A hand-held calculator program for individualized dosage adjustment of intravenous theophylline in acute asthma

Abstract

Despite general development of drug assay services and increasing interest in pharmacokinetics, proper dosage regimen calculations are not often made in routine clinical practice. This could be due, in part, to unfamiliarity with pharmacokinetic theory, the consequent difficulty of collaboration and the inevitable delay while data are processed on a computer. The present program for a hand-held calculator (TI-59) was written to minimize these problems, and was designed for the use of intravenous theophylline in the management of acute asthma, where there is a need for individualisation of dosage. Calculations are based on the one compartment open model. With 3–4 plasma drug concentrations taken early in the treatment, the program gives a measure of goodness of fit, the elimination rate constant (k_el), the volume of distribution (V_d) and the suggested infusion rate to achieve a given steady-state level. Data from 10 severe acute asthmatic patients were used to test the model and the estimated parameters were: V_d=0.26±0.029 l/kg (mean±SEM) and k_el=0.20±0.045 h⁻¹. The average standard deviation (s) for differences between the model and observations was 0.96±0.21 mg/l, of which at least 0.5 mg/l was due to assay error. In seven patients where the infusion period was extended, the predicted steady state plasma concentration agreed reasonably with that observed (r=0.83, df=5, 0.01<p<0.05).