'Train of Four' monitoring: how to carry out the procedure

The purpose of this guideline is to provide guidance of how to correctly carry out Train of Four (TOF) monitoring on a patient within the ICU environment taking into account nursing considerations and indications/contraindications for use.


Train of Four (TOF) monitoring is a widely used term for the peripheral nerve stimulation used in neuromuscular blockade monitoring (Fergusson, 2008; Yentis et al., 2009; Luckett and Hays, 2013b; Kudchadkar, et al, 2016). Neuromuscular blocking agents (NMBAs) or ‘muscle relaxants’ are commonly used as an adjunct to therapy whilst in an intensive care setting (Jones, 2003). NMBAs (Muscle relaxants) support the care administered to patients as they can facilitate endotracheal intubation and mechanical ventilation, ensure adequate neurological protection for a head-injured patient and provide surgical relaxation (Loyola and Dreher, 2003; Burmester and Mok, 2005; Luckett and Hays, 2013a). 

Critically ill patients in intensive care are at risk of developing critical illness polyneuropathy and myopathy which can be exacerbated by the use of NMBAs. The symptoms can vary in presentation from mild muscle weakness to severe quadriplegia and ensuing respiratory failure. Therefore, NMBAs are not to be administered lightly and should only be used when necessary (Luckett and Hays, 2013b).

Within intensive care, if a patient requires prolonged muscle relaxation, vecuronium is commonly used as it has less cardiovascular effects compared to other non-depolarising NMBAs (Gage, 2011). Vecuronium is also associated with having less histamine release than other NMBAs as it is an aminosteroid, making it more favourable for longer-term use (Jones, 2003). The depth of block must be monitored as certain groups of patients will require differing dose ranges: patients in renal failure will require a reduced dose as up to 35% of the drug is renally excreted, patients with hepatic impairment will also require a reduction as 50% is excreted in bile (Murray, et al, 2002).

The purpose of this guideline is to provide guidance of how to correctly carry out Train of Four monitoring on a patient within the ICU environment taking into account nursing considerations and indications/contraindications for use.

Indications for use  

  • Patient has required administration of a non-depolarising NBMA for the following reasons:
    • Endotracheal intubation 
    • Facilitation of mechanical ventilation 
    • Deterioration in gas exchange 
    • Management of  increased ICP 
    • Neuroprotection 
    • Reduction of metabolic demand
    • Reduction of shivering in temperature control
    • Post – surgery (Baumann, et al¸ 2004). (Rationale 1).
  • Patient has received adequate sedation prior to administration of NMBA. (Rationale 2).
  • Patient is > 2 months of age and > 5kg. (Rationale 3).

Contraindications for use

Preparing the child/family

As the child is in receipt of muscle relaxants or is in a condition in which they are about to be, the child may not be aware, however, the family will most likely be distressed and anxious. It is therefore important to adequately explain to them why train of four monitoring is necessary (Rationale 7). Avoid using the word paralysis as this can have negative connotations and cause more stress to the family (Luckett and Hays, 2013a).

Prepare equipment

Pre-procedure, gather all equipment necessary (Rationale 8).

You will need: 

  • Train of Four stimulator (either the Rutter RS6 or RS8 model) – ensure two leads (1 x black, 1 x red) are available (Rationale 9).
  • Two ECG electrodes (the size shown in figure 1 is preferable as may be more accurate) (Rationale 10).
  • Alcohol based skin preparation wipe in order to prepare skin (Rationale 11).
  • Aderma (or other pressure relieving equipment) (Rationale 12)

Figure 1 (O’Boyle, 2016)

Undertaking Train of Four monitoring

After explaining the procedure to parents and obtaining your equipment, follow the next steps in order to carry out the Train of Four monitoring. 

  • Obtain the ‘Train of Four’ box, for those within Great Ormond Street Hospital this can be found in the store room on PICU/CICU. 

Figure 2 (O’Boyle, 2016)

  • There are currently two machines in use. The RS6 (the black coloured model) and the newer RS8 (the blue and yellow model). It does not make a difference which one is used.

Figure 3 (O’Boyle, 2016)

  • In order to be sure that the machine is working, it will require testing (Rationale 13). There are two arrows, one facing up, one down; these need to be pressed simultaneously to ensure that the battery is functioning. There is a column (LED bargraph) (in the middle on the RS8 model and to the left side on the RS6) which has a series of numbers on either side – the left pertains to mA for TOF measurement, the right is for voltage to ascertain battery life.
  • When the two arrow buttons are pressed simultaneously, the LED bargraph will light up and there should be a minimum voltage of 3V observed. If it is 2.8V then the batteries will require changing (Rutter, 2016).

