Background and objective: Non-depolarizing neuromuscular blocking agents have a shorter duration of action on the diaphragm than on skeletal muscles. It was to be tested if this also held true for rapacuronium, a shortacting, amidosteroid non-depolarizing neuromuscular blocker, lately withdrawn from the market, using a novel technique for stimulating the diaphragm and assessing its function.

Methods: Anaesthesia was induced with propofol 2 mg kg−1 and remifentanil 1 μg kg−1, and the trachea was intubated after topical anaesthesia. Rapacuronium was given at a dose of 1.5 mg kg−1. The diaphragm was stimulated by cervical magnetic stimulation of the phrenic nerves (2 Tesla, single coil) and airway pressure responses were measured at the endotracheal tube connector. The neuromuscular effects at the adductor pollicis and orbicularis oculi muscles were measured by acceleromyography.

Results: Fifteen males and five females (ASA I and II; 27 ± 8 yr; 73 ± 13 kg; mean ± SD) were recruited. Median maximal relaxation was less (P < 0.01) for the diaphragm (89%) than for the adductor pollicis or orbicularis oculi muscles (each 100%). The time to 25% recovery was shorter for the diaphragm than for adductor pollicis or orbicularis oculi (7.5 ± 3.1 versus 14.1 ± 3.7 and 15.1 ± 3.5 min, respectively, P < 0.01). Recovery from 25 to 75% was identical for the diaphragm and adductor pollicis (9.4 ± 2.9 versus 9.1 ± 3.5 min), but longer for orbicularis oculi (13.4 ± 4.2 min, P < 0.01). The median recovery time to TOF0.8 was shorter for the diaphragm (23.9 min) than for the adductor pollicis or orbicularis oculi muscles (31.5 and 28.4 min, respectively; P < 0.05).

Conclusions: As with other non-depolarizing muscle relaxants, the duration of the clinical effect of rapacuronium was shorter for the diaphragm than for skeletal muscle. The recovery index was identical for the diaphragm and adductor pollicis.