Excitability of the Normally Beating Heart

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Abstract
The electrical and mechanical responses to single square wave stimuli applied to spontaneously beating hearts in situ were examined. Shocks were delivered through a time delay circuit triggered by the voltage of the R wave of the ECG. The silver-silver chloride electrodes were 3 mm apart and their position on the ventricular surface kept constant throughout an experiment Stimuli were separated by 20–30 heart beats. The plotted excitability curve (strength-interval) was found to be smooth without any indication of transient variations in threshold. Single stimuli of various durations (up to 100 msec.) and amplitudes (up to 25 ma) were ineffective for the production of ventricular fibrillation. Apparently excitability recovery for a small and localized myocardial area is synchronous and smooth.