QUANTITATIVE NEUROMUSCULAR MONITORING – NECESSITY OR TECHNOLOGY OVERDONE?

Author: Richard Novak, MD

The Anesthesia Consultant

How do anesthesiologists monitor the degree to which a patient’s muscles are pharmaceutically paralyzed during an anesthetic? A recent publication in our specialty’s most prestigious journal urges the use of a QUANTITATIVE neuromuscular monitoring machine to do this when general anesthetics include a paralytic drug. The article was not a prospective randomized study, but rather a retrospective (from 2016 to 2020) practice initiative from a solitary medical center. The goal of the authors (Weigel et al) was to measure the reversal of neuromuscular paralysis in all anesthetized patients at the end of their anesthetic, and to document that reversal in the patient’s chart.

Their measured goal was to document a train-of-four ratio of greater than or equal to 0.9 prior to extubation in each anesthetized patient. What is a train-of-four? A locomotive with four cars? Alas no. A train-of-four ratio is a monitor of the level of neuromuscular blockade. Four consecutive electronic stimuli are delivered along the path of a patient’s nerve. The twitch response of the muscle is measured in order to evaluate stimuli that are blocked, versus those that are delivered. Four consecutive muscle contractions of equal strength (a score = 1.0) occur if there is no neuromuscular blockade. If neuromuscular blockade is present, there will be a loss of twitch height of the final twitch compared to the first twitch, and the resulting ratio of the final twitch height/first twitch height (e.g. 4/5 = 0.8) will indicate the degree of blockade. The clinical concern is that a ratio of lower than 0.9 correlates with a weak patient who may not safely ventilate himself/herself.

The conclusion of the Weigel study boldly states, “Anesthesia providers are solely responsible for properly rescuing patients from the states of paralyses they initiate. This should occur for ALL PATIENTS as verified by QUANTITATIVE measurement and documentation of train-of-four ratios greater than or equal to 0.9.” (Capital letters added by me.)

Should the American Society of Anesthesiologists (ASA) add QUANTITATIVE neuromuscular monitoring as a standard of care?

Hmm. This would be a marked change because, to my observation, almost no anesthesia providers routinely use QUANTITATIVE neuromuscular monitoring at this time.

The authors’ goal of documenting a train-of-four ratio greater than or equal to 0.9 requires the purchase of QUANTITATIVE neuromuscular monitoring equipment in every anesthetizing location. The cost of each monitor was approximately $1,995, with the disposable costs of $20 to $25 per patient. An example QUANTITATIVE neuromuscular monitor is shown here:

 

 

 

The article states, “The dangers of paralyzing a patient with neuromuscular blocking drugs are well recognized. Despite advances in anesthetic management, approximately half of all patients arriving to the postanesthesia care unit (PACU) suffer from residual blockade defined as a train-of-four ratio less than 0.9.” They cite a previous article from Anesthesia and Analgesia in 2018 which stated: “whenever a neuromuscular blocker is administered, neuromuscular function must be monitored by observing the evoked muscular response to peripheral nerve stimulation. Ideally, this should be done at the hand muscles (not the facial muscles) with a quantitative (objective) monitor. Objective monitoring (documentation of train-of-four ratio ≥0.90) is the only method of assuring that satisfactory recovery of neuromuscular function has taken place. (Bold emphasis added by me.) The panel also recommends that subjective evaluation of the responses to train-of-four stimulation (when using a peripheral nerve stimulator) or clinical tests of recovery from neuromuscular block (such as the 5-second head lift) should be abandoned in favor of objective monitoring.”

The American Society of Anesthesiologists (ASA) sets the standard of care for intraoperative monitoring. The ASA Standard of Anesthesia Monitoring currently does not mandate any form of neuromuscular monitoring. The ASA Standard of Anesthesia Monitoring is the gold standard for all operating room monitoring, is followed by all training programs, and is referred to in courts of law as the standard of care should an adverse anesthesia outcome occur.

A 2010 survey of anesthesia providers documented that 19.3% of Europeans and 9.4% of Americans never use neuromuscular monitors. The majority of respondents from the US (64.1%) and Europe (52.2%) estimated the incidence of clinically significant postoperative residual neuromuscular weakness to be <1% (P<0.0001). Most respondents in this study reported that “neither conventional nerve stimulators nor quantitative train-of-four monitors should be part of minimum monitoring standards.”

I suggest three values in anesthetic care: Do the right thing, be safe, and Keep It Simple Stupid (the KISS principle). Rather than strapping a thumb monitor onto every one of my patients, I’m a disciple of qualitative neuromuscular monitoring—a less technologically complex form of monitoring. When I was serving my residency training in anesthesiology at Stanford in the 1980s, each resident was equipped with a MiniStim nerve stimulator, which is a qualitative neuromuscular monitor.

