Hypothermia and minimal gradient requirements for excitation of cardiac muscle

Abstract

A method is described for testing the stimulating-effectiveness of linearly rising current. It was found that cardiac muscle, like nerve fibers, spinal neurons and skeletal muscle has a minimal gradient requirement. Dog trabecular muscles showed great differences in their minimal gradients even at the same temperature but the requirements of individual muscles were rather constant. During hypothermia (24°–27°C) there was a reduction in threshold to stimulation by a rectangular pulse and a great prolongation of the minimal gradient requirement for stimulation as though a type of accommodative reaction present at normal temperatures had been suppressed. Determination of minimal gradient requirements by exponentially rising current gave the same results as obtained with linearly rising currents. Excitation occurred when the rheobasic strengths, as determined by rectangular pulses of long duration, were attained if stimuli satisfied the minimal gradient requirement. The likelihood that a number of accommodative processes occur in tissues subject to stimulatory forces is discussed.