DETERMINATION OF GENTAMICIN PHARMACOKINETICS BY BIOELECTRICAL-IMPEDANCE IN CRITICALLY ILL ADULTS

This investigation compares the accuracy of calculating gentamicin pha rmacokinetic parameters by a noninvasive body composition technique (b ioelectrical impedance analysis; BIA) with an empiric method, against the two-point method as the criterion standard. A prospective concurre nt open label design was used. The 32 medical and surgical intensive c are unit beds at Henry Ford Hospital, a not-for-profit, university-aff iliated teaching hospital, served as the setting. Twenty critically il l adults, Therapeutic Index Scoring System (TISS) = 4, who required ge ntamicin as part of their normal course of therapy for gram-negative b acillary infections, were evaluated. Gentamicin V(d) and k were calcul ated by three methods. After measurement of body composition parameter s by BIA, previously derived gentamicin dosing equations were used to predict gentamicin volume of distribution (V(d)) and elimination rate constant (k) (BIA method). Empiric estimates of these parameters (V(d) = 0.3L/kg and k derived from creatinine clearance) were compared with the BIA parameters against a criterion standard V(d) and k determined from a two-point sampling of gentamicin serum concentrations. Measure ments of BIA parameters and gentamicin serum concentrations were made in duplicate with coefficients of variation, less-than-or-equal-to 2% and less-than-or-equal-to 3%, respectively. The BIA and empiric method s produced resultant pharmacokinetic parameters (V(d) and k) not diffe rent than those measured by the two-point method. There were no statis tically significant differences in mean error (bias), or mean squared error (precision) for both V(d) and k assessed by the empiric or BIA m ethods. In this sample of critically ill adults there is no apparent a dvantage of BIA-derived predictive pharmacokinetic equations over an e mpiric method of predicting gentamicin V(d) and k. Further study with variable frequency BIA technology is needed to assess BIA applications in the critically ill.