Qualitative vs Quantitative Monitoring
Monitoring of neuromuscular function can guide the clinical management of neuromuscular blockade and help to reduce the incidence of residual neuromuscular blockade. There are different ways to assess neuromuscular function:
The neuromuscular assessment can be subjective or objective. Visual or tactile assessment of the twitch responses is considered qualitative (subjective) neuromuscular assessment, because the clinician guesses the degree of fade or recovery, instead of measuring the strength of responses. The result is therefore subjective and unreliable, regardless of the clinician’s level of experience or training. Low, but still clinically meaningful degrees of neuromuscular blockade can only be measured using a quantitative, objective, monitoring device. A residual neuromuscular block of TOFR > 0.4 cannot be reliably detected by tactile or visual means even by an experienced clinician. Clinical tests, such as 5-s head-lift, tidal volume, grip strength or 5-s leg lift do not guarantee complete resolution of neuromuscular block and should not be used to make clinical decisions about the adequacy of neuromuscular function.
Evolution of Quantitative Monitoring Techniques
Quantitative neuromuscular monitors measure and quantify the degree of neuromuscular blockade and display the results numerically.
Quantitative assessment of the train-of-four fade by neuromuscular transmission monitors is the only suitable method to identify low but clinically meaningful levels of residual neuromuscular block. To determine whether full recovery of neuromuscular function has occurred at the time of tracheal extubation, quantitative monitors are required.
Mechanical techniques such as mechanomyography (MMG), acceleromyography (AMG), and kinemyography (KMG) measure muscle responses, using physics principles such as force, acceleration, or velocity.
Electromyography (EMG) is based on the measurement of the compound muscle action potential. EMG measures an electrical event that occurs at the neuromuscular junction; the release of acetylcholine from the pre-synaptic nerve endings (a chemical process) and activation of postsynaptic receptors that converts it to a mechanical response (excitation- contraction coupling that results in muscle contraction).
For this reason, EMG is less prone to interference from presynaptic or postsynaptic events and is a better indicator of pure neuromuscular function.
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Naguib et al. Conceptual and technical insights into the basis of neuromuscular monitoring. Anaesthesia 2017;72:16-37