Strength-duration curves in the hypothermic dog heart

¹ Department of Physiology, Boston University School of Medicine, Boston, Massachusetts

Strength-duration curves were constructed from threshold measurements obtained with unipolar anodal and cathodal stimuli at various temperatures. Stimuli ranging in duration from .1 to 50 msec. were delivered during diastole via an electrode sutured to the left ventricular surface. Thresholds were measured at 37°, 29°, 23° and 19°C. Plotting thresholds against reciprocal of stimulus duration it was possible to transform the strength-duration curves into straight lines. All lines demonstrated a progressive increase in slope with low temperatures, the increase being greater for the anodal strength-duration lines. The changes in chronaxie were secondary to the slight changes in rheobase and the more important increases in slope of the strength-duration lines. It is concluded that cold causes a decrease in excitability to stimuli of short duration, this effect being greater for anodal than for cathodal stimuli.