Quantitative monitoring is now the gold standard when using muscle relaxants or reversal agents. 

In this blog, we will discuss the previous practice used, the two most widely available quantitative monitors, and how they compare. While both technologies fall under the new guidelines, we will discuss why EMG technology, and especially TetraGraph, stands out compared to other methods. 

PNS: Works by stimulating a peripheral nerve with a series of brief electrical pulses to produce a muscle response or twitch. TOF count is determined by subjectively determining the number of visible twitches. 

AMG: Measures muscle acceleration in response to nerve stimulation. AMG technology requires free movement of the thumb to capture data of nerve stimulation. Objective, measurable data. 

EMG: Measures the compound muscle action potential (CMAP), which is the first electrical signal that can be measured after neuromuscular transmission. Quantitative, direct data is measured with versatile applications. 
 

In short, EMG is an objective, direct measurement of muscle activity, sensitive to small changes, is muscle-specific monitoring and gives you data instantly. 
EMG technology allows versatile applications including the ulnar nerve with AP, ulnar nerve with ADM, and the posterior tibial. With these different application choices, it opens the door for usage in any type of case or patient.

Unlike other technologies, EMG does not need to be calibrated prior to induction. Without the hinderance of calibration, the data is readily available in seconds. EMG has also been compared to the not readily available gold standard MMG technology.  

“Electromyography and MMG monitoring are closely matched” (Rodney, 2024)

 “Electromyography reflects more accurately the response at the neuromuscular junction (where all neuromuscular blocking agents work), is not affected by es in muscle contractility and responses are independent of hand position and thumb movement” (Rodney, 2024)

“The results of this study demonstrate that three currently available electromyograph monitors produced substantially better accuracy and precision than three acceleromyographs, when measuring the train-of-four ratio” (Wedemeyer, 2023)
 

AMG has an extensive set up, cumbersome calibration required, abundance of non-disposable pieces, requires free movement of the hand, and evidence-based data has proven overestimation of baseline. These limitations produce significant roadblocks to adaptation and could cause patient harm if over-estimation is not considered upon reversal.

While non-disposable pieces have the potential to reduce cost, it’s required that these pieces remain together to adequately function. Along with preventing loss of multiple parts, sanitation from one patient to the next is imperative. With an already fast pace, high stress environment, AMG is not the ideal set up or the most accurate. 

“Three currently available acceleromyograph monitors produced overshoot (train-of-four ratio greater than 1.0) and substantial variability that could be clinically significant” (Wedemeyer)

“An important issue with acceleromyography is that baseline (also referred to as control) train-of-four ratio measurements (i.e., before muscle relaxation) often exceed 1.0. It is common with baseline values in the range of 1.1 to 1.15, but significantly higher baseline values have been reported. Therefore, the clinical definition of adequate recovery of neuromuscular function may vary when the results of monitoring with acceleromyography are not normalized, and the train-of-four ratio may recover to values greater than 1.0 ” (Thilen, 2023)

”An example, if the baseline train-of-four ratio is 1.15 and the raw postoperative train-of-four ratio is 0.95, then the normalized train-offour ratio is 0.95/1.15 = 0.83” (Thilen, 2023)

PNS is a current practice in a many operative room settings in the United States, due to habit and comfortability of the provider. This technology was the first widely used method to monitor neuromuscular blockade. Clinicians have practiced with this for years and have grown trust and comfort in this way of practice.

Unfortunately, PNS technology has been proven unreliable due to its subjective nature. Along with being subjective, many clinicians use the facial nerve instead of the ulnar. If using the facial nerve, there is a higher incidence of residual blockade. 

“Widespread use of peripheral nerve stimulators to assess blockade with the mistaken belief that “four visibly equal twitches” to train-of-four stimulation or “sustained tetanus” indicate full recovery. Several studies have established that clinically significant weakness cannot be identified with subjective assessment of the response to a peripheral nerve stimulator. Using subjective assessment of the train-of-four, fade cannot be reliably appreciated until the train-of-four ratio is less than 0.4.” (Thilen, 2023)
 

” A meta-analysis of 53 studies and 12,664 patients over four decades, showed residual neuromuscular block rates of 33.1% with no neuromuscular monitoring (management guided by clinician experience and patient clinical signs) and 30.6% when using qualitative assessment” (Rodney, 2024)

“Adults who undergo intraoperative quantitative monitoring have significantly less overall weakness and rate their quality of recovery as superior to patients who are monitored subjectively”
(Rodney, 2024)

“The ASA practice guide strongly recommends against using the eye muscles for neuromuscular monitoring. If the facial muscles are monitored, lower stimulating currents are necessary to reduce the risk of direct muscle stimulation. In addition, the relative resistance of some eye muscles to neuromuscular blocking drugs may falsely indicate presence of neuromuscular transmission, resulting in excessive dosing.”
(Rodney, 2024)

“Use of a PNS allows subjective (tactile or visual) detection of TOF count and PTC, but only provides an unreliable estimate of recovery by evaluating fade of the fourth twitch compared with the first twitch (T4/T1) response. Whereas detection of moderate (TOF count 1e3) and deep (PTC >1) block is possible using a PNS, readiness for tracheal extubation (defined as TOF ratio ≥0.9) CANNOT be determined by subjective (non-quantitative) means”
(Rodney, 2024) 
 

In the medical field, it’s important to stay relevant in the newest technologies that can improve outcomes and patient safety. 

Clinical judgment is a critical piece of anesthesia and is being used in conjunction with technology. Technology should not replace the knowledge/judgment that clinicians possess but rather enhance their practice. Evidence-based research is compiled to create protocols and guidelines to elevate patient safety and practices. Without this research, anesthesia could still be using a chloroform rag for sedation. 

Anesthesia has come a long way due to technological advancements and it will keep growing. EMG technology is one of these advancements that can help clinicians elevate their craft. When adapting this technology into practice, it’s not undermining the clinical judgement—it’s providing another layer of safety. 

“We recommend against clinical assessment alone to avoid residual neuromuscular blockade, due to the insensitivity of the assessment” (Thilen, 2023)

“Clinical tests of recovery (sustained head lift, sustained hand grip, tongue depressor tests) all fail to reliably detect residual neuromuscular block. With sensitivity rates of 10e30% and positive predictive values (precision) ~50%, residual neuromuscular block cannot be excluded unless TOF ratios are <0.4, exposing patients to considerable harm.” (Rodney, 2024)
 

TetraGraph provides clinicians with a signal strength prior to muscle relaxation. This signal strength provides the clinician with concrete data that they have good connection to the ulnar nerve. With this added confidence, clinicians can trust the data, allowing closer monitoring of muscle relaxation through procedure and safe handling after. 

TetraGraph’s algorithm is based on more than 40 years of research and is validated for accuracy against MMG (mechanomyography) at all levels of block.

A recent clinical study published in Anesthesiology demonstrated that
TetraGraph achieves accuracy and precision when measuring baseline TOF
ratios. TetraGraph produced results similar to the clinical reference standard,
mechanmyography (MMG), reinforcing its reliability for quantitative TOF monitoring. (Wedemeyer Z, et al. 2024)

Its small, portable design enables the monitor to fit into any setting. TetraGraph also has the capability to integrate into external monitors and hubs. In addition, TetraGraph provides a complete portfolio of flexible, easy-to-use sensors.

Last but not least, as it was stated in Anesthesiology journal in 2021, Nemes et al:

“TetraGraph EMG monitor is a better indicator of adequate recovery from neuromuscular block and readiness for safe tracheal extubation than the acceleromyography monitor.”