Massimo Lamperti, MD, Boris Tufegdzic, MD
The introduction of muscular relaxation into anaesthesia marks one of the most significant dates in the history of surgical anaesthesia. In 1942, Harold Randall Griffith, an anaesthesiologist at the Homeopathic Hospital in Montreal, with his resident Enid Johnson, administered curare to a young man undergoing appendectomy, inducing complete muscle relaxation in a resistant patient undergoing general anaesthesia with cyclopropane.1 Albeit the use of muscle relaxants became more frequent, the first report of monitoring the degree of neuromuscular relaxation was almost twenty years later.
In 1958, Christie and Churchill-Davidson described the use of a nerve stimulator to monitor neuromuscular blockade in a quantitative way. The method used by these two pioneers was a very basic neuromuscular monitoring device able to produce a single twitch, while the introduction of more detailed methods of neuromuscular stimulation was described only in the early 1970s.2 Neuromuscular monitoring is based on the electrical stimulation of a peripheral motor nerve and on the clinical observation or quantitative measurement of the electrical or mechanical response of the muscle innervated by this nerve. The most widely used pattern of nerve stimulation is the train-of-four (TOF) stimulation.
Although the use of neuromuscular blockade (NMB) monitoring is absolutely recommended in many guidelines3,4 as part of the standard monitoring in cases where a muscular relaxant has been used, the reality is completely different5,6 in everyday practice.
Why are anaesthesiologists still not using quantitative NMB monitors? Why are they undermining the possible consequences of residual muscular blockade? Is the financial pressure, coming from cost-cutting in healthcare, not allowing provision of neuromuscular monitors in every operating room? Do anaesthesiologists feel comfortable assessing residual NMB only by evaluating clinical signs? All these questions do not have clear answers but they are opposing anaesthesia principles of patients’ safety.7
Indications for use of neuromuscular monitoring are wide. It is used to determine the ideal time for intubation, the degree of muscle relaxation, the need for an additional dose of muscle relaxant, and to determine the ideal conditions for extubation. Certainly, the most important indication in terms of outcome and possible postoperative complications is the assessment of the residual neuromuscular blockage. Residual neuromuscular relaxation is accurately defined as the presence of signs or symptoms of muscle weakness in the postoperative period after the intraoperative administration of a neuromuscular relaxant. There is increasing evidence that residual neuromuscular block is a very common condition after general anaesthesia (up to 45% of patients),8 and that it may adversely affect patient outcome leading to critical respiratory events as it impairs the ventilatory response to hypoxia.
How can we define residual neuromuscular blockade? Previous studies concluded that the threshold for inadequate neuromuscular recovery was represented by a TOF ratio less than 0.7.9,10 More recent studies suggest that TOF ratio should be more than 0.9 to ensure full neuromuscular recovery and avoiding postoperative respiratory adverse events (ranging from 20% to 65%).8
It is now clear that monitoring neuromuscular block clinically is unreliable and only quantitative techniques should be used. At a TOF ratio of 0.7 or greater, almost all patients are able to sustain eye opening, hand grasp, and tongue protrusion, whereas 9 out of 10 are able to maintain a 5-second head lift. Reaching a TOF ratio of 0.9 significantly increases patient safety and reduces the risk of respiratory complications, but some patients may still exhibit obvious weakness despite achieving TOF ratios >0.9.
The residual effect of numerous drugs used in anaesthesia such as opioids, benzodiazepines, volatile anaesthetics, or induction agents can be misinterpreted as incomplete neuromuscular recovery. In these situations, quantitative neuromuscular monitoring can help us interpret the clinical signs in an objective way and make a differential diagnosis between residual muscular block and delayed awakening for excessive sedation. In both cases, the use of a neuromuscular quantitative monitor and the use of a processed electroencephalogram monitor can give an objective evaluation of the delayed awakening.
The role of the anaesthesiologist during general anaesthesia is to provide a safe and predictable perioperative course, monitoring the patient’s physiological state and adequacy of depth of sedation. Neuromuscular monitoring certainly must be considered as a fundamental standard monitor whenever neuromuscular blocking agents are used even if neuromuscular block reversals were administered. However, monitoring devices cannot replace clinical observation but they need to supplement it, with the goal to prevent unwanted perioperative adverse events.
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- Checketts, MR, Alladi, R, Ferguson, et al. Anaesthesia 2016;1:85–93.
- Czech Society of Anaesthesiology and Intensive Care Medicine. Practice Parameters For The Safe and Effective Use of Neuromuscular Blocking Drugs In Anaesthesia. 2010. http://www.akutne.cz/res/file/doporucenepostupy/15_Practiceparametersforthesafeandeffectiveuseofneuromuscular_blockingdrugsinanaesth.pdf.
- Phillips, S, Stewart, PA, Bilgin, AB. Anaesth Intensive Care 2013;41:374–9.
- Naguib, M, Kopman, AF, Lien, CA, et al. Anesth Analg 2010;111:110–9.
- Mellin-Olsen J, Staender S, Whitaker DK, Smith AF. Eur J Anaesthesiol 2010;27:592-7.
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