Fetal response to cardiopulmonary bypass.

The fetal heart rate (FHR) was continuously monitored by ultrasonography during a mitral valve replacement in a patient at 22 weeks' gestation. The changes in FHR were analyzed in relationship to events of cardiopulmonary bypass. The baseline FHR dropped substantially during bypass with initial loss of variability. After the pump run, a brief episode of fetal tachycardia was followed by stabilization to preoperative levels. The patient delivered at 30 weeks' gestation and the infant did well.