Plasma theophylline and caffeine and plasma clearance of theophylline during theophylline treatment in the first year of life.

Plasma concentrations of caffeine and theophylline were simultaneously determined in 17 preterm infants after oral administration of aminophylline for treatment of apnoea. The ratio of caffeine/theophylline concentration in plasma increased during the first 2 weeks of treatment, owing to the longer time required for caffeine than for theophylline to reach steady state. The caffeine/theophylline ratio at steady state was 0.57 +/- 0.03 (mean +/- SE). Thus, caffeine contributed significantly to the total methylxanthine load in the infants. The plasma clearance of theophylline was calculated from the plasma steady state concentrations. In 3 preterm infants treated with oral aminophylline, repeated sampling showed an approximately linear increase in clearance with time from 16.8 +/- 0.4 (mean +/- SE) at a postnatal age of 6-11 days to 30.9 +/- 2.5 ml/kg/h at 64-69 days. In 1 full-term infant treated with oral theophylline from 3.5 to 11.5 months of age, the plasma clearance of theophylline increased in a roughly linear manner, reaching a plateau of about 100 ml/kg/h at 6-7 months of age. This corresponds to the clearance found in 1-4 year old infants.