• Xavant Nerve Stimulator
  • Xavant Nerve Stimulator
FAQ

Why monitor NMBA's?

  • Even moderate Post Operative Residual Curarization (PORC) decreases chemoreceptor sensitivity to hypoxia.
  • PORC associated with impairment of muscles of upper pharynx and upper esophagus leading to regurgitation and aspiration.
  • Reduced upper airway volume or partial airway collapse
  • Significant risk factor for development of pulmonary complications – increased morbidity and mortality
  • Research shows that almost half of anesthesia related deaths are directly or indirectly attributable to PORC.

When are TOF, DB, PTC , Twitch and Tetanus used?

Stimulation Pattern

Onset of Action

Deep NMB Moderate NMB Neuromuscular Recovery
TOF Suitable Not Suitable Suitable Conditionally Suitable
TOF with objective monitoring Suitable Not Suitable Suitable Suitable
DB Conditionally Suitable Suitable Not Suitable Conditionally Suitable
PTC Conditionally Suitable Suitable Not Suitable Not Suitable
Tetanus Not Suitable Not Suitable Not Suitable Not Suitable


Which mode provides a reliable indication for recovery?

  • TOF with objective monitoring

What clinical indications of PORC are detectable at which objectively monitored TOF ratios?

TOF Ratio

Inability to sustain headlift for > 5 sec [n=12]

Inability to swallow normally [n=12] "Fade Visible" [n=12]
0.5 1 10 1
0.8 0 7 0
1.0 0 1 0

Eikermann et al. AM J Repir Crit Care Med. 2007;175:9-15
Berg H et al. Acta Anaesthesiol Scand. 1997; 41:1095-1103
Murphy GS et al. Anesth Analg. 2008; 107:130-137

What clinical outcomes could be expected at which TOF ratios?

Monitoring TOF Ratio on adductor pollicis

Ratio: 0.5

Ratio: 0.8 Ratio: 1.0
Pharyngeal function (Swallowing) Significantly Impaired Impaired Mostly Normal
Integrity of upper airway Significantly Impaired Impaired Mostly Normal
Hypoxic respiratory response Frequently Impaired Mostly Normal Normal

Eikermann et al. AM J Repir Crit Care Med. 2007;175:9-15
Berg H et al. Acta Anaesthesiol Scand. 1997; 41:1095-1103
Murphy GS et al. Anesth Analg. 2008; 107:130-137

What is attributed to cause the fading observed during TOF or DB?

  • The reason for this is attributed to the binding of non-depolarizing neuromuscular blocking agents to presynaptic acetylcholine receptors, resulting in inhibition of the recruitment of Ach from the reserve pool.

What makes Sugammadex more effective than other reversal agents?

  • A minimum of spontaneous recovery does not need to be present before Sugammadex can be administered. Even the most deep neuromuscular blockade can be reversed rapidly within 1-2min.
  • Fewer autonomic side effects are anticipated with Sugammadex.

The Stimpod NMS450 continually offers a reading of 100% even though the NMBA was injected – Why?

  • It could be because a depolarizing NMBA was injected.

The device showed that the patient was fully recovered, suddenly the TOF ratio dropped again – Why?

  • It could be that the core body temperature of the patient dropped again after recovery was observed. 
  • In order to prevent PORC patients should only be assessed for residual NMBA once core has reached a temperature of greater that 36 °C.

Heier T, Caldwell J E, Impact of Hypothermia on the Response to Neuromuscular Blocking Drugs, Anesthesiology 2006; 104:1070-80


The device showed a TOF ratio of 0% while the patient still had good clinical signs of not being paralyzed?

  • The location of the electrode could have moved. 
  • It could be that the supramaximal stimulating current was not first established. 
  • It could be that the electrode dried out. 
  • It could be that the polarity of the electrodes were reversed.

I am monitoring the orbicularis oculi during NMBA but I always see a TOF ratio of 100% - Why?

