A new desert island monitor

Myron Yaster MD, Lynne Maxwell MD, and Debra Faulk MD

Two weeks ago I (Myron) introduced the idea of the desert island monitor to the Pediatric Anesthesia Article of the Day readership…basically “If you were stranded on a desert island and could have only one anesthesia monitor what would it be? pulse oximeter or capnograph”? (September 8th PAAD) Into the mix comes something new: a neuromuscular blockade monitor that actually works! The standard QUALITATIVE train of four (TOF) monitors that almost all of us use in the operating rooms (and ICUs) are subjective and, spoiler alert, are essentially useless even when positioned properly on the forearm (ulnar nerve) to stimulate the adductor pollicis muscle. Indeed, when studied and with proper positioning of the electrodes on the ulna, the results that qualitative TOF monitors visually produce are really no better than 50-50 or flipping a coin, which is NOT what you want from a diagnostic monitor!. And fuggedabout placing the leads on the forehead to look for orbicularis occuli twitiching!  In our opinion, that’s a total waste of time and effort.  More often than not when doing that the muscle is directly stimulated, not the nerves supplying the muscles! A new generation of QUANTITATIVE blockade monitors are now readily available and will become as mandatory as pulse oximetry and capnography sometime in the spring or summer when the ASA issues its new neuromuscular blockade monitoring guidelines. Thus, a new desert island monitor is on the horizon. Because of the importance of this and because many of you, like me (Myron), may be unfamiliar with the pros and cons of the different types of quantitative monitors, we are going to break with PAAD tradition and discuss an editorial and original article on this subject that were published in the most recent issue of Anesthesiology separately. This first PAAD will discuss how the quantitative blockade monitors work.  The next will focus on which may be the better monitor technology to buy and use.  Additionally, we’ve asked Dr. Debra Faulk to assist us.  Deb recently published a survey on the use, monitoring, and antagonism of neuromuscular blockade by members of the Society for Pediatric Anesthesia so her insights will be invaluable.1  In that study, she and her team found that very few pediatric anesthesiologists routinely monitored the TOF and almost none used quantitative monitoring. So, as Bette Davis said in the 1950 movie classic All About Eve: “Fasten your seatbelts; we’re in for a bumpy night”.

Myron Yaster MD, Lynne Maxwell MD, and Debra Faulk MD


Bowdle, A. & Michaelsen, K. (2021). Quantitative Twitch Monitoring: What Works Best and How Do We Know?. Anesthesiology, 135 (4), 558-561. PMID: 34499107

Original Article: To be reviewed tomorrow

Nemes R, Lengyel S, Nagy G, Hampton DR, Gray M, Renew JR, Tassonyi E, Fülesdi B, Brull SJ: Ipsilateral and simultaneous comparison of responses from acceleromyography- and electromyography-based neuromuscular monitors. Anesthesiology 2021; 135:597–611

The use of paralytics in anesthetic practice is and has been ubiquitous for over 80 years. Many studies have demonstrated that as many as 60 percent of paralyzed patients remain partially paralyzed even after antagonism (“reversal”) with either neostigmine or sugammadex.2  Most of our qualitative, subjective methods to assess TOF response are simply not sensitive enough to detect residual blockade.2  Enter new quantitative monitors.3-5 There are essentially 2 types of quantitative TOF monitors based on either acceleromyography or electromyography (see figure below).

“Acceleromyography utilizes a tiny sensor called an accelerometer, a common component contained in nearly every smart phone, which is usually attached to the thumb. Depolarization of the ulnar nerve results in contraction of the adductor pollicis, which flexes the thumb, producing an acceleration detected by the sensor”.3  (See figure below) Although more commonly available, we don’t think this technology will survive and is less reliable in neonates and small infants, as small infants may not generate muscle movements forceful enough to reliably be detected by the accelerometer.  Driessen et al.6 found detecting an adequate baseline deflection problematic in one-third of infants studied.  There are 2 problems in patients of all ages: “first, the thumb must be entirely free to move, which precludes monitoring the hand that has been tucked at the patient’s side during surgery. Second it requires measurement of a baseline, unparalyzed TOF ratio (the ratio of the fourth to the first twitch of a train-of-four), which should theoretically be equal to 1, but is often greater than 1, and may be as high as 1.6.  Without this baseline calibration,  interpretation is almost useless”.3  Our take: It’s hard enough to get a pulse oximetry probe on pediatric patients before the induction of anesthesia, adding this one will be very difficult or impossible.

