Effect of stimulatory and inhibitory drugs on uterine electrical activity measured noninvasively from the abdominal surface of pregnant rats

OBJECTIVE: Our purposes were (1) to identify and analyze parameters of uter ine electrical activity that change during active term and preterm labor in response to stimulatory (oxytocin) or inhibitory (terbutaline) agents and (2) to correlate the information obtained from abdominal surface measuremen t of electrical activity with intrauterine pressure and with the electrical activity measured directly from the uterine surface in vivo. STUDY DESIGN: Electromyographic activity was acquired simultaneously from t he uterine wall and the abdominal surface by means of unipolar electrodes. Electromyographic activity was recorded in the 0.3 to 50-Hz range and digit ized at 200 samples per second. Intrauterine pressure was measured via an i ntrauterine catheter. The effect of cumulative doses of oxytocin and terbut aline on power density spectrum, amplitude, number and duration of electrom yographic bursts, and intrauterine pressure was recorded in anesthetized ra ts during spontaneous active term labor (n = 7) and induced preterm labor ( n = 6). RESULTS: Bursts of electromyographic activity recorded from the abdominal s urface mirrored those from the uterine wall, albeit at a lower amplitude. D uring active term labor, lower concentrations of oxytocin did not significa ntly affect power-density-spectrum energy, amplitude, or number of bursts p er unit time. The duration of electromyographic bursts increased dose depen dently. Myometrial contractions were phasic, with return to the baseline be tween phases. As the concentration of oxytocin increased, the energy, ampli tude, and number of bursts per unit time declined while the intrauterine pr essure continued to rise until the contraction became tetanic, without retu rn to the baseline. In rats with induced preterm labor, terbutaline inhibit ed uterine contractility by decreasing the intrauterine pressure. This was accompanied by a progressive decrease in the power density spectrum, amplit ude, number, and duration of the uterine wall and abdominal surface electri cal bursts. CONCLUSIONS: First, uterine electromyographic activity measured noninvasive ly from the abdominal surface reflects changes in uterine electrical activi ty and intrauterine pressure measured directly and invasively in term and p reterm labor, as well as during treatments to stimulate or inhibit labor. S econd, this noninvasive method may be useful in monitoring uterine activity in vivo. Third, clinical studies to evaluate this technology in human subj ects are warranted.