Hemodynamic changes in the ewe affecting fetal heart rate in utero

¹ Department of Anatomy, University of Illinois College of Medicine, Chicago, Illinois

Hemodynamic disturbances in pregnant ewes upon the fetus were studied and compared with respiratory anoxia. Vagus section and stimulation in the ewe or injection of humoral mediators produce hemodynamic disturbances in the mother, with fetal bradycardia. Comparison of active or passive congestion of the maternal placenta is accompanied by increase in umbilical vein pressure and/or umbilical artery pressure and fetal bradycardia. These hemodynamic factors on both sides of the placenta are closely interconnected. Disturbed respiration in the ewe is associated with hemodynamic responses of the maternal and fetal circulations. Increased intrauterine pressure is associated with increase in fetal blood pressures and bradycardia. With respiratory anoxia, fetal distress does not appear before hemodynamic disturbances in the ewe. The concept that hemodynamic disturbances in the maternal organism influence the fetus through changes in the uteroplacental circulation is advanced. Passive engorgement of an obstructed venous return or an active congestion resulting from an increased arterial inflow into the uteroplacental circulation affect the fetal part of the placental circulation resulting in bradycardia. The fetal vagus is involved; afferent pathways are not established.