Acceleromyography is the form of quantitative twitch monitoring that has been the most widely used over many years. It is a simple concept. An accelerometer sensor is attached to the thumb. When the ulnar nerve is stimulated, the thumb twitches, and the accelerometer measures the acceleration. According to Sir Isaac Newton (a reliable source), force is directly proportional to acceleration. However, in this case, the force is not constant, it is changing during the course of a twitch; remember that, it will be important later.
What we really want to know is the force, not the acceleration. When we measure force with a “gold standard” mechanomyograph in an unparalyzed patient, the train-of-four ratio is 1.0. The peak force of thumb movement is nearly identical for each of the 4 twitches. However, when we measure the train-of-four ratio with acceleromyography, the train-of-four ratio is often greater than 1.0, as shown in Figure 1. This is often referred to as “overshoot”, because the fourth twitch, T4, accelerates faster than the first twitch, T1. This is important because in assessing recovery or reversal, we are looking for a train-of-four ratio of at least 0.9. But if the baseline, unparalyzed train-of-four ratio is more than 1.0, then we need to look for a train-of-four ratio that is 0.9 times the baseline train-of-four ratio, e.g. 0.9x1.3=1.2. If we settle for a train-of-four ratio of 0.9 in a patient with a baseline, unparalyzed train-of-four ratio greater than 1.0, that patient will have residual neuromuscular blockade.
Over the years people have tried to get rid of the overshoot in the acceleromyography train-of-four ratio. They have tried using devices attached to the hand to add some “preload” to the thumb or used acceleromyography sensors with “multiaxial” sensing capability. But the overshoot has remained.
Kopman and Brull recently published an article in Anesthesiology demonstrating with mechanomyography and acceleromyography (on the same hand) that acceleration increases with each twitch in the train-of-four, while the peak force remains the same for each twitch. Another way to think of this is that the impulse is increasing with each twitch, where the impulse is the change in force over time. The peak force is developing slightly more quickly with each twitch; in other words, the acceleration is greater with each successive twitch. What the underlying mechanism of this is, we don’t know. Regardless of the mechanism, the key lesson here is that overshoot is “baked in” to acceleromography, as we pointed out in an editorial that accompanied the article by Kopman and Brull. By the way, we have looked at our mechanomyography train-of-four waveforms from unparalyzed patients and found the same increase in acceleration during the train-of-four as found by Kopman and Brull.
There are at least 3 serious shortcomings with acceleromyography:
There is often a lot of overshoot in the acceleromyography train-of-four ratio.
The precision of acceleromyography is poor. We demonstrated this in a recent study of three different acceleromyographs. In comparison to electromyography or mechanomyography, there was a large amount of scatter in the unparalyzed train-of-four ratio (Figure 2).
Acceleromyography requires that the thumb be free to move, effectively precluding tucking the arm with the acceleromyograph sensor.
Before electromyography with single piece electrode arrays was readily available, we had to put up with acceleromyography if we wanted quantitative twitch monitoring. Now there are several electromyographs with single piece electrode arrays, that provide us with quantitative twitch monitoring without overshoot, and with much better precision than acceleromyography. It’s time to say goodbye to acceleromyography. If you already have acceleromyographs, don’t throw them away. Measure the unparalyzed train-of-four ratio after induction of anesthesia but before administering a neuromuscular blocking drug. Normalize the results by dividing the train-of-four ratio by the baseline train-of-four ratio. But, if you are going to buy new or replacement quantitative twitch monitors, electromyography is currently the best choice.