Pressure pulses and flow velocities in central veins of the anesthetized sheep fetus

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TP position, and intravenous pressures were recorded in the UV, DV, and inferior vena cava (VC) while the TP was withdrawn.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