Effect of calcium channel blocker nifedipine on uterine artery flow velocity waveforms.

Calcium channel blockers have been used successfully in the treatment of pregnancy hypertension and premature labor. Only limited information related to their effect on uterine blood flow during pregnancy is available. In this study we measured the ratio between peak systolic to end-diastolic flow velocity (S/D ratio) in the ascending branch of the uterine artery in nine pregnant patients prior to and following a 10-mg dose of sublingual nifedipine. Another group of seven matched patients who received a placebo served as a control group. All studies were performed between 17 and 22 weeks gestation. The S/D ratio decreased shortly after sublingual nifedipine, but the change was not significant. It increased gradually afterward so that by 60 minutes it returned to the control value. The mean systolic blood pressure decreased by 8.6% to its lowest value after 25 minutes (P less than 0.01). The mean diastolic blood pressure decreased by 15.7% to its lowest value after 35 minutes (P less than 0.002). Maternal heart rate did not change significantly. No significant changes were observed in any of the measured variables in the placebo group. In conclusion, nifedipine does not induce significant changes in uterine arterial resistance in midtrimester and may be considered during pregnancy, providing that large fluctuations in maternal blood pressure are avoided.