Kinetics of terbutaline in asthmatic children.

Seven asthmatic children (8-12 years) were given terbutaline sulphate intravenously (5.5 micrograms/kg) and orally (50 micrograms/kg) one week apart. Unchanged terbutaline was measured in plasma and urine. In urine, conjugates were also assayed. The intravenous plasma concentration-time curve declined in a multiexponential manner. The terminal half-life ranged from 8.8 to 15.8 (mean 12.1) h. Body clearance (mean +/- SD) was 3.76 +/- 0.86, renal clearance 2.42 +/- 0.49 mL/min/kg. The volume of distribution at steady state was 1.57 +/- 0.19 L/kg. The extrapolated recovery of intact terbutaline in urine was 65.4 +/- 6.5% of the dose and the total recovery 80.8 +/- 8.4%. After oral administration, the recovery of intact terbutaline in urine was 6.2 +/- 1.1%. Absorption was on average 33%, but because of a mean first-pass elimination of 70%, bioavailability was 9.5 +/- 2.4%. It seems that children as a group have shorter terminal half-lives than adults and slightly higher weight-corrected clearances.