Qualitative assessment of maternal uterine and fetal umbilical artery blood flow and resistance in laboring patients by Doppler velocimetry.

The purpose of this study is to evaluate the effect of uterine contractions during labor on both the uterine and the umbilical circulations. Twenty-seven patients in active labor were studied by continuous-wave Doppler velocimetry. Umbilical, left uterine, and right uterine arterial waveforms were obtained before, during, and after peak uterine contractions, and the ratio of maximum systolic and minimum diastolic velocities was calculated. Fifteen patients showed absent flow in end diastole on the uterine artery waveform and had significantly higher intra-amniotic pressures (64.5 +/- 3.5 mm Hg) during the peak amplitude of the uterine contraction compared with the 12 patients with maintained end diastolic flow (46.5 +/- 2.6 mm Hg; p less than 0.05). During the peak amplitude of the uterine contractions the 12 patients maintaining end-diastolic flow had significantly higher systolic/diastolic ratios in the uterine artery (6.5 +/- 1.5) compared with either before or after a contraction (2.1 +/- 0.15 and 2.0 +/- 0.2, respectively; p less than 0.05). Also, these 12 patients showed a linear relationship between the systolic/diastolic ratio and the intrauterine pressure. However, no differences were observed in the umbilical artery systolic/diastolic ratios before, during, or after a contraction in the intensity range studied. On the contrary, during contractions an increase in uterine artery resistance occurs with decreased or absent end-diastolic flow, which bears an inverse linear relationship to the intensity of the contraction. This suggests that during uterine contractions the human fetus continues to have uninterrupted fetoplacental blood flow, whereas the degree of interruption or reduction in uteroplacental blood flow is dependent on the intensity of uterine contraction.