Observations on the muscle relaxant rocuronium bromide in the horse - a dose-response study

Objective. To investigate the onset and duration of neuromuscular blockade of rocuronium bromide and its associated haemodynamic effects at three doses in healthy horses. Study design. Prospective, randomized experimental study. Animals Seven adult horses aged 3-20 (mean 10.3) years and weighing 466 +/- 44 (mean +/- SD) kg. Methods. Horses were anaesthetized three times with at least 2 weeks between. They were pre-medicated with 0.6 mg kg(-1) xylazine and 0.01 mg kg(-1) butorphanol IV. Anaesthesia was induced with 2.2 mg kg(-1) ketamine and 0.1 mg kg(-1) diazepam IV. Following orotracheal intubation anaesthesia was maintained with isoflurane in 100% oxygen. Intermittent positive pressure ventilation was initiated and the horses were ventilated at a respiratory rate (fr) of 4-8 breaths minute(-1). Neuromuscular function was monitored with an acceleromyograph. The peroneal nerve was stimulated with train-of-four (TOF) mode at 2 Hz every 15 seconds. Each horse received, in randomly assigned order, one of the three doses of rocuronium: 0.2 mg kg(-1) (D02), 0.4 mg kg(-1) (D04) or 0.6 mg kg(-1) (D06) IV. Lag time, onset time, time of no response, duration of action and the TOP ratio 0.7 and 0.9 were measured. Recovery time (T1.(25-75)) was calculated. Vital parameters were recorded at 5-minute intervals on a standard anaesthetic record form. Results Rocuronium produced a dose-dependent duration of action in isoflurane-anaesthetized horses. 100% block was observed in D04 and D06 but not in D02, in which the maximum decrease of the first twitch of TOF attained was 91.5 +/- 16.5%. Time to T1(25) was 13.1 +/- 5.5 minutes, 38.6 +/- 10.1 minutes and 55 +/- 9.8 minutes in D02, D04 and D06 respectively. There was a significantly shorter time for TOFR 0.9 with 0.2 mg kg(-1) compared with 0.4 and 0.6 mg kg(-1) rocuronium. T1(25-75) in D04 and D6 was not statistically significantly different. Heart rate, systolic, diastolic and mean arterial blood pressure increased slightly during the observation period. Conclusion Rocuronium is an effective nondepolarizing muscle relaxant in horses under isoflurane anaesthesia. It had a dose-dependent onset and duration of action. Rocuronium did not produce significant changes in the measured cardiovascular parameters.