Uneven Distribution of Maternal and Fetal Placental Blood Flow, as Demonstrated Using Macroaggregates, and Its Response to Hypoxia*

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Abstract
A technique is described for studying the distribution of blood flow to the maternal and fetal placental vessels in sheep and dogs with radioactive labeled macroaggregates of albumin. When the maternal animal breathed room air the distribution of maternal placental blood flow was uneven among the cotyledons as well as within a given cotyledon. Fetal blood flow was also distributed nonuniformly among and within the cotyledons. The relation of maternal to fetal placental blood flow was also markedly uneven (coefficient of correlation, τ = 0.066). After the animal was made hypoxic by breathing 10-12% O2 the distribution of maternal, fetal, and maternal/fetal placental flows became more uniform. The coefficient of correlation of maternal to fetal flow was high (τ = 0.53, P < 0.01). While the maternal animal breathed room air, after ligation of a major branch of the umbilical artery the distribution of maternal, fetal, and maternal/fetal flows in the remaining two-thirds to three-fourths of the placenta became more uniform. The correlation coefficient for maternal to fetal flow was high (τ = 0.35, P < 0.01). It appears that under normal circumstances with uneven distribution of blood flows there is a considerable portion of the placenta that does not receive blood flow in optimum quantities to promote efficient O2 exchange. Failure to consider the influence of nonuniform maternal flow/fetal flow will result in overestimation of mean maternal-fetal oxygen tension gradients, and thus underestimation of the placental diffusing capacity for oxygen. In response to maternal hypoxia or compromise of the fetal placental circulation the distribution of maternal, fetal, and maternal/fetal flows becomes more uniform, thereby increasing the efficiency of placental O2 exchange.