The steady-state concentrations of digoxin at trough levels were studied to establish the role of infant characteristics in estimating the doses for digoxin based on routine therapeutic drug monitoring data. The data (n = 340) which showed a steady-state after repetitive oral administration in 147 hospitalized infants were analyzed using NONMEM, a computer program designed to analyze the pharmacokinetics in study populations by allowing for the pooling of data. An analysis of the pharmacokinetics of digoxin was accomplished using a simple steady-state pharmacokinetic model. The effects of a variety of developmental and demographic factors on the clearance of digoxin were investigated. Estimates generated using NONMEM indicated that the clearance of digoxin (L/hr/kg) was influenced by the demographic variables of age, the daily dose, serum creatinine, the presence or absence of congestive heart failure, and the coadministration of spironolactone in infants. The interindividual variability in the clearance of digoxin was modeled using proportional errors with an estimated coefficient of variation of 30.2%, while the residual variability was 28.2%.
