Role of the pulmonary circulation in the distribution of human fetal cardiac output during the second half of pregnancy

Background By using Doppler echocardiography, we determined the normal distribution of human fetal combined cardiac output (CCO) from the left and right ventricles. We also established weight-indexed pulmonary and systemic vascular resistances (R(Pi) and R(Si), respectively) and changes during the second half of pregnancy. Methods and Results Blood flows at the aortic and pulmonary valve annuli (LVCO and RVCO, respectively), right and left pulmonary arteries (Q(P)), and ductus arteriosus (Q(DA)) were calculated in 63 normal fetuses. Foramen ovale blood flow (Q(FO)=LVCO-Q(P)) was estimated. From 20 to 30 weeks of gestation, the proportion of Q(P) of the CCO increased (from 13% to 25%, P<.001), while the proportion of Q(FO) decreased (from 34% to 18%, P<.001). After 30 weeks, the proportions of Q(P) and Q(FO) were unchanged. At 38 weeks, the proportion of RVCO (60%) was higher (P<.05) than that of LVCO (40%). The proportion of Q(DA) did not change significantly. The correlation between RVCO calculated from blood flow through the pulmonary valve and from Q(DA) and Q(P) was good (r=.97, P<.0001). R(Pi) (P<.001) decreased from 20 to 30 weeks of gestation. From 30 to 38 weeks, R(Pi) increased (P<.0001). R(Si) increased (P<.001) from 20 to 38 weeks. The ratio of R(Pi) to R(Si) decreased (P<.01) from 20 to 30 weeks and later remained unchanged. Conclusions The human fetal pulmonary circulation has an important role in the distribution of cardiac output.