cells when initiating tachycardia, during tachycardia itself, and when premature beats were interpolated during tachycardia warrant the conclusion that a circus movement in the AV node, based on functional longitudinal dissociation of the upper AV node, was the underlying mechanism of the arrhythmia. KEY WORDS reciprocating tachycardia echo beats longitudinal dissociation transmembrane potentials atrial premature beats initiation and termination of tachycardia• On the basis of experiments on turtle hearts, Mines (1) described the basic mechanism underlying a reciprocating rhythm: "A slight difference in the rate of recovery of two divisions of the AV connexion might determine that a extrasystole of the ventricle . . . should spread up to the auricle by that part of the AV connexion having the quicker recovery process and not by the other part. In such a case, when the auricle became excited by this impulse, the other portion of the AV connexion would be ready to take up the transmission back to the ventricle. Provided the transmission in each direction was slow, the chamber at either end would be ready to respond . . . and thus the condition once

The pattern of atrioventricular (AV) nodal activation in isolated rabbit hearts was studied by recording transmembrane action potentials in about 200 AV nodal fibers per heart. A brush electrode consisting of ten microelectrodes was used. Both antegrade and retrograde spread of excitation were mapped: retrograde conduction was not an exact mirror image of antegrade conduction. The major input during antegrade conduction was from the crista terminalis, but the major output during retrograde conduction was through the interatrial septum. Some parts of the AV node were excited without directly participating in either antegrade or retrograde transmission of the impulse. Simultaneous pacing of the atrium and the His bundle generated antegrade and retrograde wave fronts which collided at different levels of the AV node. In this way, cells which transmitted the impulse in both directions were distinguished from those which did not (dead-end pathways). Two types of dead-end pathways were found: type A probably consisted of superficial atrial fibers terminating in the base of the tricuspid valve and type B branched off from cells in the middle node. The shape of the action potential of AV nodal cells strongly depended on the direction of propagation. In some parts of the AV node, the amplitude of the action potential was larger during antegrade activation than it was during retrograde activation; in other parts, the amplitude was smaller during antegrade conduction. The largest amplitudes occurred when wave fronts collided.

Interruption of anterior fibers of the left bundle branch system together with right bundle branch block was accomplished experimentally in canine and primate hearts. In both species the electrocardiographic effects included a major alteration of the mean electrical axis to a superior and anterior direction. Epicardial excitation was markedly delayed anteriorly, causing widening of the QRS complex in standard electrocardiographic leads. Normal outward intramural spread of excitation in anterior regions of the left ventricle was reversed after left anterior arborization block alone. After addition of right bundle branch block, intramural spread of excitation was again directed outwardly but was markedly delayed in endocardia] onset. Electrical effects of these blocks could be "corrected" individually or in combination by introducing synchronized electrical stimuli distal to each lesion. A scheme is proposed by which various forms of intraventricular conduction disturbance can be defined in terms of block of one or more divisions of a three-pronged system of rapid ventricular excitation.ADDITIONAL KEY WORDS intraventricular conduction disturbances left axis deviation masquerading block QRS widening bilateral bundle branch block epicardial excitation posterior precedence transmural excitation synchronous endocardial stimulation dog baboon• The electrocardiographic consequences of complete block of either the left or the right branches of the bundle of His are well established, but speculation existed regarding the significance of, subtotal interruption of these conduction pathways (1). Recent experimental studies established the effect of disturbances in anterior and posterior rami of the left branch system in the canine heart (2) and specifically confirmed the relationship This investigation was supported in part by Veterans Administration Clinical Investigator Program, Houston Veterans Administration Hospital, and by U. S. Public Health Service Research Grants HE-05435 and HE-10400 from the National Heart Institute.Accepted for publication November 27, 1967. of left anterior arborization block to left axis deviation in the primate (3). Several authors (4-7) speculated on the electrical phenomena which might derive from block of all the right bundle branch together with only a portion of the left bundle branch. However, morphologic data essential to corroboration were meager (8).The results described here demonstrate the characteristic features attending an anatomically verified combination of left anterior arborization block and right bundle branch block. Such features include not only a unique form of complexes recorded from standard electrocardiographic leads, but also marked alterations of observed epicardial and transmural patterns of excitation.Methods Studies on 24 mongrel dogs weighing about 15 kg and 10 baboons constitute the basis