  • When facial muscles are stimulated there is a risk of direct muscle stimulation.

Why do we see TOF values greater than 100%?

  • When monitoring TOF prior and post NMBA, ratios of > 100% are often noticed (As high as 147%)
  • Effect ascribed to mobilization of presynaptic acetylcholine in NMJ after T1 promoting release in synaptic cleft during subsequent stimulations.
  • Both Stimpod NMS450 and TOF-Watch SX (uncalibrated) show values greater than 100%.

Suzuki T, Fukano N, Kitajima O, Saeki S, Ogawa S, 2006.’Normalization of acceleromyographic train-of-four ratio by baseline value for detecting residual neuromuscular block.’ British Journal of Anaesthesia; 96:44–7


I have compared the readings of two Stimpods, one on each arm of a patient with both set at 50mA and they give different TOF ratios – Why?

  • The cathodes (negative electrodes) could be placed at different distances from targeted nerves.
  • It could be that the supramaximal stimulating current was not first established for both stimulators independantly.
  • It could be that one or more electrodes dried out.
  • It could be that the polarity of some elecrtodes were reversed.
  • It is a known fact that stimulation of the ulnar nerves on one patient's two arms would results in different TOF ratios if all other setup factors are significantly similar.

Will a TOF ratio of >90% reliably exclude PORC?

  • It remains unclear as to what acceleromyography TOF ratio is necessary to exclude clinically significant PORC.
  • Glenn et al PORC reduced in patients monitored with acceleromyography TOF of >90%.
Claudius, C., Viby-Mogensen, J., 2008. ‘Acceleromyography for Use in Scientific and Clinical Practice - A Systematic Review of the Evidence’. Anesthesiology; 108:1117-40
Glen S., Murphy, M. D., etc. 2011. ‘Intraoperative Acceleromyography Monitoring Reduces Symptoms of Muscle Weakness and Improves Quality of Recovery in the Early Postoperative Period

Why does the Stimpod NMS450 not require a preload device?

  • In an attempt to increase reliability of the TOF-Watch accelerometer a preload device was introduced. According to Claudius, Viby-Mogensen (2008), there is insufficient evidence to confirm or deny that the application of a preload will increase the precision of acceleromyography.
  • The preload device attempts to restrict the movement of the thumb to one dimension. With a three dimensional accelerometer this is not necessary.

Claudius, C., Viby-Mogensen, J., 2008. 'Acceleromyography for Use in Scientific and Clinical Practice - A Systematic Review of the Evidence'. Anesthesiology; 108:1117-40


Is acceleromyography the Gold Standard?

  • No. Mechanomyography is the established standard to measure neuromuscular block (NMB), however it is impractical in clinical setting and is only reserved for research purposes.

What is three dimensional acceleromyography and why should it provide a reliable answer?

  • Acceleromyography is based on Newton’s second law of motion, stating that: F (force) = m (mass) X a (acceleration).
  • Acceleromyography should be interchangeable with mechanomyography if the mass (in this case the mass of the thumb) is constant.
  • Surprisingly acceleromyography cannot be used interchangeably with mechanomyography in pharmacodynamic studies.
  • One difference between mechanomyography and acceleromyography is that the contractions observed during acceleromyography involve a three-dimensional movement involving three joints, frictional forces, and deformation of tissues.
  • Accelerometer of the TOF-Watch device utilizes a 1-dimensional accelerometer losing much of the information of the complex 3-dimensional movement.
  • The Stimpod NMS450 on the other hand uses three accelerometer sensors, each positioned perpendicular on the other two, to enable the device to measure acceleration in three dimensions.
  • A specialized algorithm processes the information provided by the three accelerometers and provides a value representing the size of the vector of the three dimensional movement.
  • Xavant is of the opinion that this feature makes acceleromyography more reliable and should bring its response curves more in line with mechanomyography.
Claudius, C., Viby-Mogensen, J., 2008. ‘Acceleromyography for Use in Scientific and Clinical Practice - A Systematic Review of the Evidence’. Anesthesiology; 108:1117-40
Baillard, C., Bourdiau ,S., Le Toumelin, P.,Ait Kaci, F., Riou, B., Cupa, M and Samama, M. 2004. ‘Assessing Residual Neuromuscular Blockade Using Acceleromyography Can Be Deceptive in Postoperative Awake Patients’, Anesthesia Analgesia; 98:854–7

Are normalization algorithms reliable?

