Pharmacodynamic effects of 51W89, an isomer of atracurium, in children during halothane anaesthesia.

51W89 is one of the 10 stereoisomers of atracurium with less propensity to release histamine than atracurium. We evaluated dose-response data and neuromuscular effects of 2 x ED95 dose and maintenance doses of 51W89 during halothane anaesthesia in 68 children, 2-12 yr old. Neuromuscular function was monitored by evoked adductor pollicis EMG. Log-probit, single-dose, dose-response data gave ED50 and ED95 values of 23 and 41 micrograms kg-1, respectively, for 51W89. Twice the ED95 dose (80 micrograms kg-1) had an onset time (time from administration to maximum effect) of 2.5 (SD 0.8) min, a clinical duration (time to 25% EMG recovery) of 31 (7) min and a recovery index (time from 25 to 75% EMG recovery) of 11.1 (1.7) min. Seventy-nine incremental doses of 51W89 of 94 (19) micrograms kg-1. Duration of effect of incremental doses remained constant within individuals reflecting non-cumulative properties. There were insignificant changes in arterial pressure and heart rate after 51W89 and no side effects were observed. We regard 51W89 as a promising, non-cumulative, intermediate-acting neuromuscular blocking agent the effects of which can be antagonized easily by neostigmine.