The preejection period of the fetal cardiac cycle. II. Maternal common internal iliac artery occlusions.

The preejection period (PEP) of the fetal cardiac cycle was studied in the chronically instrumented fetal lamb. An inflatable cuff occluder was positioned around the maternal common internal iliac artery, the pregnant ewe's main uterine blood-supplying vessel. Occlusion of the maternal common internal iliac artery produced a fall in pO2 and pH and a rise in pCO2. Fetal arterial blood pressure increased and PEP shortened, especially in the longer lasting occlusions. Fetal heart rate exhibited a bradycardia also during this type of occlusion. The fetal lamb's cardiovascular adjustments to biochemical changes caused by occlusion of the maternal uterine blood supply appear to be determined during the initial phase of occlusion by a chemoreceptor reflex-induced peripheral vasoconstriction mediated by the adrenergic nervous system. The condition is comparable to the one observed during the second part of the umbilical cord occlusions. The interaction between a supersensitive fetal myocardium and the adrenal release of catecholamines may play a dominant role during the second part of occlusion in the longer lasting experiments. Increase in peripheral resistance in the course of redistribution of blood flow during hypoxemia induces a prolongation of the PEP in the early phase of occlusion; overriding of this effect by an increase in myocardial contractility due to rising levels of adrenal catecholamines abbreviates the PEP in the later part of the longer lasting occlusions.