Physiology of Canine Intraventricular Conduction and Endocardial Excitation

Abstract

The sequence of conduction through the intraventricular conducting system and endocardial muscle was studied by microelectrode mapping of large areas of isolated canine ventricular tissue. We found that most of the endocardium of the right ventricular free wall is activated simultaneously whereas the left endocardial muscle is activated in an apex-to-base sequence. Right septal activation is from apex to base, and the left septum is activated first at the junction of the middle and lower thirds of the septum and then as a bidirectional wave front toward the apex and the base. These right-left differences occur because the sites of impulse input into muscle on the right encompass the entire free wall, base of the papillary muscle, and the lowermost septum, and on the left are primarily limited to the lower ventricular cavity, lower septum, and bases of the papillary muscles. These patterns of left ventricular excitation relate to the presence of a functionally continuous ring of conducting tissue formed by a merger of the major divisions of the left bundle branch on the upper left ventricular free wall. The "ring" itself is electrophysiologically isolated from muscle, but connects to a network of subendocardial conducting tissue extending to the apex and having input to muscle only in its lower portions. KEY WORDSAV conduction papillary muscles bundle branches conducting system Purkinje fibers cardiac electrophysiology septal activation myocardial activation ventricular muscle• The relationship between the anatomy of the intraventricular portion of the AV conducting system and the physiology of AV conduction and ventricular excitation is pertinent to our understanding of major areas of cardiac electrophysiology. The nature and location of impulse input from the intraventricular conducting system into the ventricular myocardium, the physiological interactions between conducting tissue and ordinary muscle cells, and the sequence of activation of the endocar-Downloaded from curred, but the apex-to-base muscle activation persisted. White numbers on black background = conducting cells; black numbers on white background = muscle cells.

Electrophysiological effects or procainamide in acute and healed experimental ischemic injury of cat myocardium.

“…On the day of terminal studies, the cats were anesthetized with pentobarbital (30 mg/kg, ip), and the heart was removed through a thoracotomy. The atria and right ventricle were excised, and the left ventricle opened by an incision through the free wall between the posterior papillary muscle and posterior paraseptal free wall (Myerburg et al, 1972). The mitral valve was excised and the aortic ring opened.…”

“…These electrodes were used to map endocardial surface activation of the preparation during specific stimulation of the left bundle branch, which was achieved by setting stimulus strength at 1.2-times threshold current (Myerburg et al, 1972). The surface electrograms were used to delineate areas of electrophysiological abnormality, particularly in the AMI preparations in which it was not always possible to visually identify abnormal areas.…”

“…Premature stimuli were delivered at 2.0 times late diastolic threshold strength, and diastole was scanned as previously reported (Myerburg et al, , 1972(Myerburg et al, , 1973. In those preparations in which it was not possible to initiate sustained ventricular activity by premature stimulation, we determined the effects of drug on action potential duration, local refractory period, upstroke velocity, and resting membrane potential.…”

“…Data were recorded photographically from an oscilloscope screen, using techniques which we have previously reported (Myerburg et al, 1972(Myerburg et al, ,1973, and dV/dt max , action potential duration, and local refractory periods were measured on-line Gelband et al, 1970). Statistical analyses were carried out using a two-tailed, twosample t-tests.…”

How the Purkinje system determines the ventricular activation sequence

“…While most of the P cells on the endocardial surface are electrically insulated from the underlying VM cells, the Purkinjeventricular muscle Junctions ( PVJs) are sites where sufficient electrical coupling allows action potential conduction from the P cells to the VM cells. The PVJ was first characterized as a "funnel" of increasing electrical load based on measuring 5 to 25 mSec delays in asymmetrical conduction from the P to the VM cells [1][2][3][4][5]. Subsequently, two-dimensional activation studies of the P and VM cell layers were made with surface electrodes and combined with microelectrode recordings [6][7][8] to more precisely locate junctional sites and to show that action potential conduction could be characterized as a "resistive barrier" in which transitional (T) cells coupled P cells to underlying VM cells.…”