Serial measurement of plasma arginine vasopressin in the newborn.

Plasma arginine vasopressin was measured serially in 11 preterm infants and one asphyxiated, term infant. The samples were taken through umbilical arterial catheters placed at birth for blood gas measurements because ventilatory support was required. Plasma samples of 200 to 300 microliter were taken 4 to 12 hourly for up to 100 hours after birth. Plasma arginine vasopressin was measured by a cytochemical bioassay. A significant correlation between plasma arginine vasopressin and plasma osmolality was found in only two out of six infants. The normal 'resting' concentration was between 0.5 and 2 pg/ml. In five infants, apparent bursts of arginine vasopressin secretion were seen, and in one spontaneously breathing infant with respiratory distress, continuously high concentrations (12 to 25 pg/ml) were recorded over the first 100 hours.