A QUANTITATIVE STUDY OF THE ACTION OF SYNTHETIC OXYTOCIN ON THE PREGNANT HUMAN UTERUS

The effect on uterine contractility of synthetic oxytocin given by continuous infusion at the rates of 1, 2, 4, 8, 16 and 32 mU/mm. has been quantitatively studied on pregnant women. The record of the amniotic fluid pressure gives an isometric tracing of uterine contractions. Uterine activity is measured by the product of the intensity multiplied by the frequency of the contractions. The infusion of oxytocin produces an increase of the intensity and of the frequency of the contractions. An exponential relation is found between the oxytocin infusion rate (inf) and the increase (ΔUA) of uterine activity produced by the infusion. ΔUA = (Δ max UA)(1 - e -R inf ). (Δ max UA) is the maximum increase of the uterine activity which should be produced when (inf) increases infinitely; (R) can be considered as a measure of the reactivity of the pregnant woman to oxytocin. The factors (Δ max UA) and (R) are low in early pregnancy, rise as pregnancy progresses and in late pregnancy have fairly constant values for normal cases. In some abnormal conditions they deviate significantly from the normal range and thus may be helpful in evaluating both normal and abnormal uterine functions. In normal pregnant women at term the infusion of oxytocin at the rates of 1 and 8 mU/min. respectively, increases uterine activity to values similar to those recorded at the beginning and at the end of the first stage of normal spontaneous labor; if oxytocin is the hormone controlling the contractions of the pregnant human uterus during normal spontaneous labor, a secretion rate of the same order could be predicted.