Fetal blood volume responses to intravenous saline solution and dextran

The purpose of this study was to determine to what extent the volume of blood circulating in the fetus can be increased by intravascular infusions of either isotonic saline solution or high-molecular weight dextran in saline solution. Unanesthetized, chronically catheterized fetal sheep that averaged 130 days' gestation were studied. In seven fetuses, 50 to 100 ml of warm saline solution (i.e., an average of 19.5% of the initial fetal blood volume) was infused into a vein over 5 minutes, and changes in blood volume were determined 10, 20, and 30 minutes after the infusion was begun. At these times, an average of only 15%, 9%, and 7%, respectively, of the infused volume remained in the fetal circulation. A second infusion of saline solution into these fetuses gave intravascular retentions that averaged 17%, 6%, and 6% at 10, 20, and 30 minutes, respectively. In five fetuses, a 6% solution of dextran 70 that averaged 14.9% of the initial fetal blood volume was infused over 5 minutes. The intravascular retentions averaged 80%, 63%, and 55%, respectively, at 10, 20, and 30 minutes after the infusions were started. Under comparable conditions in the adult, blood volume expansion 30 minutes after rapid infusions averages 20% to 40% of the infused volume for saline solution and 150% to 200% for 6% dextran 70. Thus, the present study suggests that intravascular infusions of crystalloid solutions have very little effect on fetal blood volume, whereas oncotic solutions cause only moderate blood volume expansion.