In vitro comparison of four tocolytic agents, alone and in combination

Objective: To compare the tocolytic effects or magnesium sulfate, ritodrine, terbutaline, and nifedipine on human myometrial strips. Methods: Myometrial strips were suspended in organ chambers for isotonic measurements and contracted with KCl. Strips from nonpregnant patients were used to obtain concentration-response curves. Myometrial strips from pregnant uteri were then exposed to the molar concentrations causing 50% relaxation in the nonpregnant tissues. Results: In strips from nonpregnant patients, nifedipine was found to be the most potent tocolytic. Using strips from nonlaboring patients, nifedipine caused relaxation similar to ritodrine, and both were more effective than magnesium sulfate or terbutaline. Combinations were more effective than single agents. These agents were found to be equally less effective in myometrial strips from laboring patients. Conclusions: Nifedipine, alone or in combination, relaxes myometrial strips more effectively than the other agents studied. Myometrial strips from laboring patients are more resistant to inhibition, with none of the agents being superior.