Absence of fetal placental waterfall phenomenon in chronically prepared fetal lambs

An electromagnetic flow sensor was placed on the distal fetal aorta (umbilical flow fraction 78.1±1.6 SEM, %), an inflatable occluder was tied around the umbilical cord, and catheters were placed in distal branches of an umbilical artery and vein, a uterine vein, and in the amniotic cavity. An inflatable occluder was tied around the vagina of some of the ewes. Control values 3 days after surgery were (mean±SE): fetal femoral artery pH, 7.37±0.01; umbilical blood flow, 186±14 ml.min^-1.kg fetus^-1; fetal arterial blood pressure, 39±3 mmHg; and umbilical venous pressure, 7.4±1.0 mmHg (above intrauterine pressure). Fetal weight at autopsy was 3.1±0.3 kg, n=11. Small increases in umbilical vein pressure caused immediate decreases in placental blood flow without decreases in fetal arterial blood pressure. The relation between venous pressure and umbilical blood flow at constant arterial pressure was that of an inert system, i.e., no evidence of a surrounding pressure (Starling resistor effect or waterfall phenomenon) could be found with increases in venous pressures of 2 to 40 mmHg. The results were not affected by increases in uterine vein pressure between 2 and 30 mmHg, nor by anesthesia and supine position of the ewe, nor by ganglionic blockade of the fetal ANS. It was concluded that surrounding pressures in the fetal placental circulation could not be demonstrated.