Urea nitrogen excretion in critically ill children.

Urinary urea nitrogen (UUN) excretion as an index of both total nitrogen excretion and protein catabolism was assayed in 32 children (aged 2 months to 15 years, median 6 years) (50% mechanically ventilated) during an intensive care unit course of one to ten days (median three days). The daily UUN excretion was 4.38 +/- 2.22 gm/sq m (171 +/- 89 mg/kg) (N = 121 patient days). The average daily UUN excretion (N = 32 children) was well described by a linear regression equation for square meters of body surface area (BSA) (milligrams of UUN = 4,421.5 x BSA; r2 = .903). This linear relationship permitted the valid comparison of both individuals and subgroups despite wide age differences. Excretion data in the mechanically ventilated vs the spontaneously breathing children, and in four diagnostic subgroups (Reye syndrome, seven; sepsis, six; elective surgery, seven; and miscellaneous, 12) were evenly distributed about the regression line for body surface area. Variability in average daily UUN excretion was on individual basis, and was independent of diagnostic or therapeutic subgroup.