In 46 normal full-term infants (0.5-36.5 hours of age) the mean systolic pressure in the lower aorta, catheterized through the umbilical artery, was 70 S.D. ± 8 mm Hg and the mean diastolic pressure 44 S.D. ± 7 mm Hg. The mean venous pressure in the thoracic inferior vena cava, catheterized through the umbilical vein and ductus venosus was 1.4 S.D. ± 2 cm H2O. Higher pressures were recorded in the portal sinus. The mean arterial O2 tension in the lower aorta was 84 S.D. ± 13 mm Hg and the mean CO2 tension 37 S.D. ± 7 mm Hg. The mean rectal temperature was 36.2 S.D. ± 0.7°C.
Arterial pressure tracings recorded during, and in the recovery period following, a 10% reduction in blood volume suggest that vasomoter baroreceptor responses to a reduction in pulse pressure are not very active during the first 2 days of life. Cardiac responses are more active.
In contrast to the adult, passive tipping of the newborn infant into the head-up position caused little change in the inferior vena cava pressure or in the arterial pressure.
An increase in intratracheal pressure of 7-10 cm H2O caused immediate bradycardia. Frequently the heart rate was halved but little fall in mean pressure occurred over 10 seconds.
Breathing hypoxic mixtures caused hypotension and tachycardia. Hypotension was observed with no change in heart rate when the arterial CO2 was raised simultaneously.
Differences in blood volume distribution, and the relative size of the circulatory beds under reflex and chemical control, possibly share with the apparent unresponsiveness of the peripheral vessels in causing the differences in responses observed between the infant and the adult.
Foramen ovale: an ultrasonographic study of its relation to the inferior vena cava, ductus venosus and hepatic veins
