Effect of environmental hypothermia on dorsal aortic blood flow in the chick embryo, stages 18 to 24.

Dorsal aortic blood flow was measured with a pulsed-Doppler meter in Hamburger-Hamilton stage 18, 21, and 24 chick embryos, and stroke volume index was calculated by dividing mean blood flow per minute by heart rate. These parameters were measured at baseline temperature 34.7 degrees C after cooling to 31.1 degrees C and subsequent rewarming to 34.2 degrees C. In stage 21 embryos, after environmental cooling, heart rate decreased from 170 bpm to 118 bpm (P less than 0.01), mean dorsal aortic blood flow decreased from 0.38 mm3/sec to 0.24 mm3/sec (P less than 0.01) but stroke volume index did not change [baseline, 0.13 mm3/beat; after cooling, 0.12 mm3/beat; after rewarming, 0.15 mm3/beat (P = N.S.)]. Similar results were observed in stage 18 and 24 embryos. The bradycardic response to environmental hypothermia was independent of functional autonomic innervation and probably mediated by a direct suppression of cell action potential dv/dt. Myocardial cell function was not adversely affected by an acute change in environmental temperature as the index of stroke volume was not altered, and all parameters returned rapidly to baseline with rewarming.