The construction and evaluation of a device for mechanomyography in anaesthetized Gottingen minipigs

Objective To devise a method for assessing evoked muscle strength on nerve stimulation [mechanomyography (MMG)] in the anaesthetized minipig. Study design Prospective observational. Animals Sixty male Gottingen minipigs weighing 10.5-26.0 kg. Methods After cadaveric studies, a limb fixation device was constructed which allowed the twitch responses of the pelvic limb digital extensor muscles to be measured by force-displacement transduction in response to supramaximal train-of-four (TOF) stimulation of the common peroneal nerve. The device was tested in 60 minipigs weighing 10.5-26.0 kg positioned in dorsal recumbency. Results The technique recorded the MMG of the common peroneal-pelvic limb digital extensor nerve-muscle unit for up to 12 hours during which twitch height remained constant in 18 animals in which single twitch duration was <300-500 ms. In 42, in which twitch duration was >300-500 ms, 2 Hz nerve stimulation caused progressive baseline elevation (reverse fade) necessitating a modified signal capture method for TOF ratio (TOFR) computation. However, T1 was unaffected. The mean (range) of the TOFR in pigs with reverse fade was 1.2 (1.1-1.3). Conclusions and clinical relevance The technique allowed MMG recording in unparalysed pigs in response to TOF nerve stimulation and revealed a hitherto unreported complication of MMG monitoring using TOF in animals: reverse fade. This complicated TOFR calculation.