Circulatory dynamics during periodic intracranial hypertension in fetal sheep

The human fetal head is periodically compressed during labor. The resulting increase in intracranial pressure (ICP) may exceed the hydrostatic increase in mean arterial pressure (MAP), thereby decreasing cerebral perfusion pressure (CPP). We determined whether the cardiovascular system of near-term fetal sheep is capable of rapidly increasing MAP during periodic increases in ICP. In 12 chronically instrumented fetuses, we produced sinusoidal oscillations in ICP with a maximum of 52 +/- 1 mmHg (baseline MAP) and a minimum of 4 +/- 1 mmHg at a 3-min periodicity by ventricular fluid infusion and withdrawal. Phasic increases in MAP and decreases in electromagnetically determined renal blood flow tracked behind ICP by 0.3-0.5 min. By the sixth cycle, tonic peripheral vasoconstriction that occurred attenuated by the reduction in CPP during subsequent ICP oscillations. By the 10th cycle, plasma catecholamines and vasopressin increased 20-fold. To more closely simulate the pattern during labor, we produced an ICP triangular pulse train with 5-min periodicity and pulse duration of 1.5 min in six other fetuses. The MAP response was nearly out of phase with this more rapid rise of ICP. Thus the phasic component of the fetal pressor response is inadequate for maintaining CPP when ICP is increased to baseline MAP in less than 0.75 min. However, when the ICP pulse duration and frequency are sufficiently high, a tonic pressor response that may be humorally mediated acts to minimize transient cerebral ischemia.