Pharmacokinetics and Pharmacodynamics of Vecuronium in the Obese Surgical Patient


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Abstract

The effect of obesity on the disposition and action of vecuronium was studied in 14 surgical patients. After induction of anesthesia with thiopental and maintenance of anesthesia by inhalation of nitrous oxide and halothane, seven obese patients (93.4 ± 13.9 kg, 166% ± 30% of ideal body weight, mean ± SD) and seven control patients (60.9 ± 12.3 kg, 93% ± 6% of ideal body weight) received 0.1 mg/kg of vecuronium. Plasma arterial concentrations of muscle relaxant were determined at 1, 3, 5, 10, 15, 20, 30, 45, 60, 90, 120, 150, 180, 210, 240, 300, and 360 min by a spectrofluorometric method. Simultaneously, neuromuscular blockade was assessed by stimulation of the ulnar nerve and quantification of thumb adductor response. Times to 50% recovery of twitch were longer in the obese than in the control patients (75 ± 8 versus 46 ± 8 min) as were 5%-25% recovery times (14.9 ± 4.0 versus 10.0 ± 1.7 min) and 25%-75% recovery times (38.4 ± 13.8 versus 16.7 ± 10.3 min). However, vecuronium pharmacokinetics were similar for both groups. When the data were calculated on the basis of ideal body weight (IBW) for obese and control patients, total volume of distribution (791 ± 303 versus 919 ± 360 mL/kg IBW), plasma clearance (4.65 ± 0.89 versus 5.02 ± 1.13 mL-min−1·kg IBW−1), and elimination half-life (119 ± 43 versus 133 ± 57 min) were not different between groups. Only when total volume of distribution and clearance are divided by patient weight (a larger value for the obese) and expressed per kilogram of actual body weight do these values appear smaller in the obese (473 ± 142 versus 993 ± 401 mL/kg and 2.83 ± 0.54 versus 5.36 ± 1.14 mL·min−1·kg−1, respectively). As obesity did not alter the distribution or elimination of vecuronium, the prolonged action seen at 0.1 mg/kg is due to an overdose when vecuronium is administered on the basis of total body weight. Clinically, ideal body weight should be used for dose calculation in the obese patient.

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