Fetal and neonatal cardiopulmonary response to histamine.

Effects of intravenous histamine and its H1 receptor blocker (Benadryl) were investigated in near-term fetal and in newborn lambs. Fetuses were studied before and after closure of the ductus arteriosus. Newborn lambs were chronically instrumented and the same animal was tested periodically from 3 to 70 days old. The results show that a) in the fetus whose pulmonary vascular resistance is already high, histamine produces a profound pulmonary vasodilation; b) in contrast, in the neonate with reduced pulmonary vascular resistance, histamine produces pulmonary vasoconstriction similar to that of the adult; c) both responses can be attenuated by Benadryl indicating that they are mediated by the same receptor; d) in the fetus, histamine produces marked constriction of the ductus arteriosus which could be partly attributed to the pulmonary vasocilatation; and e) when neonatal pulmonary vascular resistance was raised by hypoxia, histamine elicited a biphasic response, part of which was blocked by Benadryl. These findings are discussed in terms of their possible role in cardiovascular and pulmonary changes that occur after birth.