Why Quantitative NMT Monitoring is Critical in Surgical Patients and How Best to Do It
Some sort of assessment of neuromuscular transmission (NMT) is necessary in surgical patients by clinicians and anesthetists to get a feel for the depth of anesthesia. This assessment can be done using simple yet subjective clinical parameters or through more advanced and objective monitoring devices. NMT monitoring is required to
- ascertain that anesthesia is appropriate for tracheal intubation
- check the adequacy of neuromuscular blockade during a procedure
- determine the need for adjusting the dose of neuromuscular blocking agents (NMBAs)
- decide the timing and dose of reversal agents
- ensure full patient recovery before extubation
When NMT monitoring is absent, inadequate, or inaccurate, it is associated with an increased risk of
- airway obstruction
- adverse respiratory events
- pharyngeal dysfunction
- prolonged post-anesthesia stays
- unpleasant postoperative symptoms including muscle weakness
Having said that, it may come as a surprise that anesthesiologists often overlook the importance of effective monitoring. Research reveals that less than 40% of patients receive subjective assessment using nerve stimulators, while objective monitoring is performed in only 17% of patients. Furthermore, almost 40% of patients have incomplete neuromuscular recovery in the early recovery period from anesthesia. Against this backdrop, it is easy to understand why proper NMT monitoring is the need of the hour in surgical patients. We will now discuss how such monitoring can be achieved.
Current Approaches to NMT Monitoring
There are three ways to monitor the neuromuscular status:
- Clinical assessment—most commonly employed by clinicians, it is a subjective evaluation of clinical parameters such as respiratory measures and muscle function. However, none of the tests has a sensitivity greater than 0.35 or positive predictive value more than 0.52. Clearly, it’s not the most reliable approach.
- Qualitative monitoring/peripheral nerve stimulation—Qualitative monitoring uses peripheral nerve stimulators (PNSs). The evoked response of the stimulated muscle is then assessed visually or tactilely. It’s more reliable than a simple clinical assessment but less so than quantitative monitoring.
- Quantitative monitoring—this involves the use of devices that quantify the NMT blockade and display the measurements numerically. Quantitative monitoring offers the virtues of reliability, accuracy, and objectivity. We describe quantitative monitoring in more detail below.
Why Quantitative Monitoring is the Way to Go
Following are just some of the benefits of quantitative neuromuscular monitoring:
- Objective measurements—stimulation is provided to a suitable muscle and the evoked response is quantified objectively. It can be through measuring the action potentials generated within the muscle, the strength of its contraction, or even the crackling sounds associated with muscle movement. Whichever the underlying mechanism for the monitoring device, nothing is left to the subjective opinion of the clinician. Hence, the results are consistent and reproducible.
- Display of results—quantitative monitoring devices are smart and internally compute any raw data to display final results in numerical form that can then be used to guide clinical decision-making.
- Automatic processes—most devices just need their leads to be attached at the appropriate locations and then they do the rest themselves, including providing stimuli, recording responses, computing results, and displaying the same.
- Risk of PRNB nearly eliminated—the risk of postoperative residual neuromuscular blockade (PRNB) is almost completely eliminated with the use of quantitative monitors for tracheal extubation.
Quantitative NMT Monitoring Devices
There are several methods to perform quantitative NMT monitoring:
Modern quantitative NMT monitoring devices are capable of automatically providing stimulation to the muscle and recording and interpreting the response. The impulses can be given in various patterns such as train of four (TOF), double-burst (DBS), tetanic and post-tetanic count (PTC), which can be used to determine the train-of-four count (TOFC) or the degree of fade. The evoked muscle response can then be measured by using different scientific techniques giving rise to several types of devices:
- Mechanomyography (MMG)—the device detects muscle isometric force of contraction and converts it into an electrical signal. The amplitude of the signal reflects contraction strength.
- Electromyography (EMG)—it records compound muscle action potentials generated in the muscle and this electrical activity is proportional to the contraction force.
- Acceleromyography (AMG)—the monitor measures muscle acceleration via a piezoelectric sensor. The piezoelectric crystal generates voltage when it is put into motion due to muscle contraction.
- Kinemyography (KMG)—such devices quantify muscle movement via a motion sensor strip. The strip once again contains piezoelectric sensors.
- Phonomyography (PMG)—it calculates muscle response based on sounds picked up a by microphone. This is possible because muscle contraction produces low-frequency sounds.
While the above seem some fantastic methods to quantitatively monitor NMT blockade in clinical settings, MMG and PMG monitors are not commercially available and only used for research purposes, there is only one EMG monitor available for commercial use but it’s not standalone, and studies comparing KMG to MMG (the “gold standard”) have found its data to have a large bias and it cannot be used interchangeably. In the end, the AMG technology has been the most successful for commercial and clinical deployment. The Stimpod NMS 450X is a classic example of a cutting-edge AMG monitor.
Why the Stimpod NMS 450X is the Monitor of Choice for Quantitative NMT Monitoring
There are many features that make the Stimpod NMS 450X the monitor of choice for quantitative NMT monitoring. You can review these features in detail here. We will go over some of the more pertinent features of this device in light of what we have discussed so far about NMT monitoring:
- Global coverage—There is no point in reading up about the importance of NMT monitoring and researching a monitor only to find that either it’s not commercially manufactured or it’s not available in your area. Xavant’s worldwide coverage ensures Stimpod’s availability across the globe. In his comprehensive review article covering NMT monitoring in the perioperative period, Dr. Glenn Murphy, a senior anesthesiologist affiliated with the NorthShore University HealthSystem, points out, “At the present time, only one stand-alone portable device is available in the United States, the STIMPOD (Xavant Technologies, Pretoria, South Africa).”
- Fully automated—The Stimpod NMS 450X’s OneTouch NMT™ technology allows users to monitor an entire case from electrode placement to extubation by pressing a single button. The device automatically verifies optimal electrode placement, provides the appropriate supramaximal current, begins TOF monitoring and moves to PTC when a deep block is reached. Upon reversal, it automatically reinitiates monitoring until the patient is 90%+ recovered.
- 3D AMG transducer—The Stimpod NMS 450X uses a 3D AMG transducer which is most effective in capturing the full force of muscle contraction. In his seminal article, Dr. Murphy observes, “An important disadvantage of first-generation AMG monitors … is that acceleration of a muscle following nerve stimulation is only measured in a single direction (perpendicular to the face of the transducer). However, stimulation of the ulnar nerve results in isotonic contractions of the adductor pollicis that are often in three dimensions, involving three joints, frictional forces, and deformation of tissues.”
Decades of research demonstrate that AMG technology can detect residual paralysis in 97% of patients. The Stimpod NMS 450X is an embodiment of that technology and an all-in-one solution for NMT monitoring. Please contact us to know more about NMT monitoring and our solutions.
Murphy GS. Neuromuscular monitoring in the perioperative period. Anesth Analg. 2018;126:464–468.
Duţu M, Ivaşcu R, Tudorache O, et al. Neuromuscular monitoring: an update. Rom J Anaesth Intensive Care. 2018;25(1):55–60. doi:10.21454/rjaic.7518.251.nrm
Thilen SR, Bhananker SM. Qualitative Neuromuscular Monitoring: How to Optimize the Use of a Peripheral Nerve Stimulator to Reduce the Risk of Residual Neuromuscular Blockade. Curr Anesthesiol Rep. 2016;6:164–169. doi:10.1007/s40140-016-0155-8
Hemmerling TM, Le N. Brief review: Neuromuscular monitoring: an update for the clinician. Canadian Journal of Anesthesia. 2007;54(1):58-72.