Abstract
The effect of age on the determinants of the diuretic response to furosemide was evaluated in 22 infants (9 premature infants 2-6 days old and 13 infants 1 to 36 months old). The urinary excretion of furosemide, and therefore the presentation of the drug to its site of action, was significantly slower (p < 0.005) in the premature infants (t½ = 26.5 ± 31.2 h) than in the older children (t½ = 1.79 ± 1.26 h). Despite the prolonged time of drug excretion, the younger infants excreted significantly less of the administered dose in the urine (20.7 ± 17.5% vs. 49.2 ± 22.9%; p < 0.01). All but one infant in each group exhibited an increase in urine flow after furosemide administration and demonstrated a linear relationship between furosemide excretion rate and diuretic response. The increase in urine flow rate per unit furosemide excretion rate was significantly greater in the premature infants (p < 0.001). The results suggest that the immaturity in at least two aspects of renal function contribute to the pattern of diuretic response seen in very young infants. Immaturity of organic ion transport limits the rate and amount of drug reaching its site of action. However, the inability of parts of the tubule not effected by the diuretic to compensate for the increased electrolyte and water loss leads to an enhanced sensitivity to the drug which is excreted.
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