Fetal blood volume restoration following rapid fetal hemorrhage

In a previous study, we found that ovine fetal blood volume returned to normal in 3 h after a slow hemorrhage of 31% over 2 h; volume was slightly elevated at 24-25 h. In the present study, we explored the time required for blood volume restoration in late gestation fetal sheep following a rapid hemorrhage over 10 min. The rate of hemorrhage was constant within each fetus but varied among fetuses from 13.5 to 32.2%. Two fetuses that were hemorrhaged 32% of their initial blood volume over 10 min underwent cardiovascular collapse during the hemorrhage. In 10 fetuses that were hemorrhaged 21.0 ± 1.7% (SE) over 10 min 6.5 h were required for blood volume to return to control. Fetal arterial pressure, venous pressure, and heart rate decreased during and immediately after the hemorrhage and returned to normal within 1 h. Plasma arginine vasopressin (AVP) concentration and plasma renin activity (PRA) underwent large increases following the rapid hemorrhage. Volume restoration at 5-7 h posthemorrhage correlated negatively with PRA and norepinephrine (NE) concentration immediately after the hemorrhage. Three of the 10 fetuses died overnight, and in the remaining seven fetuses blood volume was 8.8 ± 3.3% below control (P<0.01) at 24-25 h posthemorrhage. The fetuses were also hypoxic, acidotic, and had greatly elevated plasma AVP and NE concentrations at this time. We conclude that ovine fetuses are less able to survive a rapid hemorrhage compared with a slow hemorrhage of the same extent. In addition, fetal blood volume restoration is delayed after rapid hemorrhage, and the impaired restoration is to the detriment of the fetus. This may be attributable, at least in part, to the larger increases in the plasma concentration of vasoactive hormones after rapid rather than slow hemorrhage, presumably because the vasoactive hormones constrict the blood vessels to maintain arterial pressure rather than promote blood volume restoration.