The preejection period of the fetal cardiac cycle. I. Umbilical cord occlusions

The preejection period (PEP) of the fetal cardiac cycle was studied in the chronically instrumented fetal lamb. A specially constructed device permitted the occlusion of either both umbilical veins, or of both umbilical arteries, or of the total umbilical cord. A prolongation of the PEP together with an increase in blood pressure was a consistent finding during any type of cord occlusion. In poorly oxygenated fetal lambs a two-step increase in the duration of the PEP was observed during the occlusion. It was accompanied by a simultaneous two-step increase in fetal arterial blood pressure. A correlation was found between the steady-state fetal p O 2 before the occlusion and the increase in fetal arterial diastolic blood pressure during the blockade of the umbilical circulation. During occlusion the main determining factors of the PEP prolongation appear to be the increase in afterload and the decrease in preload of the fetal heart. The second step in the prolongation of the PEP during the occlusions in poorly oxygenated fetal lambs is attributed to a chemoreceptor-induced reflex, resulting in a redistribution of cardiac output in the fetal lamb.