Doppler study on pulmonary venous flow in the human fetus

The fetal pulmonary circulation is different from the postnatal circulation: the flow is small. The arterial pressure is almost systemic, and the vascular resistance is high. Moreover, lungs are collapsed in the fetus. However, the pulmonary venous flow in the fetus has not been studied. The aims of this study were to look at flow patterns of pulmonary venous system in the fetus and to see whether there are any changes in Doppler parameters with fetal growth.

The pulmonary venous flow velocity was analyzed in 56 normal human fetuses. The fetal gestational age ranged from 17 to 37 weeks. Pulsed-Doppler echocardiography was used to investigate the right upper pulmonary vein in four-chamber view. After the pulmonary venous signal was obtained, by enlarging the sample size, the pulmonary arterial signal was superimposed for timing. Heart rate velocities at systolic peak (S), at diastolic peak (D), at nadir between S and D (O), and at nadir between D and the next S (X) and velocity-time integral were measured.

The pulmonary venous signal showed basically continuous flow from pulmonary vein toward left atrium through all the cardiac cycles, and it had biphasic peaks: one during the systolic and the other during the diastolic phase. The systolic peak of pulmonary vein occurred slightly earlier or later than the systolic upstroke of pulmonary arterial waveforms. The systolic peak of pulmonary vein was higher than the diastolic peak in 45 out of 56 cases. Velocities of systolic and diastolic peaks and velocity-time integral of the pulmonary vein have significantly increased with fetal growth.

In the fetus, the flow pattern of the pulmonary vein was similar to that of the postnatal pattern with the exception of absence of atrial reversal. Lower velocities may reflect a decreased pulmonary flow volume.