The most common drug used to halt preterm labor, ritodrine, is poorly tolerated and causes significant maternal and fetal tachycardia, as well as maternal hypotension. Recently, nitroglycerine (glyceryl trinitrate) has been suggested as a tocolytic agent because it showed no significant side effects in animal studies and limited human studies. In theory, the vasodilating effect of nitroglycerine could cause significant hypotension and secondary tachycardia. Black and colleagues compare intravenous ritodrine with transdermal nitroglycerine in the treatment of preterm labor and report the effects on maternal and fetal cardiovascular function.

Sixty women in preterm labor with intact membranes who were between 24 and 31 weeks, six days of gestation were enrolled in the multicenter study. None of the women was hypotensive at the time of enrollment. Participants were randomly assigned to receive transdermal nitroglycerine or intravenous ritodrine. Participants were closely monitored with fetal heart rate, maternal pulse and maternal blood pressure measurements taken every 15 to 30 minutes during the acute phase of treatment and less frequently if readings were stable.

The 31 women assigned to the ritodrine-treatment group were on average nearly three years older than the 29 women who were assigned to the nitroglycerine-treatment group. Before receiving treatment, both groups were comparable in all other significant aspects, including mean maternal heart rate, fetal heart rate and arterial pressure. The mean change from baseline in maternal heart rate was 21.1 beats per minute lower in the nitroglycerine-treatment group. The mean change from baseline in fetal heart rate was 9.2 beats per minute lower, and the mean fetal heart rate was significantly lower at 6.9 beats per minute in the nitroglycerine-treatment group. No episodes of clinically significant hypotension occurred in either group. Ritodrine had a significant hypotensive effect on mean arterial pressure; however, this was not significantly different over the treatment course.

The authors conclude that at the dosages recommended for treatment of acute tocolysis, intravenous ritodrine showed a more pronounced effect on maternal and fetal cardiovascular systems. Transdermal nitroglycerine showed minimal effects on fetal heart rate, and maternal pulse and blood pressure. The authors suggest that nitroglycerine is a safer and more acceptable drug in the treatment of preterm labor.