Tissue distribution of caffeine in premature infants and in newborn and adult dogs.

Caffeine concentrations were measured in postmortem tissues of two premature infants who had received caffeine for the prevention of apnea. The distribution of caffeine was not uniform: Up to a 2.1-fold difference in caffeine concentrations existed among the various tissues in one infant and up tu a 4.3-fold difference in the other. This variability was only slightly reduced when caffeine concentrations were corrected for the water content of the respective tissues. Caffeine was also analyzed in tissues of newborn and adult dogs 3, 10, and 36 hr after single doses of caffeine (50 mg/kg IV). The decrease in caffeine concentrations was slower in the puppy than in the adult dog. The ratio tissue water caffeine/plasma water caffeine decreased with time after administration of this drug: It ranged between 0.9 and 1.1 at 3 hr and between 0.5 and 1.0 at 10 hr in adult dogs, and it varied from 0.8 to 1.3 at 3 hr and from 0.5 to 0.8 at 36 hr in puppies. Greater caffeine concentrations in newborn than in adult tissues following a given dose are consistent with the known slow elimination rate of caffeine in the young organism.