E. E. Suckling

Vulnerability of the Dog Ventricle and Effects of Defibrillation

Vulnerability to ventricular fibrillation has been studied in 20 dogs by means of test stimuli of known strength and duration in order to ascertain the relationship between the vulnerable period and the recovery of excitability and to determine the effects of countershock defibrillation. The relative refractory period is found to be the only portion of the cycle during which fibrillation could be produced by this technique. Defibrillation was accomplished without producing significant changes in vulnerability or excitability.

The phenomenon of accommodation in the ventricular myocardium

SummaryThe ventricular and auricular myocardium was found to possess an ability to accommodate to applied electrical current pulses.Accommodation to cathodal current flow was observed even when nonpolarizable electrodes were employed and also when tests were made with chronically implanted metal electrodes in intact anesthetized dogs.Accommodation tended to attain its maximum within 40 to 45 msec after the beginning of the conditioning pulse. It reduced the facilitatory action of the conditioning pulse by 50 to 60%.Post-cathodal and post-anodal changes in excitability occurred. They were of 10 to 15 msec duration following a 70 msec conditioning current flow. They were of lesser magnitude then the average residual facilitation remaining just before termination of conditioning stimuli.Evidence that various circumstances can modify the ability of cardiac tissue to accommodate to intrinsic and applied stimuli was discussed. The possible importance of this phenomenon to the maintenance of effective action of the heart under abnormal circumstances was considered.