Heart rate response of fetal and adult sheep to hemorrhage stress.

The responses of heart rate and blood pressure to hemorrhage of 15% of estimated blood volume were studied in 24 chronically instrumented fetal lambs of gestational ages 81-144 days (0.55-0.98 gestation) and 6 conscious adult nonpregnant ewes. A transient tachycardia of 25 +/- 3 beats/min (+ 15 +/- 2%) was noted in the fetuses after removal of about 5% blood volume and reduction of mean arterial pressure of 2 +/- 1 mmHg (8 +/- 2%). With 15% reduction of blood volume fetal mean arterial pressure fell 12 +/- 1 mmHg (27 +/- 2%), venous pressure fell from 3.5 +/- 0.5 to 2.0 +/- 0.5 mmHg, and heart rate dropped by 39 +/- 8 beats/min (22 +/- 4%). The fetal bradycardia during the hemorrhage period was reversed by atropine administration. In adult ewes, mean arterial pressure decreased 12 +/- 3 mmHg, venous pressure was unchanged but heart rate increased by 25 +/- 6 beats/min (29 +/- 8%). These studies indicate the presence of a functional baroreflex by 81 days (0.55 gestation).