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Neuromuscular Research Group (NRG), Department of Anesthesiology, Université de Montréal, Canada
To the Editor:
We read with interest the article by Dahaba et al. (1), which evaluated the clinical use of the M-NMT (Datex-Ohmeda, Helsinki, Finland) neuromuscular monitor. However, we have several technical comments and believe that their conclusion that the M-NMT could be a reliable clinical monitor in daily anesthesia practice is not supported by their findings.
1. Information about the physical basis of the M-NMT sensor is incomplete. Dahaba et al. describe it as a "piezoelectric motion sensor measuring the signal generated from the bending and deformation of a piezoelectric sensor wafer strip." What does that mean? Does it measure movement, and, if so, the extent of the movement, the amplitude of the evoked signal being proportional to the actual extent of the movement of the thumb, or does it simply measure acceleration like acceleromyography?
Paloheimo (2) at the Datex-Ohmeda home page describes it as "kinemyography (KMG)" and compares it in one patient with mechanomyography. In this patient, KMG measurements are more sensitive than mechanomyographic measurements, which is in contrast to Dahaba’s findings, the M-NMT monitor being less sensitive than mechanomyography during recovery. Recently, Schwaerzler et al. (3) presented an abstract at the 7th International Neuromuscular meeting, which showed in 20 patients that TOF-ratios of more than 0.7 during recovery of neuromuscular blockade after rocuronium derived using the Datex-OhmedaTM Mechanosensor and mechanomyography of the thumb can be used interchangeably. The term "mechanosensor" is confusing because it gives the impression that, like mechanomyography, the force is measured. The question remains: what does it actually measure?
2. In a previous study (4), Dahaba et al. presented another piezoelectric train-of-four neuromuscular monitor, the illustration of which is not very different from the current M-NMT mechanosensor. Do the authors know in which way these two devices are related? And do they have any explanation why in the former study, TOF ratio of 0.7 or greater derived using both methods could not be used interchangeably in contrast to the findings in the current study?
3. The most important problem we have with the M-NMT sensor is the fact that only TOF ratio of 0.7 or higher can be used interchangeably with mechanomyography. Although this is better than previously reported for acceleromyography (5), the rest of the pharmacodynamic comparisons with mechanomyography are disappointing. A mean difference of -0.3 min and limits of agreement of 0.4 and -0.2 min for onset time with a mean onset time of 1.5 min can matter clinically and is not superior to other methods such as acceleromyography or electromyography. Furthermore, it would be interesting to determine the agreement with neuromuscular blocking agents of longer onset time, such as cisatracurium. Even more disappointing is the fact that there were important differences between the M-NMT sensor and mechanomyography for the recovery times, such as DUR10or DUR 25, which are important for maintaining surgical neuromuscular blockade and determination of the time for "rescue" reversal. This finding contradicts the conclusion that the M-NMT sensor could be a reliable clinical monitor in daily anesthetic practice. The mere fact that it is integrated into the anesthesia monitor is not enough to qualify it as reliable or even useful.
We agree that the M-NMT sensor can be used to determine a TOF ratio of more than 0.7 during recovery of neuromuscular block. However, the device cannot be used to determine accurately neuromuscular blockade for surgery (a neuromuscular blockade of 10–25 % of control twitch height) or early recovery of neuromuscular blockade and can therefore not be considered as a reliable clinical monitor in daily practice.
References
Department of Anaesthesiology and Intensive Care Medicine, Karl Franzens University, Graz, Austria
In Response:
1. We do indeed understand that the technical details of the different principles on which different neuromuscular monitors are based could sometimes be rather confusing. Allow me to start by first clarifying what the M-NMT is not. The M-NMT is indeed not an acceleromyograph, since it does not measure acceleration by any means. Furthermore, Dr. Hemmerling used, in his title and in his letter, the same terms the manufacturer used to commercially describe the M-NMT as a "kinomyograph," "KMG," and "mechanosensory." Such terms we have completely refrained from using in our article, as they could obviously imply that the M-NMT is some sort of a force displacement mechanomyograph. Anesthesiologists who are familiar with different neuromuscular monitors would immediately realize that it is not, since it applies no preload on the thumb nor does it quantify any force displacement. As detailed in our article on page 592, the M-NMT uses a piezo strip to generate an electric signal proportional to the degree to which the piezo strip is bent. So when we describe the motion as a "bending motion," it is just that in its simplest form. To clarify matters further, if one would simply take the M-NMT sensor in one’s hand and quickly move the sensor (as a whole), no signal will be generated whatsoever. When the sensor strip is bent or folded, only then a signal will be generated. This clearly demonstrates that the sensor is neither an acceleromyograph nor a force displacement sensor. This is a completely different application and one of the numerous versatile uses of piezo sensors in medicine and other fields of science.
Dr. Hemmerling cites, in references 2 and 3, that when the M-NMT is compared to mechanomyography, it is "more sensitive" and "could be used interchangeably," descriptions which are both in contrast to the title he chose for his letter. Reference 2 is a handpicked selected case on the home page of the manufacturer obviously intended for demonstration purposes. This hardly constitutes a scientific study with an appropriate sample size. Using terms such as "more sensitive" lacks the scientific bases of quantifying how "sensitivity" is being measured. As for reference 3, until the peer-reviewed publication of a full study, we feel that it is always hard to render a judgment based only on an abstract.
2. In reference 4, Dr Hemmerling mentions the ParaGraph® (Vital Signs, NJ), another commercially available neuromuscular monitor which similarly quantifies the neuromuscular block based on the same "bending motion" of a piezo strip. In that regard, the two monitors are similar. Alhough based on the same principle, the piezo sensor of the ParaGraph is attached to the hand by its adhesive pad, while the M-NMT uses, for the piezo strip attachment, the malleable plastic semicircular rings shown in the photo on page 592. This limits the "drift" in recovery from neuromuscular block, as we explained on page 595.
3. Dr. Hemmerling is clearly misquoting our study when he states that "the data could be used interchangeably." As a matter of fact, we stated in the recovery phase, and again in the conclusion on page 595, the exact opposite. We stated that "the limits of agreement are unacceptably wide to allow the values given by the two monitors to be used interchangeably for individual patients," as monitors based on different phenomena would not be expected to yield identical results.
The widely acceptable Bland and Altman statistical analysis (1) that we used in our study considers both monitors equally liable to experimental error, rather than assuming that one of the monitors is the correct true "standard," thus rendered "reliable," while the other is not. Our study showed that M-NMT could be used to indicate the onset time, times to repeat dose administration, and full recovery, and to that effect the M-NMT could be clinically reliable in a daily anesthesia setting. On the other hand, we objectively demonstrated the differences between the two monitors. We strongly feel that how one should regard the significance of these differences should be left entirely to the discretion of each anesthesiologist, who might choose, for instance, to take a parameter difference of 0.3 min into consideration rather than have a bulky mechanomyograph in the operating room.
Reference
This article has been cited by other articles:
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  T. M. Hemmerling and N. Le Brief review: Neuromuscular monitoring: an update for the clinician: [Article de synthese court : Monitorage neuromusculaire : une mise a jour pour le clinicien] Can J Anesth, January 1, 2007; 54(1): 58 - 72. [Abstract] [Full Text] [PDF]
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 T. M. Hemmerling, G. Michaud, S. Deschamps, G. Trager, and Y. Saitoh Monitoring neuromuscular blockade in diabetic patients using electromyography: an opportunity missed Br. J. Anaesth., October 1, 2003; 91(4): 608 - 609. [Full Text] [PDF]
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