Cardiac function in hypothermia

Hypothermia retards cardiac contraction and prolongs the subphases of the cardiac cycle in varying degrees. Six anaesthetized beagle dogs were catheterized and cooled between ice bags until the aortic blood temperature was 25 degrees C and then rewarmed to normothermia. The speed of relaxation decreased to a half from its value in normothermia as indicated by the time constant of exponential isovolumic ventricular pressure fall and by the change in the negative dp/dt. It is suggested that retardation of relaxation is connected with temperature dependent changes in calcium kinetics. Decrease of cardiac output was mediated mainly by decreased stroke volume indicating sympathetic tone in spite of cold narcosis.