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Original Article

Comparison between the Datex-Ohmeda M-NMT® module and a force-displacement transducer for monitoring neuromuscular blockade

Motamed, C.; Kirov, K.; Combes, X.; Duvaldestin, P.

Author Information
European Journal of Anaesthesiology: June 2003 - Volume 20 - Issue 6 - p 467-469


The Datex-Ohmeda neuromuscular transmission module (M-NMT)® is a new monitor and part of the AS/3® anaesthesia monitor. The M-NMT® module incorporates a mechanosensor that is a piezoelectric polymer attached to the hand. The piezoelectric probe produces a signal directly proportional to the motion of the thumb caused by stimulating the ulnar nerve. The measures of neuromuscular blockade are the train-of-four (TOF) ratio, TOF count, single-twitch amplitude, double-burst stimulation and post-tetanic count. The module takes <1 min to initialize, and there is no need for preload or immobilization of the hand. We aimed to assess the agreement between this device and a force transducer, which is still the standard method of monitoring neuromuscular blockade.


The study was approved by the local Research Ethics Committee. We sought informed consent from patients scheduled for elective surgery that would require general anaesthesia and neuromuscular blocking agents.

Anaesthetic management

Premedication was at the discretion of the anaesthetist. We had free access to both upper limbs. Anaesthesia was induced with fentanyl 3–5 μg kg−1, propofol 3–4 mg kg−1 and muscle relaxants were not given before the trachea was intubated. Anaesthesia was maintained with isoflurane and nitrous oxide in oxygen.


A force transducer (Entran®; Entran SA, Les Clayes-Sous-Bois, France) with a preload of 300 g was attached to one arm and the M-NMT® monitor to the other arm. The ulnar nerve was stimulated at the wrist using surface electrodes and neuromuscular blockade assessed from the corresponding adductor pollicis. Supramaximal (60 mA) TOF stimulation was delivered every 12 s. When readings from both devices had been stable for 3 min, rocuronium 0.2 mg kg−1 was injected intravenously over 5 s.

The following variables were recorded from the force transducer: the maximal neuromuscular effect; the onset of maximal blockade, defined as the time from injection to maximum blockade; and the duration to clinical recovery, defined as the time to reach 25% of first twitch height of a TOF delivered every 12 s (15 patients), or of double-burst stimulation delivered every 20 s (15 patients). The thenar temperature of both hands was monitored by surface probes and the hand maintained >32°C with a warming blanket during the whole procedure.

Statistical analysis

Repeatability was assessed for each monitor and for each type of stimulation, 10 times, by two consecutive measurements, between 10 and 95% recovery. Bias and limits of agreement were calculated as described by Bland and Altman [1]. For each subject, the difference between pairs of corresponding measurements was plotted against the average. The corrected SD was calculated from repeated measurements. The upper and lower limits of agreement were defined as 2 SD either side of the mean. Analysis was by Stat-View® software (v.4.5, Abacus Concept, Berkley, CA, USA).


Thirty consecutive patients gave informed consent to be recruited into the study. There were 19 females and 11 males aged 56 ± 12 yr, weight 64 ± 10 kg, height 165 ± 9 cm (means ± SD). There were no adverse events related to anaesthesia or to the monitoring devices. Valid data were collected from all patients.

The force transducer recorded a mean onset time of 124 ± 42 s, a maximum blockade of 95 ± 5%, and a mean time to clinical recovery of 20 ± 8 min. The coefficient of repeatability for the force transducer was 1.64% (95% CI 1.58–1.70%) for the recovery of the TOF ratio, and 2% (95% CI 1.9–2.1%) for the recovery of double-burst stimulation. The NMT module had a coefficient of repeatability of 1.9% (95% CI 1.8–2.0%) for the recovery of the TOF ratio, and 2.1% (95% CI 2.0–2.2%) for the recovery of double burst stimulation.

The bias for the recovery of the train-of-four ratio was +1.3% (95% CI 1.21–1.39%), i.e. the NMT module measured slightly higher than the force transducer, with an upper limit of agreement of 14.2% and a lower limit of agreement of −12.9% (Fig. 1). The bias for the recovery of the double-burst stimulation ratio was +1.09% (95% CI 0.89–1.29%) with an upper limit of agreement of 17% and a lower limit of −16% (Fig. 2).

Figure 1.
Figure 1.:
Plot of the difference between TOF ratios, measured from the NMT monitor and force transducer, plotted against the logarithm of the average of the two measurements (Bland and Altman [1]) during the recovery of neuromuscular blockade (15 patients). The upper and lower limits of agreement are ±2 SD of the mean difference.
Figure 2.
Figure 2.:
Plot of the difference between the double-burst stimulation ratio, measured from the NMT monitor and force transducer, plotted against the logarithm of the average of the two measurements (Bland and Altman [1]) during the recovery of neuromuscular blockade (15 patients). The upper and lower limits of agreement are ± 2 SD of the mean difference.


A research prototype of a piezoelectric mechanosensor, similar to the one used in the Datex module, has been evaluated [2], but it did not become commercially available. We have shown that the Datex M-NMT® module gives repeatable measurements, and compares well with a force transducer for measuring the TOF ratio and double-burst stimulation during recovery. The limits of agreement between the two devices mean that the module is suitable for clinical practice but, with limits of >10%, the module is unsuitable for the more precise and accurate needs of research.

The module can measure post-tetanic count, but we did not study it because double-burst stimulation can also be used to assess profound neuromuscular blockade [3]. We did not study the onset of blockade because rocuronium produces too rapid an onset for assessment of coefficient of repeatability.

The piezoelectric device detects movement between the thumb and index finger. This is indirectly related to the force of contraction, so it is not necessary to immobilize the hand. Unlike accelerography [4], the N-NMT® does not generate a TOF ratio >1. The stabilization time is shorter than with electromyography [5].

From previous work using vecuronium [6], we think it unlikely that our results could have been influenced by different responses between the arms. We conclude that the Datex-Ohmeda M-NMT® is adequate for clinical practice, but for research into neuromuscular blockade, the greater accuracy of the standard force transducer is still necessary.


1. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 8: 307-313.
2. Kern SE, Johnson JO, Westenskow DR, Orr JA. An effectiveness study of a new piezoelectric sensor for train of four measurement. Anesth Analg 1994; 78: 978-982.
3. Kirkegaard H, May O. Double burst stimulation for monitoring profound neuromuscular blockade: a comparison with posttetanic count and train of four. Acta Anaesthesiol Belg 1992; 43: 253-257.
4. Jensen E, Viby Mogensen J, Bang U. The accelerograph: a new neuromuscular transmission monitor. Acta Anaesthesiol Scand 1988; 32: 49-52.
5. Viby Mogensen J, Engbaek J, Erikson LI, et al. Good clinical research practice (GCRP) in pharmacodynamic studies of neuromuscular blocking agents. Acta Anaesthesiol Scand 1996; 40: 59-74.
6. Merle JC, Jurczyk M, Dhonneur G, Ruggier R, Duvaldestin P. Onset of neuromuscular block is the same if the ipsilateral limb or contralateral limb to the injection site is used for monitoring. Br J Anaesth 1995; 74: 333-334.

ANAESTHESIA AND ANALGESIA; neuromuscular blockade; MONITORING; PHYSIOLOGICAL; monitoring; intraoperative

© 2003 European Society of Anaesthesiology