Normal fetal pulmonary venous blood flow velocity.

Our objectives were to describe the normal pulmonary venous blood flow velocity waveform and to establish reference ranges for the second half of pregnancy in healthy human fetuses. A total of 123 women with uncomplicated pregnancies of between 20 and 40 weeks were examined, using a combined color-coded Doppler and two-dimensional real-time ultrasound system. Pulsed Doppler flow velocity waveforms of pulmonary venous drainage into the left atrium were obtained from a transverse cross section of the fetal chest at the level of the cardiac four-chamber view. All waveforms were obtained during fetal apnea. The success rate in obtaining the pulmonary venous waveform was 81%. The waveform displayed a biphasic forward flow profile with a systolic and diastolic component. Peak systolic, peak diastolic and time-averaged velocities demonstrated a gestational age-related rise, whereas the peak systolic/peak diastolic ratio showed a gestational age-related reduction. The nature of the fetal pulmonary venous flow velocity waveform pattern suggests positive pressure towards the left atrium throughout the cardiac cycle. We speculate that an increase in volume flow and the pulmonary venous pressure gradient play a role in the gestational age-related changes in pulmonary venous flow velocities.