To the Editor:—
A recent editorial in this journal has suggested that
“… it is time to … introduce objective neuromuscular monitoring in all operating rooms, not just those occupied by researchers and aficionados of muscle relaxants… . objective neuromuscular monitoring is an evidence-based practice and should consequently be used whenever a nondepolarizing neuromuscular blocking agent is administered… . there are strong reasons to believe that its use can improve patient outcome.”1
Our department strongly supports this position and recently introduced one such monitor, the S/5 M-NMT NeuroMuscular Transmission Module (Datex-Ohmeda, Madison, WI) into all of our operating rooms. Although we are generally pleased with this device, we have identified one aspect of the user interface that delivers a confusing message to the clinician. We think this issue may have clinical ramifications and is important enough that an alert to other users of this module is warranted.
The M-NMT’s transducer (MechanoSensor; Datex-Ohmeda) consists of a strip of piezoelectric polymer that is applied to a boomerang-shaped spring, which is placed between the thumb and the forefinger. Mechanical movement of the thumb results in a redistribution of the electrical charge on the sensor membrane, a change that can be quantitated. The M-NMT is a movement sensor, and the method is not acceleromyography. To differentiate this monitoring approach from acceleromyography, the company refers to their monitor as using kinemyography. The calculated train-of-four (TOF) ratio is displayed numerically (as a percent) on the patient monitor (S/5 Anesthesia Monitor; Datex-Ohmeda). In addition, the unit displays a bar graph of all four responses so that this ratio may be appreciated visually.
Unfortunately, the bar graph and the displayed TOF ratio may appear to disagree (fig. 1). It is a common occurrence during recovery of nondepolarizing neuromuscular block to encounter a numeric TOF ratio indicative of inadequate recovery (e.g. , < 0.70) at a time when all four responses displayed graphically appear identical. The clinician who merely glances at the bar graph without reading the displayed value may incorrectly assume that neuromuscular recovery is complete when this is clearly not the case. The explanation for this disparity between the displayed graphic and numerical values follows.
In the absence of neuromuscular block, repeated rapid indirect muscle stimulation produces a steady increase in mechanical twitch height. This is a function of altered muscle contractility and is unrelated to changes in neuromuscular transmission. This phenomenon is known as the “staircase effect.”2Therefore, after 10–15 min of TOF stimulation control, T1 (twitch height) may increase by 50–100%. In the research setting, this is why investigators wait several minutes before doing a final calibration of their baseline values. However, in the clinical setting, anesthesiologists rarely if ever take this effect into account. The usual sequence is (1) induce anesthesia, (2) calibrate the M-NMT, and (3) administer muscle relaxant, all within 2 min or less. This failure to establish stabile baseline values usually results in T1 values far in excess of 100% when spontaneous or induced recovery is complete. Therefore, at the end of the case, the sensor might record the following values: T1 = 165%, T2 = 145%, T3 = 130%, and T4 = 120% with a calculated TOF ratio of 0.73. This value will be corrected displayed numerically on the monitor screen. However, the bar graph attempts to display the absolute value (relative to control) of all four responses. Unfortunately, all values greater than 120% are truncated or “chopped off” at the top. The result in the above hypothetical case is a bar graph displaying a total absence of fade.
This situation is unsatisfactory for several reasons. Although the T1 value as a percent of control is only displayed in the monitor’s display trend mode, values greater than 100% will be confusing to most clinicians. When data that “don’t make any sense” are presented to the clinician, there is an understandable tendency to conclude that there must be something wrong with the monitor. If this happens often enough, the offending unit will eventually be destined to occupy to a dusty shelf in a back storage room.
Of greater concern, the conflicting information presented by the monitor may contribute to improper clinical decisions. Some clinicians may opt to believe the bar graph rather than the numerical TOF ratio. The result is failure to antagonize residual block when reversal is clearly indicated. We have found the M-NMT module to be a useful addition to our monitoring armamentarium. However, its current user interface needs to be rethought by the manufacturer.
New York Medical College, Valhalla, New York. firstname.lastname@example.org