H-00969-2002 R2 Pressure pulses and flow velocities in central veins of the anaesthetized sheep fetus

The pressure drop and pressure pulses in the isthmus of the ductus venosus in fetal sheep have not been measured directly and related to flow. In eight acutely anaesthetized fetal sheep, a 3F tip pressure transducer (TP) was inserted from the external jugular into the umbilical vein (UV). Ultrasound Doppler flow velocities, TP position and intravenous pressures were recorded in the UV, ductus venosus (DV) and inferior vena cava (IVC) while the TP was withdrawn. Flow was steady in the UV but small pressure fluctuations (< 0.4 mm Hg) could be detected. Time-averaged pressure dropped 1.9 (0.5-3.3) mm Hg (mean, 95% confidence interval) across the DV isthmus. Pressure pulses increased from 1.7 (1.2-2.1) mm Hg in the DV to 3.9 (1.8-6.0) mm Hg in the IVC. The pressure wave from the heart arrived later (0.053 sec [0.025 -0.080]) in the isthmus of the DV than in the diaphragmatic IVC indicating a wave velocity of approximately 1.1 m sec. At all locations, pressures and flow velocities were inversely related.