Fetal blood volume responses to acute fetal hemorrhage.

The purpose of this study was to explore the changes in fetal blood volume that occur after fetal hemorrhage. Fifteen unanesthetized, chronically catheterized fetal sheep averaging 130 +/- 3 (SD) days gestation were studied 4 to 6 (average 5) days after catheter implantation. The fetuses were hemorrhaged by continuous withdrawal from an arterial catheter over 5 minutes. On the average (n = 15), 13.9 +/- 5.3% of the initial volume was removed. Fluid gradually entered the fetal circulation during and after the hemorrhage. Thirty minutes after hemorrhage had been initiated, 53.0 +/- 22.0% restitution of the lost volume occurred. Thus, short-term fetal blood volume restitution after fetal hemorrhage averaged about twice that of the adult. Hemorrhages averaging 7.3 +/- 2.3% (n = 3), 12.9 +/- 1.0% (n = 8), and 21.0 +/- 3.3% (n = 4) were followed by 55.7 +/- 13.3%, 54.7 +/- 26.4%, and 47.7 +/- 19.9% restitution, respectively. Thus, fractional volume replacement appears independent of the shed volume over the range of 5-25% volume loss. The protein concentration in the fluid which entered the fetal vasculature averaged 53% of the plasma protein concentration, suggesting that the fetal interstitium was the primary source of the fluid. In summary, the data suggest that the fetus is able to replace rapidly about half of the volume lost due to rapid hemorrhage, and appears considerably better at controlling its blood volume immediately after rapid hemorrhage than the adult.