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October 16, 2012
8:00:00 AM - 11:00:00 AM
Room Hall C-Area B
Repeatability and Performance of the T4-EMG and TOF-Watch Evoked Neuromuscular Responses in Volunteers
Sorin J. Brull, M.D., Jolanda A. Witteveen, M.Sc., David R. Hampton, Ph.D.
Mayo Clinic, Ponte Vedra Beach, Florida, United States
Background. Three types of quantitative monitors are used to assess neuromuscular function. Mechanomyography (MMG) measures force of contraction of the adductor pollicis muscle following ulnar nerve stimulation. MMG responses are precise and reproducible, but because of complex and bulky equipment, MMG is no longer available commercially. Acceleromyography (AMG) measures acceleration of muscle (usually thumb) in response to nerve stimulation. Electromyography (EMG) devices measure compound muscle action potentials in response to nerve stimulation. Due to high manufacturing costs and interference from electrical devices in the operating room, stand-alone EMG monitors are not available commercially. The TOF-Watch® (Merck Inc.) is an AMG device that is small, portable, and designed for intraoperative use and is considered the “established standard” for clinical use. Routine use of the TOF-Watch® by clinicians has been limited by high acquisition costs ($800-$2,100) and limitations of the technology in the operating room (requirement for baseline measurements and normalization, the staircase effect of nerve stimulation, reduced precision in awake patients)(1).

An easy-to-use and accurate quantitative neuromuscular monitor is needed in the clinical setting. The aim of this IRB-approved clinical investigation was to compare evoked responses from a battery-operated prototype EMG monitor (T4-EMG) to those obtained from the AMG-based TOF-Watch monitor.

Methods. In 10 consenting volunteers (8F/2M, aged 26-44 yo, ASA 1-3, Wt. 122-215 lbs), train-of-four (TOF) responses were recorded with the prototype monitor and TOF ratios were calculated. Simultaneously, evoked responses (TOF ratios) were recorded from TOF-Watch AMG device. Stimulating surface electrodes were placed along the median nerve on the distal volar right & left forearms, and recording surface electrodes were placed on the thenar eminence to record EMG responses from adductor pollicis muscle (APM). The AMG piezoelectric transducer was attached to the thumb for AMG recording of TOF responses. Four sequential sets of TOF stimuli were delivered with a current intensity of 5 mA below the maximal current (Max, defined as the current level at which the evoked response reached a plateau), in order to minimize the potential discomfort from high-current stimulation. The applicability (ease of use, equipment need, etc.), repeatability (precision or internal consistency), and performance (agreement with established standard, bias) of the T4-EMG compared to the AMG were determined in unpremedicated volunteers. The study followed Good Clinical Practice Guidelines for monitoring (2).

Results. The mean AMG- and EMG-recorded TOF ratios were 103.0±13.3, and 98.1±2.6, respectively. The repeatability and performance of the 2 monitors are shown in Fig 1 and 2.

Discussion. The initial data indicate that the new T4-EMG monitor may have a clinical application, since it shows similar bias and better repeatability.


1) Anesthesiology 2008;108: 1117-40

2) Acta Anaesthesiol Scand 2007;51: 789-808
Figure 1
Figure 2

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