The effects of age, isoflurane and sevoflurane on atracurium in lambs

Objective To determine the effects of age, sevoflurane and isoflurane on atracurium-induced neuromuscular blockade in 316 week-old lambs. Study design Prospective randomized experimental trial. Animals Twenty-six Scottish blackface ewe-lambs were anaesthetized for spinal surgery when either 36 (mean age 4.6 weeks; n = 18) or 1216 weeks (mean age 13.7 weeks; n = 15) of age; seven animals were anaesthetized at both ages. Methods After intramuscular injection of medetomidine (10 mu g kg-1) anaesthesia was induced in the younger lambs either with isoflurane or sevoflurane in oxygen delivered by mask, and in the older lambs with ketamine (4 mg kg-1), and midazolam (0.2 mg kg-1) administered intravenously (IV). In both groups anaesthesia was maintained with fixed end-tidal concentrations of either sevoflurane (2.8%) or isoflurane (1.8%) delivered in oxygen. Before surgery meloxicam (0.6 mg kg-1), morphine (0.5 mg kg-1) and ketamine (1 mg kg-1 followed by 10 mu g kg-1 minute-1) were administered IV. The lungs were ventilated mechanically to maintain normocapnia. Neuromuscular block was achieved with a loading dose (LD) of atracurium (0.5 mg kg-1 IV). The peroneal nerve was stimulated (train-of-four every 12 seconds). Evoked responses in the digital extensor muscles were evaluated by palpation and observation. Maintenance doses (MD) of atracurium (0.17 mg kg-1 IV) were administered when the first twitch (T1) returned. The onset and duration of LD action (T1 absent) and the duration of MD were recorded. Data were analysed using Students t test, MannWhitney U test, repeatedmeasures anova, Wilcoxons matched pairs test or Pearson correlation coefficient as relevant (p <0.05). Results Onset of LD action developed significantly (p <0.05) more rapidly in isoflurane compared with sevoflurane-anaesthetized lambs (55 +/- 18 cf. 80 +/- 37 seconds). Duration of action of LDs and MDs was longer (p <0.05) in lambs aged 1216 than 36 weeks (33 +/- 5.4 cf. 25 +/- 6.4 and 26 +/- 4.2 cf. 18 +/- 5.5 minutes) but were independent of the anaesthetic used. Conclusions and clinical relevance The effect of atracurium is age-dependent in lambs being prolonged in older animals. The onset of neuromuscular blockade is more rapid in isoflurane compared with sevoflurane-anaesthetized lambs.