Total blood volume is maintained in nonhydropic fetuses with severe hemolytic anemia

Fetal alloimmune anemia is associated with increased blood flow velocities and cardiomegaly. In severe cases, hydrops can develop. We investigated whether the decrease of red blood cell volume is associated with a reduction or expansion of plasma volume.

In 86 alloimmunized fetuses that received a first intrauterine transfusion, we calculated fetal total blood volumes (i.e. fetoplacental blood volumes) using a dilutional principle of fetal hemoglobin with adult hemoglobin. The relation between total blood volume and estimated fetal weight, severity of anemia and hydrops was analyzed.

Gestational age ranged from 17 to 35 weeks. Mean hemoglobin deficit was 6.8 standard deviations (range 2.1-11.7) below the normal mean. Fetal total blood volume was significantly related to estimated fetal weight (p < 0.001). Mean total blood volume in nonhydropic fetuses was 123 ml/kg (n = 74) and in hydropic fetuses 144 ml/kg (n = 12). There was a significant relation between total blood volume per kg body weight and hydrops (p = 0.035); however, there was no relation with severity of anemia (p = 0.94).

In the human nonhydropic fetus with severe hemolytic anemia, total blood volume is maintained: the decrease in red blood cell volume is thus compensated by an increase in plasma volume. In hydropic fetuses, however, total blood volume seems to be increased. This is in accordance with the hypothesis that congestive heart failure plays a role in the pathophysiology of hydrops in anemic fetuses.