Effect of atracurium-induced neuromuscular block on cortical motor-evoked potentials

The effect of nondepolarizing neuromuscular block (NMB) with atracurium on transcranial magnetic motor-evoked potentials (tcMMEP) was examined in 10 adult cynomolgus monkeys. The monkeys were anesthetized initially with ketamine (15 mg/kg intramuscularly) and maintained with a continuous ketamine infusion (10-15 mg.kg-1 x h-1). NMB was then adjusted from no block to profound block and then back to no detectable block by adjustment of an atracurium infusion (0-7.25 mg.kg-1 x min-1). NMB was assessed by measuring the peak-to-peak amplitude of the evoked electromyographic (EMG) activity of the thenar muscles and by measuring the ratio of the 4th to 1st peak in the train of four (TOF) of the mechanical action from direct stimulation of the median nerve (direct EMG). NMB was adjusted incrementally, whereas cortical magnetic motor-evoked potentials were assessed by measuring the onset latency and amplitude of the thenar EMG response. Cortical stimulation was accomplished with a Cadwell MES-10 magnetic stimulator. tcMMEP and direct EMG amplitude and the mechanical response were reduced with increasing NMB. The average tcMMEP amplitude remained near baseline values (no NMB) until the direct EMG decreased < 0.4 of the unblocked value and when the TOF ratio decreased < 0.3. Further increases in NMB resulted in decreasing amplitude with a statistically significant reduction below baseline when the direct EMG decreased < 0.2 of baseline and TOF ratio decreased < 0.1. The tcMMEP onset latency was unchanged from baseline with TOF or with direct EMG reductions above 0.1 of baseline.(ABSTRACT TRUNCATED AT 250 WORDS)