Historical assessment of the level of muscle paralysis
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.
A growing global consensus
The introduction of NMBA to anesthesia practice represents a significant pharmacologic advancement that has at the same time also introduced iatrogenic complications related to RNMB.
Postoperative residual weakness is a patient safety threat. An international panel of experts recently developed a consensus statement strongly recommending quantitative monitoring and many societies have developed strong national guidelines, for example the Association of Anaesthetists in UK & Ireland (AAGBI)
“Quantitative neuromuscular monitoring should be used whenever neuromuscular blocking (NMB) drugs are administered, throughout all phases of anaesthesia from before initiation of neuromuscular blockade until recovery of the train-of-four ratio to > 0.9 has been confirmed. “