[Effect of halothane, enflurane and isoflurane on the pharmacodynamics of mivacurium in children].

INTRODUCTION AND OBJECTIVE

Mivacurium is a new non-depolarising muscle relaxant with a relatively rapid onset and short duration of action. In children, intravenous injection of 0.2 mg/kg produces satisfactory relaxation. Because inhalational anaesthetics have been found to enhance the potency of muscle relaxants we determined if onset or recovery times following mivacurium are influenced by inhalation of halothane (HAL), enflurane (ENF) or isoflurane (ISO).

METHODS

After intramuscular induction, 36 surgical children (2-6 years, ASA I) were randomly assigned to inhale either HAL (n = 12); ENF (n = 12) or ISO (n = 12). The train-of four (TOF) response was determined electromyographically (Relaxograph, Datex) at 20-second time intervals. Following ten minutes of inhalation of either HAL, ENF or ISO (0,8; 1,2; 1,0 vol% respectively) in N2 O/O2 (2:1), 0.2 mg/kg of mivacurium was injected intravenously. Patients were intubated at maximal T1-suppression and the intubating conditions were judged according to a graded score. Upon recovery of T1 = 25%, six patients in each group were antagonised with 30 micrograms/kg of neostigmine and 15 micrograms/kg of atropine; the recovery indices were compared with those from non-antagonised patients. Differences between groups were tested with multifactorial analysis of variance (p < 0.05).

RESULTS

Intubating conditions were graded as "excellent" or "good" in all patients but one who showed moderate breath holding following the tube passage. Onset times of mivacurium were not different between patients receiving HAL: 2.4 min +/- 0.52 (+/ SD); ENF: 2.4 min +/- 0.55 or ISO: 2.6 min +/- 0.68. Time to T1 = 25% was 7.6 min +/- 2.91 (HAL); 7.9 min +/- 1.55 (ENF) and 8.6 min +/- 2.30 (ISO). Recovery indices were not significantly different between groups. Total duration of action in non-antagonised patients was 13.0 min +/- 3.32 (HAL); 14.3 min +/- 4.01 (ENF) and 19.6 min +/- 5.17 (ISO), whereas antagonised duration of action was 13.4 min +/- 5.11 (HAL); 13.3 min +/- 1.97 (ENF) and 15.6 min +/- 4.25 (ISO). The shorter total duration of action in patients receiving neostigmine (average 0.5-2 minutes) was statistically insignificant.

DISCUSSION AND CONCLUSION

Following injection of 0.2 mg/kg of mivacurium, no clinically relevant differences in onset or recovery times were found between children receiving halothane, enflurane or isoflurane. No differences in heart rate or blood pressure were found between groups. Compared to previous investigations with mivacurium, we noted a 30-60% longer mean onset time and a 30% shorter mean spontaneous recovery time. This may be explained by the lower mean age of our patients, which correlates with a relatively higher volume of distribution, resulting in lower plasma concentrations if the dose is calculated per kilogramme body weight. The reduction of the mean recovery time by 2 minutes following neostigmine injection seems to be clinically irrelevant. Similar to adult, a twofold ED95 produces satisfactory surgical muscle relaxation in children receiving mivacurium. Thus, its onset time is comparable to that of vecuronium or atracurium. The shorter duration of action offering a tighter control over relaxation may be of clinical advantage in this age group.