Fetal cardiovascular and endocrine responses to prolonged fetal hemorrhage

Twelve chronically catheterized fetal sheep averaging 131 ± 1 (SE) days gestation (term = 145-150 days) were hemorrhaged an average of 30.8 ± 1.8% of their initial blood volume over 2 h by removing blood at 10-min intervals. During the hemorrhage, fetal blood volume decreased by 14.3 ± 1.4%, and arterial pressure (AP), venous pressure (VP), and heart rate (HR) did not change significantly, although fetal plasma renin activity (PRA), arginine vasopressin (AVP), and norepinephrine (NE) were elevated to 1.5-2.5 times their initial values (P < 0.05). Circulating levels of PRA, AVP, and NE began to rise when 5-10, 10-15, and 20-30%, respectively, of the initial blood volume was removed. Three to five hours after the hemorrhage, blood volume had returned to normal, AP was reduced by an average of 6 mmHg, VP was unchanged, and HR was elevated by an average of 20 beat/min; PRA, AVP, and NE averaged two to three times control (P < 0.05). Twenty-two hours after the hemorrhage, blood volume was 5.4 ± 2.4% above control; AP and HR returned toward normal; VP was elevated by an average of 2 mmHg; PRA and NE levels remained elevated (P < 0.05), but AVP was not different from control. Plasma concentrations of epinephrine, dopamine, and prolactin showed little change during or after the hemorrhage. Thus these studies indicate that the fetus rapidly returns its blood volume to normal after a substantial loss of blood. In addition, the fetal cardiovascular and endocrine responses to a prolonged fetal hemorrhage of moderate volume are substantially less than those that occur after rapid hemorrhage. There is a dissociation between plasma levels of AVP, NE, and PRA and cardiovascular function after hemorrhage in that blood volume, AP, and HR are normal in the presence of elevated circulating hormone concentrations. Multivariate regression analysis indicates that this dissociation appears to be due to the effect of the blood gases on hormone release.