The incidence of maternal and infant morbidity and mortality has led to a continued study into the physiology and pathophysiology of the placenta, the seat of life for the developing fetus. For several decades investigation has been made into the morphology and exchange functions of the placenta and blood flow in the uterus and placenta, but only recently has compliance begun to have been studied. Recognizing the important part it plays in affecting circulation, compliance has been studied in the fetal circulation of the placenta, the fetal placental vascular bed, and the umbilical vasculature, The purpose of the present study was to determine maternal placental vascular compliance.
Through ethical considerations, the study was carried out on 20 anesthetized near-term New Zealand white rabbits, as their placental structure is similar to that of humans. Erythrocytes and plasma were labeled with 51 Cr and 125I in order to determine placental maternal blood volume. Maternal pressure was measured by means of catheters placed in femoral arteries and veins and recorded on a piethysmograph. Adjustable ties were placed around the maternal inferior vena cava and aorta so that pressures could be altered. A fetus with its placenta was snared at four different pressures, the order being randomized: one at normal, one with the aorta fully occluded, one with the aorta partially occluded, and one with the inferior vena cava occluded. Whole placentas were placed in vials for isotope counting by a gamma well counter, and plasma, erythrocyte, and whole blood volumes determined. Intervillous space pressure was estimated from maternal arterial and venous pressures assuming a venous to total resistance ratio of 0.02. The pressure-volume relationship in the placenta was plotted, and compliance was calculated from the slope of the curve.
At normal maternal arterial (Pa) and venous (Pv) pressures of 71.8 and 5.5 mm Hg, placental blood volume was 0.447 (±0.051 SEN!) ml/g placental tissue. When venous pressure was raised (Pa = 45.5, Py= 12.2 mm Hg) by occluding the inferior vena cava, blood volume increased to 0.729 (±0.068 SEM) ml/g, a significant 63 percent 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. Over the intervillous space pressure range from normal to high, placental compliance was 0.0471 ml/mm Hg/g placental tissue. The results suggest that maternal placental volume would be maintained during transient hypotension and would increase when venous pressure is elevated.
A sidelight from the study was that maternal placental hematocrit averaged 27 percent, appreciably less than the circulating hematocrit of 38 percent, possibly due to plasma skimming where erythrocytes are directed preferentially into faster flowing streams. The rise in hematocrit seen when the inferior vena cava was occluded may have been due to increased blood vessel diameter and slower flow with consequently less preferential erythrocyte distribution and plasma skimming. The low placental hematocrit indicated that erythrocytes transit the placenta faster than plasma, a finding that may have implications for gas exchange.
The results reveal the placenta to be a very compliant organ under normal conditions, with a structure that provides for maternal fetal exchange under a variety of pressures. The findings lend support to the nurse in the clinical setting in reassuring the concerned mother-to-be of her body's adaptation and resilience under normal circumstances in providing for the developing fetus. The study lays a foundation for further study into how compliance may be affected in certain disease states and conditions that make pregnancies high risk.
Clarice W. Woodward
Gordon G. Power
Evelyn L. Elwell
Master of Science in Nursing (MSN)
Year Degree Awarded
Date (Title Page)
Library of Congress/MESH Subject Headings
Placenta -- physiology; Maternal-Fetal Exchange
Loma Linda University Libraries
This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.
Brownfield, Gwen Matthews, "Maternal Vascular Compliance of Placentas in Rabbits" (1979). Loma Linda University Electronic Theses, Dissertations & Projects. 651.
Loma Linda University Electronic Theses and Dissertations
Loma Linda University. Del E. Webb Memorial Library. University Archives