Variation and correlation in human fetal umbilical Doppler velocities with fetal breathing: evidence of the cardiac-placental connection.

Doppler velocity waveforms in the human fetal umbilical vein and artery were analyzed during episodes of fetal breathing. Heart rate, systolic and diastolic velocities were measured from the umbilical artery waveform. Diastolic velocity varied the most with a mean (+/- SD) coefficient of variation of 16.0% +/- 5.0%. The coefficient of variation of systolic velocity was 7.8% +/- 2.4% and of heart rate was 5.0% +/- 1.8%. We also found that umbilical arterial flow was related to umbilical venous flow, implying an interdependence between fetal cardiovascular blood flow and placental blood flow. During breathing, venous flow varies because of changes in intrathoracic pressure in the fetus. This variation in umbilical venous velocity may affect the umbilical arterial diastolic velocity through alterations in placental filling, and may affect the umbilical arterial systolic velocity through alterations in ventricular filling, which by the Frank-Starling mechanism changes stroke volume. The interdependency of umbilical venous and umbilical arterial blood flow velocities must be considered in the interpretation of the significance of umbilical artery Doppler velocity measurements.