PHARMACOKINETICS AND PHARMACODYNAMICS OF VECURONIUM IN THE OBESE SURGICAL PATIENT

The effect of obesity on the disposition and action of vecuronium was studied in 14 surgical patients. After induction of anesthesia with th iopental 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 vecuro nium. Plasma arterial concentrations of muscle relaxant were determine d 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, neuromu scular blockade was assessed by stimulation of the ulnar nerve and qua ntification of thumb adductor response. Times to 50% recovery of twitc h were longer in the obese than in the control patients (75 +/- 8 vers us 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 b oth groups. When the data were calculated on the basis of ideal body w eight (IBW) for obese and control patients, total volume of distributi on (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 gr oups. Only when total volume of distribution and clearance are divided by patient weight (a larger value for the obese) and expressed per ki logram of actual body weight do these values appear smaller in the obe se (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 d istribution or elimination of vecuronium, the prolonged action seen at 0.1 mg/kg is due to an overdose when vecuronium is administered on th e basis of total body weight. Clinically, ideal body weight should be used for dose calculation in the obese patient.