Relationship of bioelectrical impedance to pharmacokinetic parameters of theophylline in healthy males.

Bioelectrical impedance is a reliable, safe, non-invasive and valid method of determining body composition, using measures of resistance and reactance by passing a low voltage alternating current through the body. This study investigates relationships between the pharmacokinetics of theophylline and parameters of bioelectrical impedance in 15 non-smoking, healthy, adult male volunteers. After an overnight fast, subjects received 5 mg/kg of aminophylline intravenously over 30 minutes. Blood samples were obtained serially over a period of 12 hours. Bioelectrical impedance measurements were made in triplicate, using a 4-electrode plethysmograph. Sera were assayed in duplicate by enzyme-mediated immunoassay (coefficient of variation less than 5%), and data were fitted to a non-compartmental regression program. An all-subsets multiple-regression technique was employed to arrive at predictive equations for theophylline clearance (CL) and volume of distribution at steady-state (Vss) using age, height, weight and mean bioelectrical impedance parameters. Equations for Vss and CL revealed p-values of less than 0.001 and coefficients of variation of 8.5 and 13.33% respectively. Although the equations display some degree of colinearity they account for 95 and 86.5% of the variability in Vss and CL respectively, and represent an innovative approach to the estimation of pharmacokinetic parameters.