  • TOF-Watch and TOF-Watch S will calculate and display T2/T4 and if this value exceeds 100% it would simply limit the display to 100%.
  • TOF-Watch SX has a well implemented normalization porcedure but it takes 20 minutes to perform.
  • Xavant is currently researching a practical way for implementing a normalization algorithm.

Claudius, C., Viby-Mogensen, J., 2008. ‘Acceleromyography for Use in Scientific and Clinical Practice - A Systematic Review of the Evidence’. Anesthesiology; 108:1117-40


Stimpod switched off during surgery – Why?

  • The Stimpod has an automated switch off function which is activated if the device has not been in use for more than 10 minutes.

Stimpod showed “Open Circuit Detected” with the ECG clips are connected - Why?

  • It could be because electrodes have dried out to the extent that they resemble an open circuit.
  • There could be a problem with the cable.
  • There could be a problem with the device.

Stimpod showed “Insert Cable” but the cable is inserted - Why?

  • There could be a problem with the cable.
  • There could be a problem with the Stimpod.

Stimpod showed “No Accel” – Why?

  • The Stimpod is in ‘TWI’ mode. In this mode the device does not communicate with the accelerometer.
  • The Stimpod is in ‘TET’ mode. In this mode the device does not communicate with the accelerometer.
  • There could be a momentary interference that interrupted communication between the accelerometer and Stimpod.
  • There could be a problem with the accelerometer cable.
  • There could be a problem with the Stimpod.

Is Stimpod Immune to Cauterization?

  • No. In order for Stimpod to receive its certification it has to pass certain tests to prove its immunity to electromagnetic interference (EMI) and electrostatic discharge (ESD). All versions of Stimpod have passed this criteria.
  • In beginning 2012 the Stimpod was redesigned in order to increase its immunity to EMI and ESD.
  • The new version of Stimpod has proven its immunity to cauterization in the OR under general conditions, however, extreme intensities of cauterization could still impact of the accuracy and functioning of the Stimpod.
  • All devices in the OR are generally affected by extreme intensities of cauterization.

A 'Burn' mark was reported under an ECG electrode - what are the possible causes?

  • Erythema (skin condition characterised by redness or rash due to congestion of underlying capillaries).
  • Adverse drug reaction.
  • Pressure necrosis
  • Disease
  • Allergic and irritant contact dermatitis.
  • Micropunctate burns.
  • Electrode burns accepted as a reality of the perioperative environment, especially in the presence of cauterization.

What must be known to properly assess a “Burn”?

  • Patient specific considerations
  1. Underlying medical conditions or medication that may predispose a patient to skin injury.
  2. Use of cosmetic products that may not have been removed prior to surgery and applications of the electrodes
  3. Known allergies or contact dermatitis
  • Skin preparation i.e. solution used to clean the skin underneath the electrode.
  • Condition of the electrode itself.
  • The exact settings used on the device in question.
  • The setup of the device for use.
  • Duration of use of the device.
  • Surgical factors i.e. setup and use of electro-surgery units (ESU). As well as possible spillage of surgical cleaning solution.
  • General electrical safety of the theatre where the incident took place.
  • Other electrical devices that may have been in use during the procedure.
  • Photos of the “wounds” as well as of the electrodes
  • If possible, the electrodes must be forwarded to Xavant for investigation
  • Any other information pertinent to the investigation
  • If possible, patient follow ups (photos etc) if authorised.

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