Objective: To investigate the influence of different degrees of neuromuscular blockade on motor evoked potential (MEP) of forearm muscles under the same condition of anesthesia depth and analgesia.
Methods: Forty ASA I - II patients aged 20 - 60 underwent general anesthesia via intravenous injection of propofol, fentanyl, and vecuronium to achieve the bispectral index (BIS) at the level of (50 + 10) during neurosurgical operation. The patients were all given fentanyl infusion at a rate of 2 microg/kg/h during the operation and continuous vecuronium injection while the value of train of four stimulations (TOF) was monitored under 3 different levels: TOF = 4 (TOF(4)), TOF = 2 (TOF(2)), and TOF = 0 (TOF(0)). The values of amplitude and latency at the 3 forearm muscles, forearm flexors (FF), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) were monitored and compared under stable hemodynamic status.
Results: There was no significant differences in the latency of compound muscle action potential (CMAP) between the anesthesia status TOF(4) and anesthesia status TOF(2) in FF, APB, and ADM (all P > 0.05). The amplitude of CMAP deceased gradually as the degree of neuromuscular blockade increased in FF, APB, and ADM (all P < 0.05). CMAP could not be recorded when the value of TOF was 0 (P < 0.01).
Conclusion: Neuromuscular blocking drugs significantly depress the MEP in a dose-dependent manner. Intra-operatively it may be preferable to maintain the value of TOF not less than 2 during the critical periods of MEP monitoring.