Cardiovascular effects of acute hemorrhage in fetal lambs.

The effects of acute hemorrhage were studied in two groups each with six fetal lambs (100-116 amd 128-147 days gestation) 3-4 days after we implanted catheters. Fetal blood pressures, heart rate, arterial blood gases and pH, and combined ventricular output and its distribution (radionuclide-labeled microsphere technique) were measured before and 5 min after removal of 15% of fetal-placental blood volume measured by 125I-albumin dilution. Because there were no differences in responses in the two age groups, the data were pooled. Fetal arterial mean pressure fell significantly (50.7 +/- 2.5 to 45.5 +/- 2.6 mmHg) as did heart rate (186 +/- 6 to 151 +/- 13 beats/min) and arterial blood pH (7.39 +/- 0.02 to 7.30 +/- 0.02); arterial blood carbon dioxide tension rose (39.7 +/- 29 to 44.1 +/- 4.4). Combined ventricular output fell from 610 +/- 58 to 448 +/- 45 ml . kg-1 . min-1 (P < 0.05). Blood flow to the umbilical-placental circulation, as well as to the fetal body, fell significantly. Blood flow to the kidneys, gastrointestinal tracts, and lungs also fell, but flow to other organs was maintained. Blood volume reduction in the fetus markedly influences blood gas exchange, because it results in a reduction of umbilical-placental blood flow associated with the fall in arterial pressure.