“Electromyography directly measures the compound action potential of the adductor pollicis muscle”.3 (See figure below) It’s like motor evoked potential monitors used in posterior spine surgery. “No movement is required for this measurement to be made. The hand can be tucked at the patient’s side without any significant effect on the electromyogram”.3 Indeed, this may be particularly useful in children because access to the arms is often limited in the operating room and this lack of access is often the reason many use the forehead as a monitoring site. “In addition, a baseline, unparalyzed train-of-four ratio is not required”.3  The Nemes et al. paper4 compares these two modalities and we will discuss the results in a forthcoming PAAD.

Why should we care? Indeed, why is the ASA going to produce guidelines requiring the use of these monitors? Residual blockade results in airway obstruction, atelectasis with hypoxemia, reintubation, postoperative respiratory complications, ICU admission, intraoperative and postoperative awareness with attendant psychological distress. And it turns out residual paralysis is very common in both adults and pediatric patients. Aside from the respiratory complications the psychological effects would be devastating…we don’t think any one of us would want to feel the effects of paralysis or partial paralysis while awake!  The cost of treating these medical complications is substantial, much more than the cost of the monitors and their disposables.  Further, without knowing the extent of paralysis. it is hard to know how much reversal agent is required or if it is even necessary whether using neostigmine or sugammadex.  Think about it: if the quantitative measure shows a TOF = 1, reversal is not necessary. Again, using a quantitative TOF monitor, we can avoid possible complications from an unnecessary reversal agent and/or produce substantial cost savings by allowing lower doses of reversal agents.

Almost all the studies looking at these monitors in the OR, PACU, and ICU have been done in adults.  Pediatric studies are urgently needed. 

Myron Yaster MD, Lynne Maxwell MD, and Debra Faulk MD

“(Upper left) An example of an acceleromyograph showing stimulating electrodes over the ulnar nerve and the accelerometry sensor on the thumb. (Upper right) An example of a disposable electromyograph electrode. This is the electrode used in the study by Nemes et al.”3


1.           Faulk DJ, Austin TM, Thomas JJ, Strupp K, Macrae AW, Yaster M: A Survey of the Society for Pediatric Anesthesia on the Use, Monitoring, and Antagonism of Neuromuscular Blockade. Anesth Analg 2021; 132: 1518-1526

2.           Brull SJ, Kopman AF: Current Status of Neuromuscular Reversal and Monitoring: Challenges and Opportunities. Anesthesiology 2017; 126: 173-190

3.           Bowdle A, Michaelsen K: Quantitative Twitch Monitoring: What Works Best and How Do We Know? Anesthesiology 2021; 135: 558-561

4.           Nemes R, Lengyel S, Nagy G, Hampton DR, Gray M, Renew JR, Tassonyi E, Fülesdi B, Brull SJ: Ipsilateral and Simultaneous Comparison of Responses from Acceleromyography- and Electromyography-based Neuromuscular Monitors. Anesthesiology 2021; 135: 597-611

5.           Naguib M, Brull SJ, Kopman AF, Hunter JM, Fülesdi B, Arkes HR, Elstein A, Todd MM, Johnson KB: Consensus Statement on Perioperative Use of Neuromuscular Monitoring. Anesth Analg 2018; 127: 71-80

6.           Driessen JJ, Robertson EN, Booij LH: Acceleromyography in neonates and small infants: baseline calibration and recovery of the responses after neuromuscular blockade with rocuronium. Eur J Anaesthesiol 2005; 22: 11-5