Physiological characteristics of inferior vena cava pulse waveform during foetal development.

Non-invasive measurements of pulsatile diameter changes (pulse waveform) in the foetal inferior vena cava (IVC) during foetal development using an ultrasonic phase-locked echo tracking system with high sampling frequency (3000 Hz) are described. Successful human foetal IVC pulse waveform recording was achieved cross-sectionally in 56 out of 68 (82%) women at 20-23, 24-27, 28-31, 32-35, and 36-39 weeks' gestation. The pulse waveform in the foetal IVC consisted of four waves (A, X, V, and Y waves). An increase in the depth of the X and Y descents was observed during the second trimester. It was considered that this was due to the decrease in placental resistance. A slightly decreased foetal heart rate during the third trimester affected the time for right ventricular diastolic filling. Our findings indicate that the alterations in resistance to right ventricular afterload produced by growth in the placenta and right atrioventricular filling patterns with advancing gestational age are reflected in the foetal IVC pulse waveform. In addition, foetal systemic and placental vascular resistance profoundly influence right atrioventricular filling patterns. It is possible to record the foetal IVC pulse waveform simply, using a quite different ultrasound methodology (phase-locked echo tracking) from that used for normal pulse echo or Doppler measurements. Such studies provide important physiological information about the normal foetus.