Effects of acute hypoxemia on umbilical arterial Doppler indices in a fetal ovine model.

Changes in umbilical arterial Doppler waveform in response to acute maternal hypoxemia were assessed in a chronic ovine model. Maternal heart rate, blood gases, fetal heart rate and umbilical arterial Doppler indices measured at baseline conditions and during periods of maternal hypoxemia were compared. The indices measured were systolic-diastolic ratio (S/D), pulsatility index (PI) and resistance index (RI). Fetal heart rate decreased (less than 80 beats/min) and umbilical arterial Doppler indices increased (P less than 0.001) during the periods of hypoxemia. Furthermore, the Doppler indices were highly but negatively correlated with alterations in the fetal heart rate (FHR) (P less than 0.001). Analysis of the Doppler waveform phase intervals revealed nearly constant systolic intervals while diastolic intervals varied inversely with the heart rate alterations. Moreover, the umbilical arterial Doppler indices, when corrected for FHR changes, were relatively unchanged from baseline measurements. This study indicates that maternal hypoxemia induced changes in the umbilical arterial Doppler indices may be primarily attributable to the changes in FHR.