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The Datex-Ohmeda M-NMT Module: A Potentially Confusing User Interface

Paloheimo, Markku M.D., Ph.D., F.E.A.A.

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In Reply:—

We thank Dr. Kopman for the user feedback and appreciate the possibility to comment on his reasonable concern about the possible conflict between the bar graph display and numerically expressed train-of-four ratio (TOF%) when the MechanoSensor® (kinemyography) is used to monitor the degree of neuromuscular block with the Datex-Ohmeda (Helsinki, Finland) NMT module (M-NMT). Dr. Kopman suspects that the phenomenon of increased responses is due to the “staircase effect,” a typical feature of the acceleromyographic method (e.g., see page 702 in the October 2005 issue of Anesthesiology;TOF% is > 100% before the block and in full recovery1). It is important to note that the technology in the M-NMT module is different than the acceleromyographic method, as Dr. Kopman correctly states. Equally important is to notice that the staircase effect and the fade in the responses are different phenomena.
The measurement of TOF% begins by pressing the Start-up button on the M-NMT module or in the NMT parameter menu. The monitor will start the measurement by automatically setting the stimulus current (maximum 70 mA). With an unrelaxed patient, the TOF% is 100. Nondepolarizing relaxants cause a fade in the responses, indicated by a lower TOF% and a slope in the bar graph. Depolarizing relaxants result in an equal decrease in all responses, without fade. In deep neuromuscular block, the monitor displays the number of detected responses. What Dr. Kopman finds confusing is that in certain situations, the TOF% and the bar graphs seem to give conflicting information. This situation occurs when the reference level is set so that the bar graphs go over the measurement range, i.e., are “chopped off” when the value exceeds 120%. Therefore, the user cannot see the fade. The chopping of the bar graph in the M-NMT is a user interface design inconvenience. Of utmost importance is to note that the TOF% is the primary source of information, and the bar graphs are an additional visual aid.
The M-NMT development was based on the Datex Relaxograph®, where the default mode of function depended, after the automatic determination of a supramaximal stimulation current, on the determination of an unrelaxed reference value (Tref). During recovery, the relative behavior of the first response (T1%) to this reference value was calculated and displayed in print. It soon became evident that the electromyographic reference baseline shifted to smaller values within the first 15–20 min of anesthesia and usually stayed at this level until full recovery. This was also explained in the Relaxograph User’s Guide published in the 1980s. When force transducers were used with the Relaxograph®, the calibrated baseline tended to grow, and the recovered T1% was well over 100% without fade.2 These phenomena may result from anesthesia-induced increase in muscular blood flow and temperature.3,4 In addition, the T1% is prone to artifacts, e.g., if the position of the sensor shifts.
Because of ample customer feedback, Datex decided that the TOF% would be the default mode of analysis in the M-NMT. The measurement of TOF% does not require a reference level, because it is the percentage of T4/T1 in each TOF stimulation sequence. During module software development, the search for supramaximal current and determination of the mean reference value during Start-up were not separated. I have, when testing the movement sensor in an unrelaxed patient, been able to ascertain a slight increase in the evoked TOF response (1/20 s) and a gradual increase to around 110% during single stimulation at 1 Hz for several minutes, which apparently represents the original “staircase phenomenon.”5 If one presses the Start-up button and restricts the sensor movement during the determination of the reference level, one can demonstrate how the responses shoot over 120% during unrestricted hand stimulation. Such manipulation is, however, against the manufacturer’s instructions of use for the M-NMT.
A solution for the user interface disagreement reported by Dr. Kopman would be to access the service pages of the S/5 monitor and adjust the NMT parameter settings so that both the display of T1% and the automatic reference search are set to OFF. This causes the monitor to perform the supramaximal current search automatically and to display the bar graphs on a relative scale (T1 is scaled to a fixed value and T2–T4 are scaled relative to the T1 value). The stimulation current can also always be set manually from the NMT parameter menu. The TOF% trend is available on the trend pages, and, if one intends to do serious research, original response values in bits are available using the S/5 Collect program on a standard personal computer or laptop. This data can be further processed in, for example, Excel.
Regarding Dr. Kopman’s concern of a possibility of the M-NMT to contribute to improper clinical decisions, we disagree. The users should bear in mind that the TOF% is the primary source of information. The reported phenomenon of the bar graphs “chopping off” at values above 120% is clinically not valid if the measurement is used according to manufacturer’s instructions for use.
In conclusion, it is evident that monitoring the level of neuromuscular block has clinical benefits. Quantitative NMT monitoring facilitates optimal and cost-effective administration of neuromuscular blocking agents, enables follow-up and prediction of recovery, and helps in avoiding residual block. As Dr. Kopman states, with some understanding of the principles in NMT monitoring, the M-NMT can be an important addition to the monitored parameters.
Markku Paloheimo, M.D., Ph.D., F.E.A.A.
GE Healthcare Finland Oy, Finland; Eye Hospital, Helsinki University Hospital, Helsinki, Finland.
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