Acceleromyography vs. electromyography: an ipsilateral comparison of the indirectly evoked neuromuscular response to train‐of‐four stimulation

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Abstract
There is a considerable body of evidence which suggests that data obtained using acceleromyography (AMG) cannot be used interchangeably with observations obtained by mechanomyographic (MMG) or electromyograhic (EMG) methods. All previous such studies evaluated the responses from contralateral limbs. This investigation was undertaken to determine if these previously described differences were in part a function of observing the responses from opposing limbs. We compared the ipsilateral EMG and AMG response to an ED(95) bolus of atracurium in 50 subjects. In half of the individuals the thumb was free to move freely; in half, a small elastic preload was applied to the thumb. Train-of-four (TOF) recovery was followed until a TOF ratio >0.90 was recorded by both monitors. Acceleromyography vs. EMG differences and the resultant 95% confidence limits for twitch height (T1) and the TOF ratio were determined. When the AMG TOF value had recovered to a value of 0.72 +/- 0.03; the simultaneously evoked EMG value averaged only 0.59 +/- 0.08. This difference was statistically significant (P < 0.001). Although the mean difference AMG vs. EMG was little more than 0.10, differences in an individual might be twice that amount. When the AMG TOF value had recovered to 0.90, the simultaneously evoked EMG value averaged 0.85. Again the 95% confidence limits for individual observations was very wide. With EMG, once the TOF ratio returns to a value of 0.70, T1 has returned to 95% of control. In contrast with AMG, return of T1 -95% of control requires a TOF ratio of almost 0.90. Addition of an elastic preload to the thumb decreased control TOF variability without effecting the relationship between twitch height and the TOF ratio. Acceleromyographic TOF values tend to overestimate the extent of EMG recovery. Acceleromyographic TOF values <0.90 are indicative of incomplete neuromuscular recovery.