Maternal placental vascular compliance in rabbits

We studied compliance on the maternal side of the placenta of 20 New Zealand white rabbits, using 51Cr and 125I labels to determine erythrocyte, plasma, and whole-blood volumes per gram of placental tissue under varying maternal pressure conditions. At normal maternal arterial (Pa) and venous (Pv) pressures of 71.8 and 5.5 mmHg, placental blood volume (mean +/- SE) was 0.447 +/- 0.051 ml/g placental tissue. When venous pressure was raised (Pa = 45.5, Pv = 12.2) by occluding the inferior vena cava, blood volume increased to 0.729 +/- 0.068 ml/g, a significant 63% rise. However, when arterial pressure was lowered by occluding the aorta in two steps, dropping to Pa = 33.8, Pv = 7.0, and Pa = 13.5, Pv = 5.4, volume did not decrease significantly. We estimated intervillous space pressure (Pivs) from arterial and venous pressures assuming a ratio of venous to total resistance of 0.02. Compliance calculated from the slope of Pivs vs. volume was 0.0471 ml/mmHg per g. Maternal placental hematocrit averaged 27%, appreciably less than the circulating hematocrit of 38%. Overall, the results suggest that placental volume would be maintained during hypotension and would increase when venous pressure is elevated.