Mivacurium for caesarean section in hypertensive parturients receiving magnesium sulphate therapy

The interaction between mivacurium and magnesium sulphate was investigated in a group of parturients undergoing caesarean section under general anaesthesia. Thirty parturients were studied; 10 normotensive controls (group NT), 10 hypertensive controls (group HT) and 10 hypertensives who received magnesium sulphate (group HTM). At induction group HT received 30 microg/kg of alfentanil and group HTM 10 microg/kg of alfentanil and 30 or 60 mg/kg of magnesium sulphate. Neuromuscular function was monitored by electromyography. Mivacurium 0.15 mg/kg was given after 60% recovery of T1 following succinylcholine. Magnesium concentrations and plasma cholinesterase activity were significantly elevated in group HTM (1.57 +/- 0.53 mmol/1 and 4.60 +/- 1.27 kU/1) compared with group HT (0.71 +/- 0.18 mmol/1 and 3.44 +/- 0.97 kU/1) and group NT (0.60 +/- 0.07 mmol/1 and 2.86 +/- 0.82 kU/1) (P < 0.005). Time to maximal recovery, and time from 25-75% of maximal recovery from mivacurium, were significantly prolonged in group HTM (60.9 +/- 15.3 min and 16.8 +/- 5.6 min) compared with group HT (34.9 +/- 7.6 min and 7.6 +/- 3.6 min) and group NT (37.4 +/- 14.4 min and 8.5 +/- 3.4 min) (P < 0.01). Time to 25% recovery was prolonged in group HTM (35.1 +/- 7.4 min) compared with the other two groups (HT: 21.6 +/- 6.4 min and NT: 22.8 +/- 10.2 min) (P < 0.01). Whilst the duration of action of mivacurium, determined by electromyography, is prolonged by subtherapeutic serum magnesium concentrations, of the available non-depolarizing relaxants mivacurium would seem to be most appropriate for caesarean section.