Changes in umbilical venous velocities with physiologic perturbations

OBJECTIVE: The purpose of this study was to determine the direction of transmission of umbilical venous Doppler flow velocity changes in human fetuses. STUDY DESIGN: Strip chart recordings of simultaneously measured umbilical arterial and venous velocities were examined at two sites in the umbilical cord, one near the fetus (proximal) and one near the placenta (distal). Fetuses with venous pulsations or breathing episodes were included. At both locations time from venous pulsation to arterial systole was measured in fetuses with venous pulsations and duration of phase delay between arterial diastolic velocity minimum and venous velocity minimum was measured in fetuses with breathing episodes. RESULTS: In 21 fetuses with venous pulsations the pulsations occurred earlier in the cardiac cycle at proximal sites (0.12 ± 0.04 second before systole) and later at distal sites (0.02 ± 0.04 second before systole; P < .001). Phase delays in venous velocities in the 5 fetuses with breathing episodes were also longer at distal sites than at proximal sites (P < .011). CONCLUSION: Changes in umbilical venous velocities occurred earlier at sites that were closer to the fetus. These findings suggest that changes in umbilical venous velocities originate in the fetal venous system and are transmitted to, rather than from, the placenta.