Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients.

Aminoglycoside pharmacokinetics were determined in 30 normal weight patients and 30 morbidly obese patients (greater than 90% overweight). All had normal renal function and a gram-negative infection (documented by cultures, fever and elevated white blood cell counts) which was treated only with aminoglycoside antibiotics. The normal weight and morbidly obese patients were matched with respect to the following criterion: age, sex, ideal body weight (IBW), serum creatinine, site of infection, and type of aminoglycoside antibiotic (gentamicin, tobramycin, or amikacin). The results were similar for all 3 drugs. Average half-life was 2 h for both the morbidly obese and normal weight patients. The mean volumes of distribution and clearances were significantly larger in the morbidly obese (23.3 l and 135.8 ml/min for gentamicin, 29.9 l and 162.4 ml/min for tobramycin, and 26.8 l and 157.3 ml/min for amikacin) than in normal weight patients (17.0 l and 95.9 ml/min for gentamicin, 18.3 l and 101.3 ml/min for tobramycin, and 18.6 l and 99.2 ml/min for amikacin). As a result of altered aminoglycoside pharmacokinetics, morbidly obese patients required significantly larger mean doses (540 mg/d for gentamicin, 690 mg/d for tobramycin and 1970 mg/d for amikacin) when compared to the normal weight patients (380 mg/d, 420 mg/d and 1420 mg/d, respectively; p less than 0.005) in order to achieve comparable serum concentrations.