Neuromuscular and cardiovascular effects of mivacurium chloride in surgical patients receiving nitrous oxide-narcotic or nitrous oxide-isoflurane anaesthesia

The neuromuscular and cardiovascular effects of mivacurium chloride were studied during nitrous oxide-oxygen narcotic (fentanyl) (n = 90) and nitrous oxide-oxygen isoflurane (ISO) anaesthesia (n = 45). In addition, a separate group (n = 9) received succinylcholine during fentanyl anaesthesia to compare its neuromuscular effects with mivacurium. Mivacurium was initially administered as a single bolus in doses from 0.03 mg · kg^-1 to 0.25 mg · kg^-1 to study the dose-response relationships, as well as the cardiovascular effects of mivacurium. Neuromuscular block (NMB) was measured by recording the twitch response of the adductor pollicis muscle following ulnar nerve stimulation (0.15 Hz, 0.2 ms supramaximal voltage). The ED₉₅ values for mivacurium were estimated to be 0.073 mg · kg^-1 and 0.053 mg · kg^-1 in the fentanyl and ISO groups respectively. The duration of block (time from injection to 95 per cent recovery) for a dose of 0.05 mg · kg^{- 1} mivacurium was 15.3 ± 1.0 min and 21.5 ± 1.3 min for fentanyl and ISO anaesthesia, respectively. The recovery index (25-75 per cent) between initial bolus dose (6.1 ± 0.5 min), repeat bolus doses (7.6 ± 0.6 min), mivacurium infusion (6.7 ± 0.7 min) and succinylcholine infusion (6.8 ± 1.8 min) were not significantly different. There was minimal change in mean arterial pressure (MAP) or heart rate (HR) following bolus doses of mivacurium up to 0.15 mg · kg^-1. Bolus administration of 0.20 mg · kg^-1 or 0.25 mg · kg^-1 of mivacurium decreased MAP from 78.2 ± 2.5 to 64.0 ± 3.2 mmHg (range 12-59 per cent of control) (P < 0.05). The same doses when administered slowly over 30 sec produced minimal change in MAP or HR.