[Resistance to vecuronium in burnt patients. Influence of the burnt surface on the effectiveness of the dose 95]

Abstract

Objective: To assess the neuromuscular blocking effect of vecuronium in adult burn patients, to draw dose-response curves, to determine the ED 95 according to burn surface area, to analyze the time course of this pattern in order to recognize the development of a resistance according to the length of postinjury period.

Study design: Prospective open study, extending over a 12 month period.

Patients: Sixty-three consecutive adult burn patients in an acute phase and 13 control patients who had been thermally injured at least 500 days before their inclusion in the study.

Methods: Anaesthesia was achieved with thiopentone, fentanyl and vecuronium in patients undergoing excision and autograft surgery. Neuromuscular blockade was assessed by thumb adduction, measured by electromyography using evoked train of four responses to ulnar nerve stimulation. Dose-response curves were determined using the single dose method from only one predetermined dose of vecuronium per patient on each day of the study. Dose-response curves were compared using linear regression and ED 95 were calculated from log-probit data.

Results: In the control group, ED 95 was 53 mg.kg-1. Before the 7th postinjury day, patients did not differ from controls. Between the 7th and the 70th postinjury day the ED 95 increased significantly. Patients with a burn surface area (BSA) of less than 20% had a ED 95 of 69 mg.kg-1, between 20% and 40% of BSA the ED 95 was 103 mg.kg-1, between 40% and 60% BSA the ED 95 was 134 mg.kg-1 and patients with a BSA over 60% had a ED 95 at 154 mg.kg-1. The onset of action increased in all groups and was significantly different from control group.

Conclusion: Acutely burn patients become resistant to the neuromuscular blocking effect of vecuronium. This resistance is related to the magnitude of burn injury. The mechanism of resistance is related to an increase in nicotonic acetylcholine receptors. In these patients, the dose of vecuronium must be titrated to achieve effective muscular paralysis: the correcting factor is 1.3 for a BSA under 20%, 1.9 for a BSA between 20 and 40%, 2.5 for a BSA between 40 and 60%, and 2.9 for a BSA above 60%.