Relationship of metabolic acidosis to urinary sodium excretion in the newborn infant.

The relationship between birth weight, postnatal age and acidosis-induced urinary sodium excretion was studied in 43 one-week-old newborn infants with birth weights of 1000-4300 g and gestational age of 28-41 weeks; and in 13 premature infants with birth weights of 1000-1970 g (mean 1620 g) and gestational age of 29-35 weeks (mean, 31.4 weeks) during the first six weeks of life. Developmental changes were found in acidosis-induced urinary sodium excretion. Newborns with higher birth weight and postnatal age excreted significantly more sodium in response to acidosis than their lighter and younger matches. It is suggested that the degree of acidosis must be taken into account when estimating the sodium requirement of newborn infants different in birth weight and postnatal age.