Intramuscular versus intravenous ritodrine hydrochloride for preterm labor management

A prospective, interinstitutional comparative trial was undertaken to examine the efficacy, safety, and pharmacodynamics of different administration routes of ritodrine hydrochloride for the management of preterm labor. Forty-five subjects between 20 and 36 weeks' gestation received either intravenous (n = 24) or intramuscular (n = 21) therapy. Successful tocolysis occurred in 14 of 21 (67%) patients in the group treated intramuscularly and in 16 of 24 (67%) patients in the group treated intravenously. A greater mean dose (8.6 versus 3.3 mg/hour) and a higher mean serum concentration (38.9 versus 24.7 micrograms/ml) were needed to achieve successful tocolysis in the intravenous group as compared with the intramuscular group. Patients who did not respond to tocolytic therapy in both groups had levels of ritodrine in the blood either equivalent to or greater than those of subjects who were successfully treated. Analysis of ritodrine levels in the successfully treated intramuscular group demonstrated significant differences in blood levels depending on muscle group used. These differences can be at least partially attributed to higher mean doses administered to patients receiving vastus lateralis injections as compared with those receiving gluteal muscle injections. The results suggest that intramuscular administration of ritodrine is an efficacious and safe route of drug delivery. Additional studies are needed to better define dose-response curves for the intramuscular administration of ritodrine hydrochloride.