Inferior vena cava diameter pulse waveforms in the human fetus: relationship with flow velocity waveform

Our purpose was to evaluate the hemodynamic significance of fetal inferior vena cava (IVC) flow velocity waveform indices during fetal development in relation to the diameter pulse waveform. Doppler ultrasound and a phase locked loop echo tracking system were used to measure flow velocity waveform and diameter pulse waveform, respectively. Twenty-seven normal singleton pregnancies were examined from 20 weeks until term at 4-week intervals. The diameter pulse waveform consisted of four waves (A, X, V, and Y waves). The A value (end-diastolic diameter) was associated with the end-diastolic pressure, which causes reverse flow during right atrial contraction. In normal fetuses, a weight-related lower end-diastolic diameter suggested that the end-diastolic pressure was decreased. Except for a positive correlation between the percent reverse flow during atrial contraction and the A value per unit fetal weight, no correlations were found between diameter waveform indices and blood flow velocity waveform indices. The peak velocity index of the velocity waveform significantly correlated with the umbilical artery flow velocity waveform systolic/diastolic ratio. The fetal IVC velocity waveform indices did not yield unequivocal information as to changes in central venous pressure, for which purpose the diameter pulse waveform analysis would seem to be the only available method.