DETERMINATION OF EXCITABILITY OF MAMMALIAN HEART AT INTERVALS THROUGHOUT CARDIAC CYCLE

Abstract

Apparatus is described which permits control of the rate of heart beat by introducing driving stimuli through non-polarizable electrodes attached to tne surface of the heart. Square-wave stimuli, adjustable in duration and strength, can be applied similarly at any desired period of the cycle. This technic permits a highly accurate testing of cardiac excitability and the effects of drugs and other influences upon this property of cardiac muscle. Recovery of excitability following each beat is not a steady uniform process. A period of relative hyperexcitability occurs early in the relative refractory period. The duration of the refractory periods as detd. by stimuli of various durations differs markedly. Stimuli of long duration (5-13 msec.) produce a much greater excitatory after effect than do stimuli of short duration (0.1 msec.).