Myocardium of hypothermic rats with and without administration of dextran. Electrocardiographic and electron microscopic studies.

Electrocardiographic and electron microscopic alterations in the myocardium were investigated in rats subjected to hypothermia with and without injection of dextran. Twenty-two animals were divided into four groups and studied. The first group of five rats served as the control group. The second group of six rats, which were subjected to total body hypothermia developed arrhythmia (from first degree atrioventricular block to complete heart block) at a mean rectal temperature of 18 degrees C., with prolongation of P,P-R, and QRS duration, as well as a marked separation of intercalated discs, articularly at the level of the fascia adherens. The third group of six rats was subjected to hypothermia and to an injection of dextran. The resulting threshold temperature tthe temperature at which the arrhythmia appeared) was lower (16 degrees C.) than in the preceding group (p less than 0.005), but neither advanced atrioventricular block nor remarkable subcellular structural changes developed. The fourth group of five rats was sacrificed 18 to 24 hours following recovery from hypothermia and, at that time, showed no significant electrocardiographic or electron microscopic alterations.