Assumption that uterine blood redistributed into maternal circulation after delivery challenged

In an examination comparing the effects of two drugs on blood pressure, pulse, heart rate and cardiac output in women having elective delivery by cesarean section, an old assumption that uterine blood is redistributed into the maternal circulation after delivery was challenged, according to a study in the September issue of Anesthesiology.

"Little is known about changes to the mother's hemodynamics following delivery by cesarean section," said Leiv Arne Rosseland, M.D., Ph.D., professor, Faculty of Medicine, Institute of Clinical Medicine, University of Oslo and Department of Anesthesiology, Division of Emergencies and Critical Care, Oslo University Hospital, Norway. "We began this study to determine the side effects of oxytocin and carbetocin on a mother's hemodynamics after delivery by cesarean section, and ended up questioning the assumption that uterine blood transfuses into the maternal circulation."

The study included 76 patients divided into three groups: the first received oxytocin, commonly used to contract the uterus and reduce bleeding, the second carbetocin, a synthetic analog of oxytocin, and the placebo group sterile saline. The drugs or saline were administered to the mother just as the baby's head and shoulders were delivered. The mother's arterial blood pressure, heart rate, stroke volume, cardiac output and systemic vascular resistance were recorded in beat-by-beat measurements.

The study found that oxytocin and carbetocin caused comparable transient decreases in the mother's blood pressure. In the oxytocin group, the blood pressure decrease was greatest after 80 seconds, while the carbetocin group had the greatest decrease at 63 seconds. However, 2.5 minutes after administration of both drugs, the decrease in blood pressure was nearly undetectable. In all three groups, there were increases in the mother's heart rate, stroke volume and cardiac output. The heart rate elevation after carbetocin lasted slightly longer than after oxytocin, which may be of clinical interest for pregnant women with increased risk of cardiac events, according to the study.

Interestingly, a finding that the placebo group had stable cardiac stroke volume during delivery by cesarean section challenges the theory that blood volume increases in the mother's circulation due to the autotransfusion of uterine blood.

Oxytocin and carbetocin are drugs given to mothers to control postpartum hemorrhage, which is the leading cause of maternal deaths in the United States and throughout the world. The hormone oxytocin and carbetocin, a synthetic version of the hormone with a close structural resemblance and similar effects, can prevent postpartum hemorrhage, but may have dangerous cardiac side effects. The study found that the side effects of these two drugs in the first minutes after delivery by cesarean section are similar. More importantly, it uncovered information from the placebo group that provides new insights into maternal physiology.

Carbetocin is not approved by the U.S. Food and Drug Administration.