Clinical use of mivacurium in the cat

Abstract

This study investigated the characteristics of neuromuscular blockade (NMB) produced by mivacurium in the cat. Mivacurium is a short acting nondepolarizing muscle relaxant, which undergoes rapid hydrolysis by plasma cholinesterase and is expected to have a predictable effect of short duration (Maehr et al. 1991; Gyermek et al. 1999). Thirteen cats (seven females, six males) aged 2–14 years, body mass 3.5–5.9 kg, undergoing corneal surgery were anaesthetized with propofol (3–6 mg kg−1 IV) after pre-anesthetic medication IM with an opioid alone or in combination with acepromazine (0.02 mg kg−1) or medetomidine (0.01 mg kg−1). Anaesthesia was maintained with isoflurane (13 cats) or halothane (1 cat) in oxygen (33%) and nitrous oxide (66%) with mechanical ventilation. Cats were monitored using an ECG, side-stream capnography, and a Doppler flow sensor which was positioned on the pedal artery to estimate systolic blood pressure. The peroneal nerve was stimulated with a train-of-four (TOF) and Double Burst (DBS) pattern using a peripheral nerve stimulator, to assess the degree of NMB. Mivacurium IV (0.1 mg kg−1) was administered over 10 seconds. The TOF stimulation was applied every 12 seconds until no twitch was apparent (onset time) and continued until the TOF ratio was considered to be 1. Response to DBS stimulation was then assessed until the ratio was 1. The same procedure was followed after recovery of a DBS ratio of 1, if administration of a further dose of mivacurium (0.05 mg kg−1) was required. Times of onset of NMB, return of the 1st and 4th twitches, TOF and DBS ratio of 1, were recorded, and descriptive analyses performed. The effects of mivacurium on heart rate (HR), systolic blood pressure (SAP) and variations in onset and recovery time of NMB between the first bolus and the subsequent doses were determined using a paired Student t-test. The HR and SAP did not change significantly after administration of mivacurium. The onset time was significantly shorter (p < 0.0001) after administration of the second dose of mivacurium (48 ± 8 seconds) compared to the initial dose (107 ± 17 seconds). The time to return of the first twitch (11 minutes 12 seconds ± 3 minutes 26 seconds) and to DBS 1 (16 minutes 41 seconds ± 4 minutes 21 seconds) were longer after the first dose than after subsequent doses (8 minutes 56 seconds ± 2 minutes 2 seconds and 18 minutes 28 seconds ± 7 minutes 19 seconds, respectively). However, a second dose of mivacurium was administered in only three cats. Reversal of NMB was never required. Mivacurium is a short acting muscle relaxant in the cat, without apparent cumulative effect after repeated dosing. The time to complete recovery in 97.5% of patients after a single bolus is estimated to be 25 minutes (mean ± 2 SD).