Electrical threshold of the human heart.

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Abstract
Under the conditions of this investigation the human heart was slightly more sensitive to endocardial than to intramural stimulation, and considerably more sensitive to intramural than to epicardial stimulation. At the threshold for myocardial contraction, current and voltage varied inversely with impulse duration, the energy being constant over a wide range. Threshold requirements rose initially for all types of stimulation, but stabilized at fairly low levels in the absence of infection. Final values for threshold energy were approximately 5 microjoules for endocardial stimulation and 20 microjoules for epicardial stimulation; the initial values were about 1/10 of the final values. The heart showed similar sensitivity to unipolar cathodal stimulation and bipolar stimulation. The threshold for unipolar anodal stimulation was considerably higher. Voltage and current strength-duration curves for the human heart are presented, and values for rheobase and chronaxie are given.