What is the mechanomyograph (MMG)?
The MMG is the historical standard for
neuromuscular monitoring. Peripheral nerves are stimulated with a suffi-cient
current to depolarize all axons of a motor nerve. This current is usually on
the order of 50–60 milliamps and is called a supramaximal stimulus. Increasing the stimulus beyond supramaximal
does not further increase the response. The current applied to a nerve causes a
wave of depolarization, which results in acetylcholine release from nerve
terminals with subsequent endplate and motor depolarization and muscle
contraction. Mechanical twitch tension can then be measured using a force
transducer. The force vector must be properly aligned with a transducer, or the
measurement will not be accurate. Furthermore, the muscle must be preloaded
with a specific tension (100–300 g) to ensure optimal contractile force.
Clearly, MMG monitors must be carefully applied to ensure accu-rate and
consistent measurements. MMGs are designed for preselected sites, usually ulnar
nerve stimulation and adductor pollicis brevis force measurements.