 

A qualitative neuromuscular monitoring device is simple to use. When the two terminals are applied to the facial nerve lateral to the eye of a sleeping patient and the green button is pushed, the orbital muscles will twitch if unparalyzed, and they will not twitch if paralyzed. With experience one can easily discern whether the patient is paralyzed or not, and one can estimate the degree of paralysis. The MiniStim also has a tetanus feature. When the two terminals are applied to the facial nerve lateral to the eye of a sleeping patient and the red button is pushed, a sustained electrical energy is emitted between the two terminals. The orbital muscles will show a sustained contraction if unparalyzed, and will not contract at all if fully paralyzed. If partially paralyzed, the muscles will contract and then the contraction will fade away in seconds. With experience, one can estimate to what degree the patient is paralyzed. The qualitative neuromuscular monitor does not give you the exact data, i.e. a decimal number between 0.0 (totally paralyzed and 1.0 (no paralysis) that a QUANTITATIVE neuromuscular monitor does.

I still carry a MiniStim, and have used one for the entire 38 years I’ve practiced anesthesia, and for the 30,000 patients I’ve anesthetized. I would not start a case without a neuromuscular qualitative monitor. I would not want to be a patient receiving a neuromuscular paralytic drug if the anesthesiologist did not utilize a neuromuscular monitoring device similar to the MiniStim. The MiniStim is no longer manufactured, but other similar qualitative neuromuscular monitors are easily purchased, e.g. as depicted below, for $251, with no additional disposable costs.

 

Why is the topic of reversing neuromuscular blockade seeing this kind of scrutiny in 2022? Residual neuromuscular paralysis is less a problem now than at any time since the paralyzing medications were discovered. Why? Because in 2015 the United States Food and Drug Administration (FDA) approved the new intravenous drug sugammadex, a reliable, specific, and safe agent for the reversal of neuromuscular paralysis. Sugammadex can eliminate neuromuscular paralysis rapidly. A rocuronium molecule, bound within sugammadex’s lipophilic core, is rendered unavailable to bind to the acetylcholine receptor at the neuromuscular junction, and paralysis is reversed in seconds.

Prior to 2015, the only reversal agent for pharmaceutical paralysis with a non-depolarizing neuromuscular blocker such as rocuronium was the drug neostigmine. Neostigmine can cause the side effect of severe bradycardia (slowing of the heart rate), and had to be administered intravenously in combination with glycopyrrolate (Robinul) or atropine. If a surgery was concluding and the patient had residual neuromuscular paralysis, the anesthesia provider needed to administer the combination of neostigmine/Robinul well before the wakeup-time, because the peak effect of neostigmine occurs at 10 minutes after administration.  If the patient was markedly paralyzed, e.g. the qualitative neuromuscular monitor showed no significant twitch or tetanus activity, neostigmine could not adequately reverse the neuromuscular paralysis in a short time. Sometimes it took 20-30 minutes before a deep neuromuscular paralysis could be reversed with neostigmine. If an anesthesia provider erroneously chose to awaken a patient prior to the time their neuromuscular paralysis was reversed or worn off, the patient would be too weak to breathe normally. A medical complication of hypoventilation or of awake paralysis could occur.

Because of sugammadex, the risk of untreated residual neuromuscular paralysis has never been lower. Unreversed neuromuscular paralysis at wake-up should be a never-event now that sugammadex exists. There is virtually no circumstance in which an attending anesthesia provider should have unreversed neuromuscular paralysis at the present time. Why, in 2022, should we advocate for a QUANTITATIVE neuromuscular monitor which is bulky, expensive, and can only be strapped onto the thumb? The thumb location is a disadvantage, because many anesthetics, for example laparoscopies, require the arms to be tucked at a patient’s sides during surgery, and a thumb monitor is not practical. The qualitative neuromuscular monitors work on any peripheral nerve: e.g. the ulnar nerve at the wrist, the facial nerve lateral to the eye, or the posterior tibial nerve in the ankle, and provide a more versatile monitor than the QUANTITATIVE neuromuscular thumb monitor.

Qualitative neuromuscular monitoring is useful, easy, versatile, and inexpensive. QUANTITATIVE neuromuscular monitoring has the appeal of a score—a number between 0 and 1.0—that can be added to the already burdensome printout of the Electronic Medical Record (EMR), and may seem satisfying to those addicted to the dubious wonders of the EMR, or to those who want to see QUANTITATIVE neuromuscular monitors reported in the medical literature. But the addition of QUANTITATIVE neuromuscular monitoring to the required ASA list of monitors at this time is premature.

Where is the science? Where is the prospective, randomized trial of QUANTITATIVE neuromuscular monitoring versus qualitative neuromuscular monitoring in the age of sugammadex? Does anyone really believe that qualitative neuromuscular monitoring will be inaccurate and lead to significant anesthetic complications in an era when sugammadex is available?

Qualitative neuromuscular monitoring was always a solid idea. I made this point twelve years ago when I wrote, “During residency or during the years afterward, a MiniStim and a stethoscope are arguably the only tools of your own you need to carry into an operating room to conduct a 21^st-century general anesthetic.”

Until prospective scientific evidence demonstrates that QUANTITATIVE neuromuscular monitoring improves outcomes, mandating the extra technology of QUANTITATIVE neuromuscular monitoring as a required standard is not the correct path for the ASA to take in 2022 or at any time in the future.