Effects of mivacurium on the diaphragm evaluated by cervical magnetic stimulation of the phrenic nerves

Background and objective: Non-depolarizing neuromuscular blocking agents have differential effects on the diaphragm and skeletal muscles. We employed a new method to study the effects of mivacurium on the diaphragm and compared the results obtained with this method with published data. Methods: Anaesthesia was induced and maintained with propofol and alfentanil and the trachea was intubated after topical anaesthesia. Contractions of the diaphragm were induced by cervical magnetic stimulation of the phrenic nerves and quantified by measuring airway pressure responses. The neuromuscular effects on skeletal muscles were measured by acceleromyography of the adductor pollicis muscle. Mivacurium (0.15 mg kg - 1 ) was injected and neuromuscular responses were recorded until the effects had waned. Results: Eleven male and 10 female patients (ASA I-II; 57 ′ 16 yr; 78 ′ 13 kg; mean ′ standard deviation) participated. Median maximal reduction of twitch response was less (P < 0.05) for the diaphragm (89%) than for the adductor pollicis (100%). Time to 25% recovery was shorter for the diaphragm than for the adductor pollicis (8.8 ′ 2.2 min vs. 22.6 ′ 5.0min, P < 0.05). The difference between the recovery index of the diaphragm (7.3 min (3.6-18.4)) and the adductor pollicis (8.2 min (4.4-20.9) (median (range)) just missed our chosen level of statistical significance (P = 0.06). The recovery time to train-of-four 0.8 was shorter for the diaphragm (median and 95% confidence interval 25.1 ′ 10.2 min) than for the adductor pollicis (median and 95% confidence interval 37.5 ′ 9.4 min, P < 0.05). Conclusions: The duration of the clinical effect of mivacurium on the diaphragm is markedly shorter than on the adductor pollicis muscles but there was only a small difference in the recovery index of the two muscles. These effects and the time courses determined with the new method closely resemble the results obtained with different methods in other studies.