Cardiovascular and shivering responses in hypothermic and rewarmed young calves.

Aortic blood pressure, ECG, electromyogram, and heart rate were recorded in cold-stressed and rewarmed young Holstein bull calves. The calves were anesthetized and then cold-stressed by immersion in cold water until their core body temperature (colonic) was lowered 10 C. Hypothermia was continued for 1 additional hour and then the calves were rewarmed by 3 external rewarming methods or were allowed to recover naturally (unassisted). Aortic blood pressure began to decrease in cold-stressed calves by the time their core body temperature had decreased 2 C and continued to decrease during cooling. Heart rate initially increased then decreased with cooling. Blunting of the systolic blood pressure peaks and appearance of extraneous waveforms that obscured the normal component waveforms of the ECG complex were also observed during cooling. Aortic blood pressure and heart rate of cold-stressed calves increased soon after the start of recovery and eventually returned to base line even though the rate of recovery varied depending on the method of rewarming. The component waveforms of the ECG complex became more discernible as rewarming of the cold-stressed calves progressed.