DIRECT RELATIONSHIP OF FETAL CARBOXYHEMOGLOBIN WITH HEMOLYSIS IN ALLOIMMUNIZED PREGNANCIES

Because carbon monoxide (CO) is a byproduct of heme degradation and be cause placental diffusing capacity of CO is limited, we hypothesized t hat the concentration of CO transported in fetal blood as carboxyhemog lobin (HbCO) would correlate with the severity of fetal hemolytic dise ase. Fetal blood was obtained by cordocentesis and HbCO was measured b y gas chromatography. The two primary study groups included control fe tuses (n = 26) and fetuses of Coombs-positive mothers before in utero transfusion (n = 15). Compared with controls, fetuses with hemolytic d isease had higher HbCO levels (0.0111 +/- 0.0014 versus 0.0159 +/- 0.0 072 fraction of total Hb, mean +/- SD, p < 0.002). In contrast, HbCO l evels in simultaneously sampled maternal blood samples were not differ ent in the control and alloimmune groups [0.0110 +/- 0.0025 (n = 20) v ersus 0.0115 +/- 0.0021 (n = 11)]. There was a significant inverse cor relation observed between fetal HbCO and Hb concentrations in the grou p with hemolytic disease (r = -0.73, p < 0.002) but not in controls. I n fetuses with hemolytic disease, HbCO and bilirubin were highly corre lated (r = 0.88, p < 0.0001). Data from four anemic fetuses who were C oombs negative, three of whom had no evidence of hemolysis, indicated normal HbCO and normal plasma bilirubin levels. A fourth fetus with an emia had viral sepsis and elevated HbCO and plasma bilirubin levels. W e conclude that elevated HbCO levels detected in fetuses of nonsmoking mothers with erythrocyte alloimmunization are likely the result of ac celerated hemolysis.