Electromyographic TOF Twitch Monitors Shown Closest to Manual Palpation

Author: Michael Vlessides

Anesthesiology News

Although peripheral nerve stimulation is commonly used to assess neuromuscular transmission when dosing neuromuscular blocking agents and reversal drugs, quantitative twitch monitors have been shown to be more accurate than palpation for measuring the train-of-four (TOF) ratio.

But which type of monitor—mechanomyography, acceleromyography or electromyography—is most accurate?

New research has concluded that the three techniques have substantial agreement, but electromyography most closely emulates manual palpation and acceleromyography frequently underestimates twitch counts.

Monitors Compared

“There are a lot of data out there with respect to quantitative train-of-four assessment tools,” said Kelly Michaelsen, MD, PhD, an anesthesiology resident at the University of Washington’s UW Medicine Health System in Seattle. “Nevertheless, there are limited data comparing these assessment methods to manual palpation.

As Dr. Michaelsen explained, mechanomyography—the gold standard of twitch assessment in the 1960s—uses direct-force measurement of the thumb pressing on a sensor. “These devices are insanely cumbersome and bulky to use,” she said. “They’re no longer commercially available because they’re just too challenging to use.”

Acceleromyography may be the most widely used quantitative technique today, but it too has disadvantages. “The biggest drawback to acceleromyography is that it requires the thumb to be out and freely mobile, which can be difficult in cases where the arms may be tucked,” Dr. Michaelsen said.

To assess the relative accuracy of the three methods, Dr. Michaelsen and her colleagues enrolled 70 patients undergoing general anesthesia with neuromuscular blockade in the prospective, observational trial.

“The patients were equipped with as many devices as we could put on them,” Dr. Michaelsen said. In each case, the investigators compared manual palpation with mechanomyography, acceleromyography and/or electromyography. TOF was measured every five minutes, except after drug administration or reversal.

Substantial Agreement Found

A total of more than 1,600 measurements were performed. The team used Cohen’s kappa statistic to compare the quantitative devices with manual palpation, where a value of 1.0 indicated complete agreement and 0.0 indicated no agreement between measurement techniques.

As Dr. Michaelsen reported at the 2019 annual meeting of the International Anesthesia Research Society (abstract F201), substantial agreement was found between the three quantitative assessment techniques and manual palpation (overall kappa >0.60). Specifically, electromyography yielded a kappa value of 0.80, followed by mechanomyography at 0.67, and acceleromyography at 0.63.

“It was reassuring to see that these devices are, in fact, working,” Dr. Michaelsen noted.

Even though the three devices largely agreed with one another, there were occasions when they differed (Table). “We wanted to try to understand where the differences existed between manual palpation and the devices themselves,” Dr. Michaelsen said.

“With acceleromyography, we often have a fewer number of twitches as compared with manual palpation,” Dr. Michaelsen noted. “By comparison, this is the opposite of what we saw with mechanomyography.”

Indeed, acceleromyography yielded 36% fewer twitches than manual counts, compared with 12% for electromyography and 9% for mechanomyography. Conversely, mechanomyography measured 31% more twitches than manual palpation, compared with 15% for electromyography and 3% for acceleromyography.

“These differences may be clinically significant, although we haven’t explored it further,” Dr. Michaelsen said. It is likely that the higher twitch counts obtained with mechanomyography are due to the device’s greater sensitivity, she added.

“In conclusion,” Dr. Michaelsen said, “we do see substantial agreement between the three devices and manual palpation, which is what we expected. Nevertheless, the electromyography device most closely resembles manual palpation, whereas acceleromyography frequently underestimated twitch counts. Future areas of study include comparing outcomes after use of these various devices as well as comparing other available devices.”

Session co-moderator Michael S. Avidan, MD, a professor of anesthesiology and surgery at Washington University School of Medicine in St. Louis, questioned the timing of the study. “You noted there are more ways than ever to reverse neuromuscular blockade, but why do we need to know this when now we have sugammadex, which allows us to completely reverse neuromuscular blockade? Was there not more of an urgent need before than there is now for this research?”

Dr. Michaelsen replied that financial considerations regarding sugammadex played an important role in the decision. “One of the primary reasons would be an email I recently received from my hospital … saying sugammadex now costs 10 times more than neostigmine. I also think there is some benefit to using the appropriate technique based on your patient’s situation.”