Plasma and urinary kinetics of furosemide in newborn infants

Plasma and urinary furosemide kinetics were assayed by high-power liquid chromatography in six newborn infants receiving furosemide (1 mg/kg body weight IV) for the treatment of fluid overload. Mean +/- SD for plasma half-life, apparent volume of distribution, and plasma clearance were, respectively, 9.5 +/- 4.4 hours, 173 +/- 28 ml/kg, and 15.3 +/- 8.4 ml/hr/kg. There was close correspondence between plasma and urinary half-lives and between plasma clearance and renal clearance. In the first 24 hours, mean estimated urinary recovery of unchanged furosemide was 90% of the injected dose (range 61% to 106%). The results suggest that in the newborn infant furosemide is virtually all excreted unchanged in the urine and that the absence of significant nonrenal elimination, together with the immaturity of neonatal renal function, accounts for its prolonged half-life in newborn infants.