Effects of Hypothermia on Contractility of the Intact Dog Heart

¹ From the Anesthesia Laboratory of the Harvard Medical School at the Massachusetts General Hospital, Boston, Massachusetts

The effects of hypothermia on ventricular contractility were studied in intact dogs, directly, by attachment of a strain gauge arch to the myocardium and indirectly, by observation of left and right atrial, pulmonary and femoral arterial pressures. Marked increments in heart contractile force and reduction in the rates of contraction and relaxation occurred in all experiments during cooling. Myocardial depression occurred in several experiments, but only at temperatures near 20°C, and in the presence of extreme bradycardia. Total sympathetic block produced by epidural injections of 0.45% procaine solution and large doses of hexamethonium did not prevent development of the increments in heart contractile force, but did further decrease the rates of contraction and relaxation at low body temperatures. Epinephrine administration resulted in pronounced increments in heart contractile force and increases in rates of contraction and relaxation, often in the absence of chronotropic effects. No relationship was found between occurrence of ventricular arrhythmias and magnitude of heart contractile force. Spontaneous occurrence of fused contractions, with apparent summation and incomplete tetanus was observed in several experiments at low body temperatures.

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