MATERNAL KINETICS OF MORPHINE DURING LABOR

The disposition of parenterally administered morphine was investigated in 13 nulliparous parturients in comparison with six healthy non-pregnant women of child-bearing age. Morphine was administered intravenously or intramuscularly and repeated venous blood samples were taken up to 360 minutes after the dose, or until delivery. At delivery samples were taken from the umbilical artery and vein. The plasma concentrations of morphine and M3G (morphine-3-glucuroide) were determined. The elimination half-life of morphine was shorter (43 +/- 19 versus 84 +/- 40 min) and the plasma clearance larger (3.4 +/- 1.4 versus 2.0 +/- 0.51/min) in the parturients than in the non-pregnant women. There was no difference in the apparent volume of distribution of morphine between those two groups. The time to peak plasma concentration of M3G was shorter (11 +/- 3 versus 21 +/- 6 min) and the M3G/morphine concentration ratio at 10 minutes higher (6.4 +/- 1.0 versus 3.4 +/- 0.6) in parturients than in non-pregnant women. In all but one infant, three of whom were born within three hours after the dose, no morphine was detectable. The rapid elimination of morphine by the parturients, resulting in only a short period of intrauterine exposure of the fetus to this drug, may be of clinical importance in the choice of obstetric analgesic agent.