A pharmacodynamic basis for the peak effect of vecuronium: peak twitch versus peak tetanic fade.

ABSTRACT

Double burst stimulation (DBS), a tetanic test, shows two types of changes during nondepolarising neuromuscular block (NMB) viz, amplitude (D1) suppression and fading of second response (D2), quantified as DBS ratio (D2/D1). During subclinical dose effect of vecuronium bromide both parameters show peak suppression at two distinct intervals. To evaluate, which of the two is the true peak effect of vecuronium, twenty-two ASA 1 patients were given im buprenorphine (5 micro/kg) premedication and iv diazepam (0.1 mg/kg). Vecuronium bromide (0.015 mg/kg) effect was monitored by stimulating ulnar nerve at the wrist. Adductor pollicis response of supramaximal DBS stimuli was recorded on myograph. DBS ratio was calculated with each DBS stimuli, using pocket calculator. In randomly allocated group 1 (n = 11) patients, repeat dose of vecuronium (0.08 mg/kg) was given at the peak D1 suppression and in group 2 (n = 11) at peak DBS ratio suppression. The onset time of repeat dose of vecuronium monitored by one Hz stimuli, to '0' response in group 2 (37.3 +/- 6.65 second) was significantly (p < 0.01) shorter than in group 1 patients (46.8 +/- 9.3 second). It was noteworthy that at the repeat dose of vecuronium while D1 showed recovery in group 2 patients, DBS ratio was concomitantly and significantly lower (0.37 +/- 0.10) (more intense NMB) than in group 1 (0.49 +/- 0.17) patients, with quicker onset of repeat dose. These findings suggest that as the NMB agents show two types of changes during clinical monitoring, DBS test seems to be a better clinical pharmacodynamics-monitoring test for NMB agents. In addition, the peak tetanic fade (peak DBS ratio suppression) correlated with peak effect of vecuronium than the usually measured peak twitch suppression.