Venous Doppler velocimetry in relationship to central venous pressure and heart rate during hypoxia in the ovine fetus.
Characteristic changes in ductus venosus (DV) blood velocity and pulsations in the umbilical vein (UV) have been described during imminent fetal asphyxia. The aim of this study was to examine fetal venous blood velocity in relationship to pressure gradient across the DV during hypoxia in a fetal lamb preparation. In general anesthesia, a cesarean section was performed on seven pregnant ewes, the fetus was exteriorized and put into a heated waterbath with uninterrupted umbilical circulation. Pressure measurements in the UV and inferior vena cava (IVC) were performed with the catheter tips on both sides of the DV. Fetal hypoxemia was induced by giving the ewe 12% oxygen in inhaling air. Pressure across the DV and Doppler velocimetry were repeatedly measured during hypoxemia. Blood velocity was recorded in the DV and UV by Doppler ultrasound. Before hypoxia the median pressure gradient across the DV was in systole 1 mmHg and 0.31 mmHg in end-diastole and during hypoxemia 1.5 mmHg and zero, respectively. The pressure difference across the DV was constant during hypoxemia irrespective of the presence of umbilical venous pulsations or heart rate. IVC-pressure was greatly influenced by fetal heart rate (FHR). A small but linear fall in systolic IVC pressure was seen with increasing FHR. In end-diastole the IVC pressure changed in a parabolic fashion, with increasing pressure during brady- and tachycardia. Pulsations in the UV also showed a parabolic relationship to FHR and central venous pressure. DV end-systolic and end-diastolic blood velocity changed during hypoxemia in direct relationship to FHR and central venous pressure, but without direct relationship to fetal blood gases. The pressure gradient across the DV is constant during hypoxemia. Changes in central and umbilical venous pressure are directly related to FHR. Umbilical venous and DV blood velocity changed in direct relationship to FHR and central venous pressure.