The effect of tocolytics on systolic to diastolic (S/D) ratios used to assess downstream vascular resistance and obtained from umbilical and uterine velocity waveforms has not been studied to date. Continuous-wave Doppler studies were performed on the umbilical and uterine arteries of 60 consecutive patients with the diagnosis of preterm labor. Forty patients receiving intravenous magnesium sulfate and 20 patients receiving intravenous ritodrine tocolysis underwent Doppler studies before therapy, during incremental therapy, and four hours post-therapy. There was a statistically significant (P <.001) progressive decrease in the uterine and umbilical S/D ratios along with a concomitant increase in maternal and fetal heart rate associated with increasing infusion doses of ritodrine. Both the S/D ratios and heart rates returned to pretherapy range four hours after discontinuing therapy. Magnesium sulfate tocolysis was not associated with any significant change in S/D ratios or maternal and fetal heart rate. We conclude that intravenous ritodrine affects umbilical and uterine vascular resistance as measured by S/D ratios. This response may be secondary to changes in maternal and fetal heart rate.
(C) 1988 The American College of Obstetricians and Gynecologists