Tb. Sloan et R. Erian, EFFECT OF VECURONIUM-INDUCED NEUROMUSCULAR BLOCKADE ON CORTICAL MOTOREVOKED-POTENTIALS, Anesthesiology, 78(5), 1993, pp. 966-973
Background. Neuromuscular blockade (NMB) is a frequent component of an
esthetic techniques used during surgery in which monitoring of the ner
vous system is desirable. Because NMB should affect the evoked muscle
response to transcranial magnetic stimulation (tcMMEP), their relation
ship in a primate model was characterized. Methods. Transcranial magne
tic stimulation was characterized during NMB using an infusion of vecu
ronium in ten adult cynomologous monkeys during anesthesia with a cont
inuous ketamine infusion. Neuromuscular blockade was measured by peak-
to-peak amplitude of the evoked muscular activity (compound muscle act
ion potential [m-response]) of the thenar muscles and mechanical muscl
e action (ratio of the fourth to first peak in the train of four [TOF]
) after direct stimulation of the median nerve. Neuromuscular blockade
was increased incrementally to complete block and then allowed to dec
rease until complete resolution of measurable block. Transcranial magn
etic stimulation was assessed by measuring the onset latency (time fro
m stimulation to beginning response) and amplitude of the thenar EMG r
esponse. Cortical stimulation was accomplished using a Cadwell MES-10
magnetic stimulator at 80% of full output (1.6 Tesla). Results: The tc
MMEP, m-response amplitude, and mechanical muscle action, unblocked, w
ere reduced with increasing NMB. Transcranial magnetic stimulation amp
litude was more variable than was onset latency. Transcranial magnetic
stimulation amplitude reduction from the baseline value did not achie
ve statistical significance until the m-response amplitude was reduced
to 0.2 of baseline or until the TOF ratio was reduced to 0.1. Transcr
anial magnetic stimulation onset latency prolongation from baseline wa
s not significantly affected by declining TOF ratios, but was prolonge
d when the m-response declined to 0.1 of baseline. Conclusions: This s
tudy indicates that tcMMEP onset latency is not significantly affected
by NMB if the degree of blockade in the muscles used for tcMMEP monit
oring is not extreme (greater than 0.2 of baseline by m-response ampli
tude or a TOF ratio of 0.1 or greater). If monitoring of tcMMEP amplit
ude is desired, partial neuromuscular blockade may be acceptable. Howe
ver, amplitude reduction may occur during partial NMB. Maintenance of
a constant degree of NMB is suggested to minimize amplitude fluctuatio
ns.