The effect of drugs on uterine activity

Uterine activity was measured for a 30-minute preinjection period and a 30-minute postinjection period for six drugs commonly used in labor. Uterine activity was quantitated by measuring the area below the uterine pressure curve with an on-line technic utilizing a voltage control oscillator. Uterine activity readings were obtained every 2.5 minutes and reported in uterine activity units (UAU). Unmedicated labor demonstrated a relatively static increase in UAU per unit of time in labor. On comparing total UAU before and after injection, only magnesium sulfate was noted to cause a decrease in total UAU. Utilizing 10-minute summations of UAU, the pre- and postinjection periods were compared to the calculated expected UAU for the 30 minutes following the preinjection period (assuming no drug had been used). For each of the other drugs, the positive slope of the calculated expected UAU shifted to a negative slope of the actual observed UAU following drug administration.