Fetal blood volume, vascular pressure, and heart rate responses to fetal and maternal hyperosmolality

Our purpose was to explore the fetal cardiovascular responses to osmotic hydration or dehydration of the fetus. Chronically catheterized pregnant sheep with a single fetus, averaging 130 days gestation (term, 145-150 days), were studied. After an intravenous injection of 20 ml of 9% NaCl into the fetus (n = 6), fetal osmolality increased by a peak of 20 +/- 2 (SE) mosmol/kg and returned to control in 1.5-2 h. Fetal blood volume, mean arterial pressure, venous pressure, and heart rate increased by 17 +/- 4 ml, 5 +/- 1 mmHg, 3 +/- 1 mmHg, and 19 +/- 6 beats/min, respectively, at 2 min postinjection. These variables returned to normal within 20-60 min. After an intravenous injection of 240-300 ml of 9% NaCl into the ewe (n = 7), maternal osmolality increased by a peak of 48 +/- 4 mosmol/kg; fetal blood volume decreased by 36 +/- 6 ml, fetal vascular pressures were unchanged, and fetal heart rate decreased by 43 +/- 9 beats/min at 15 min postinjection. Fetal blood volume and heart rate returned to normal within 1 h even though fetal and maternal osmolalities were elevated by 20-25 mosmol/kg. With transplacental fluid movements in opposite directions following the fetal versus maternal hypertonic injections, these studies show that fetal blood volume is well regulated following osmotic hydration or dehydration, except for short-term transients lasting less than or equal to 1 h. Although fetal vascular pressures increased in parallel with blood volume following osmotic hydration, their lack of change following osmotic dehydration suggests a constriction of the fetal vasculature.(ABSTRACT TRUNCATED AT 250 WORDS)