British Journal of Anaesthesia, Vol 76, Issue 2 278-283, Copyright © 1996 by The Board of Management and Trustees of the British Journal of Anaesthesia
C. Lee, X. Zhang and W. F. Kwan
In spite of its well known propensity to cause accidental paralysis by a
specific mechanism of action, Mg(2+)-induced neuromuscular block has not
been examined systematically for its characteristics of muscle response to
nerve stimulation. We examined in seven anaesthetized domestic pigs the
mechanomyographic (MMG) and neurally evoked compound electromyographic
(ncEMG, EMG) responses of the tibialis anterior muscle to stimulation of
its motor nerve, at baseline and during three levels of neuromuscular block
induced by infusion of MgSO4 (at approximately 25%, 50% and 75% depression
of the 0.1-Hz EMG). We observed that: at 0.1 Hz, the MMG tended to be more
depressed than the EMG; the train-of-four (2 Hz) was essentially
non-fading; the tetanic force (50 Hz) showed tetanic ascent instead of
tetanic fade and reached its baseline control value at 5 s in spite of
depression of the twitch; the EMG counterpart of the tetanus showed
escalation of the train of ncEMG, so that the fourth ncEMG was much greater
than the first; and the post-tetanic twitch was also relatively spared from
Mg(2+)-induced neuromuscular block. Sparing of the tetanus and post-tetanic
twitch resulted in large gains in the tetanus:twitch ratio and the
post-:pre- tetanic twitch ratio, which increased at the 75% level of
depression from 2.8 (SD 0.7) to 11.5 (4.0), and from 1.5 (0.3) to 4.6 (1.4)
(P < 0.01), respectively. These characteristics of neuromuscular block
by Mg2+ reflect its prejunctional mechanism of action by depression of
transmitter release.
LABORATORY INVESTIGATIONS
Electromyographic and mechanomyographic characteristics of neuromuscular block by magnesium sulphate in the pig
Department of Anesthesiology, Harbor-UCLA Medical Center, Torrance, CA 90509-2910, USA
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