Cardiorespiratory responses to graded reductions of uterine blood flow in the sheep fetus.

Pregnant sheep were chronically instrumented with fetal and maternal catheters and an inflatable occluder and electromagnetic flow transducer were placed on the uterine artery. Uterine blood flow was reduced for approximately 15 minutes to 25 percent, 50 percent, or 75 percent of control uterine blood flow. Fetal blood gases, arterial blood pressure, heart rate and regional distribution of blood flow (by radioactive microspheres) were measured. With progressive reduction of uterine blood flow there was an increasing degree of fetal asphyxia, as measured by blood gases and acid base state. At moderate degrees of asphyxia the fetus responded by redistribution of blood flow to certain organs, namely heart, brain, and adrenal gland, thus preserving oxygenation of these organs. During the most severe degree of asphyxia induced by reduction of uterine blood flow to 25 percent of control there is a reduction of fetal blood flow due to generalized vasoconstriction of essentially all organs. We hypothesize that this is due to the inability of the vasodilator mechanisms to sufficiently oppose the vasoconstrictor mechanisms. Also, because the oxygen consumption of the "vital" organs would be decreased this can be described as the stage of decompensation.