Summary

Introduction: The influence of the twitch height of the adductor pollicis muscle during baseline measurements on the pharmacodynamic parameters of mivacurium was prospectively evaluated. Patients and methods: Fifty adult patients were anaesthetized with propofol and alfentanil. Neuromuscular function was monitored mechanomyographically by measuring the force of the adductor pollicis muscle following stimulation of the ulnar nerve. Following a stabilization period of 20 min, the individual twitch height of the adductor pollicis muscle was determined before a single bolus of mivacurium (75 μg kg−1) was administered. Patients were divided into two groups. The data of patients whose thumb adduction force was below the median value of all patients were the ‘low force’ group (9.1 ± 1.4 N) and the data of all other patients were the ‘high force’ group (13.7 ± 1.8 N). Results: In the ‘high force’ group, maximum neuromuscular blockade of mivacurium was deeper (0.97 ± 0.05 vs. 0.93 ± 0.06; P < 0.05) and onset faster (2.9 ± 1.1 min vs. 4.0 ± 1.2 min; P < 0.05). Neuromuscular recovery did not differ between the groups. Conclusion: The different onset speeds reflect either different sensitivity to neuromuscular blocking agents with respect to patients' muscle power or a problem of the mechanomyographic measuring technique.

Summary

Background: Short-acting agents for neuromuscular block (NMB) require frequent dosing adjustments for individual patient's needs. In this study, we verified a new closed-loop controller for mivacurium dosing in clinical trials. Methods: Fifteen patients were studied. T1% measured with electromyography was used as input signal for the model-based controller. After induction of propofol/opiate anaesthesia, stabilization of baseline electromyography signal was awaited and a bolus of 0.3 mg kg−1 mivacurium was then administered to facilitate endotracheal intubation. Closed-loop infusion was started thereafter, targeting a neuromuscular block of 90%. Setpoint deviation, the number of manual interventions and surgeon's complaints were recorded. Drug use and its variability between and within patients were evaluated. Results: Median time of closed-loop control for the 11 patients included in the data processing was 135 [89–336] min (median [range]). Four patients had to be excluded because of sensor problems. Mean absolute deviation from setpoint was 1.8 ± 0.9 T1%. Neither manual interventions nor complaints from the surgeons were recorded. Mean necessary mivacurium infusion rate was 7.0 ± 2.2 μg kg−1 min−1. Intrapatient variability of mean infusion rates over 30-min interval showed high differences up to a factor of 1.8 between highest and lowest requirement in the same patient. Conclusions: Neuromuscular block can precisely be controlled with mivacurium using our model-based controller. The amount of mivacurium needed to maintain T1% at defined constant levels differed largely between and within patients. Closed-loop control seems therefore advantageous to automatically maintain neuromuscular block at constant levels.

Summary

Background and objective: Volatile anaesthetics inhibit nicotinic acetylcholine receptors at clinically relevant concentrations with higher affinity for the neuronal nicotinic receptor. The inhibitory effects of propofol on nicotinic receptors have only been documented at supraclinical concentrations. The aim of this study was to determine recovery properties and train-of-four (TOF) fade of mivacurium during sevoflurane and propofol anaesthesia, in order to examine any differences both in the enhancement of the neuromuscular block (postjunctional effects) and in TOF fade (prejunctional effects).

Methods: Twenty ASA I–II adult patients were randomly allocated to maintenance of anaesthesia with sevoflurane (end-tidal concentration 2%) or propofol. Neuromuscular block was assessed by acceleromyography and a single dose of mivacurium (0.15 mg kg−1) was administered (in the sevoflurane group after 30 min of exposure to sevoflurane). We measured time for recovery of the first twitch of the TOF (T1) from 25–75%, time from 25% recovery of T1 to achieving a TOF ratio (TOFR) of 0.8, TOFR at 50%, 75% and 90% recovery of T1, and height of T1 at TOFR of 0.7 and 0.9. Data were tested using t-test for independent samples.

Results: Recovery times (mean (95% confidence interval, CI)) of mivacurium in the sevoflurane group (T1 25–75%, 11.3 (8.1–14.5) min; T1 25%-TOFR0.8, 19.1 (15.7–22.5) min) were significantly longer (P < 0.05) than in the propofol group (T1 25–75%, 6.5 (5.2–7.7) min; T1 25%-TOFR0.8, 11.3 (7.8–10.3) min). No differences were found in the relations between TOFR and T1 or vice versa, between the groups.

Conclusions: Recovery times after a single dose of mivacurium were prolonged by sevoflurane compared with propofol but no differences in TOF fade were observed between the two anaesthetics.