Fetal circulation times and their implications for tissue oxygenation.

In an effort to understand O2 delivery to fetal tissues, we measured circulatory transit times using dye dilution methods in near-term fetuses of 19 ewes. Times from dye injection to peak response (+/-1 sec) were: femoral vein to carotid artery 2.2, to femoral arter 2.6, to umbilical artery 3.7; jugular vein to carotid artery 1.7, to femoral artery 2.6; umbilical vein to carotid artery 1.9, to femoral artery 3.4, to umbilical artery 5.1, and umbilical artery to umbilical vein 5.4 sec. These rapid transits suggest that changes in placenta and peripheral tissues will quickly affect one another. The time for a complete circuit of blood through the fetus was 12.6 (+/-1.0 SEM) sec. Following dye dilunjections into an umbilical vein double peaks, separated by 4.9 sec, were recorded in peripheral arteries (44 observations), and the divergence localized to the ductus venosus. Based on relative areas under the two peaks, ductus flow averaged 0.425 +/- 0.052 SEM of total flow returning from the placenta with the remainder taking a slow course through liver parenchyma.