Anodal Excitation of Cardiac Muscle1

CRANEFIELD, PAUL F., BRIAN F. HOFFMAN AND ARTHUR A. SIEBENS. Amdal excitation of cardiac muscle. Am. J. Physiol. Igo( $33-390. 1957. -The strength-interval curve of dog ventricular myocardium has been measured with anodal. and cathodal stimulation. During diastole the anodal threshold is higher than the cathodal. As anodal stimuli are applied progressively earlier the anodal threshold first rises above and then falls to levels below the anodal diastolic threshold. During most of the relative refractory period the anodal threshold is lower than the cathodal threshold. At all times during the late relative refractory period and throughout diastole excitation of double origin (anodal and cathodal) is evoked by sufficiently strong stimuli; this simultaneous origin of excitation at two points does not evoke fibrillation. During the early relative refractory period, however, only the anode is able to excite. Differences between anodal and cathodal thresholds are not attributable to asynchronous repolarization at the two electrode sites. The ‘no-response’ phenomenon occurs only when the anodal threshold is markedly lower than the cathodal. T HE CLASSICAL CONCEPT that a propagated response arises near the cathode when the heart is excited electrically was questioned by Orias (I), who showed that during the ‘dip’ of the strength-interval curve cardiac muscle is preferentially excited at the anode. This observation was recently confirmed by Van Dam (2, 3) who further showed that during diastole a sufficiently strong stimulus will excite at both anode and cathode simultaneously. Van Dam used a rather special arrangement of electrodes (transmural) ; he was led to conclude that the dip and associated anodal origin of excitation were artefacts of that special electrode arrangement and were critically dependent upon earlier activation and recovery of tissue under the anode. It has therefore seemed desirable to extend such studies and at the same time to investigate the ‘no-response’ phenomenon (4), another remarkable feature of cardiac excitability observed during the dip of the strength-interval curve. Received for publication February II, x9$7. l Supported in part by a grant from the Life Insurance Medical Research Fund. METHODS In general, methods previously described (5) were used in the present series of studies. The exposed hearts of mongrel dogs were driven at regular rates by electrical stimulation of the auricle or ventricle. The stimulus generator which supplied d.c. pulses of chosen duration (usually IO msec.) at any desired interval after the R wave has been fully described in the reference mentioned above. The stimulating and recording electrodes (fig. IF) were stitched to the epicardium of the right ventricle. The use of these electrodes in which IO leads were incorporated in a single piece of plastic 7 by 20 mm permitted the anode, cathode and four pairs of recording electrodes to be Iocated along a straight line 17 mm