Figure 4 (O’Boyle, 2016)

  • Ensure that the black wire is to the negative port and the red wire is to the positive port. (On the RS8 model, it is colour coded at the point of insertion, on the RS6 model, it is marked – or +) (Rationale 14).
  • If the batteries require changing, please send to biomedical engineering (Rutter, 2016).

If the machine is ready to be used, prepare patient:

  • The ulnar nerve is the nerve to be used for peripheral nerve stimulation (Rationale 15).
  • Ensure that the arm to be used is appropriate (Rationale 16). 
  • Lie patients’ arm with palm facing upwards (Rationale 17).

Figure 5 (O’Boyle, 2016)

  • Wash hands (Rationale 18) and don Personal Protective Equipment (PPE) if required.  
  • Prepare skin using appropriate alcohol based wipe (Rationale 11).

Figure 6 (O’Boyle, 2016)

  • Apply first ECG electrode to ulnar nerve as shown in diagram and picture below (Rationale 19).

Figure 7 (O’Boyle, 2016)

Figure 8 (O’Boyle, 2016)

Figure 9 (O’Boyle, 2016)

  • Apply second ECG electrode to ulnar nerve as shown in picture above (Rationale 20).
  • Next apply leads – remember that red is for heart and place the red electrode further up the arm (Rationale 21). If close monitoring required, also take into account that aderma may be required (Rationale 12).

Figure 10 (O’Boyle, 2016)

  • Using arrows, select the lowest mA, this will be 10mA and will be highlighted as selected as the bottom LED light will be lit.
  • Press the TOF button - see figure 11 below. (the other two buttons; DBS and Tet are not required for use and will not be discussed in this guideline) 

Figure 11 (O'Boyle, 2016)

  • The train of four impulse will be seen in the thumb (Rationale 22).
  • If four twitches are not observed, press the up arrow and increase the mA until four twitches are observed. Increase until the largest response is seen e.g. – If 50mA was selected, and the thumb twitch is large and pronounced, increase to 60mA, and observe again – if the response is the same as at 50mA, decrease back to the 50mA – this is called the Supramaximal Stimulation Point (Baseline) (Rationale 23).
  • Ensure that this is documented in the patients’ health care record (Rationale 24).
  • When carrying out  Train of Four monitoring – position yourself as indicated in Figure 12, in order to not only visualise but also feel the train of four response (Rationale 25).

Figure 12 (O’Boyle, 2016)

Interpretation of Train of Four monitoring

Number of twitches Percentage of receptors blocked Interpretation
0 100%

May be desired for intubation

Patient is over-paralysed

Decrease dose of NMBA

1 90% Appropriate level of NMBA
2 80% Appropriate level of NMBA
3 75%

Inadequate neuromuscular blockade

Increase dose of NMBA

4 0-75%

Inadequate neuromuscular blockade 

May see clinical signs of moving

Increase dose of NMBA 

(Fergusson, 2008)

In order to be able to interpret the TOF result, a baseline should be obtained pre-muscle relaxation, and documented. However, if in an emergency, safety should take priority (Rationale 26).

If the baseline was not obtained and there is a TOF of 0 after using all mA settings – reduce the dose of NMBA and use all other clinical observations to assess depth of muscle relaxation (Rationale 27).

Document all TOF results in the patients’ health care record and respond accordingly (Rationale 24).

Current unit guidelines:

If your patient has a TOF of 0 – be sure that you observe for all four twitches as note that when patient is starting to recover from neuromuscular blockade following dose reduction, it is the fourth twitch that will return first (Rationale 29).

For each patient receiving a neuromuscular blockade, ensure that on the morning ward round you as the bedside nurse ask what level of blockade your patient requires 

ASK: “What should my Train of Four be?”

Document and titrate your infusion accordingly (Rationale 30)

When you have finished with the equipment, ensure that it is cleaned with a sanitising wipe (e.g. green Clinell wipes). (Rationale 31). 

Remove PPE and wash hands or use alcohol gel (Rationale  18).


Rationale 1: Nondepolarising neuromuscular blocking agents compete with acetylcholine for binding sites on nicotinic acetylcholine receptors. The prevention of this binding ensures the receptors at the neuromuscular junction are blocked. This means that voluntary muscle movement is inhibited, therefore causing a pharmacologically induced paralysis (Burmester and Mok, 2005; Luckett and Hays, 2013a). 

Rationale 2: In order to administer an NMBA, appropriate sedation should be administered first. This is to ensure that a patient is not pharmacologically paralysed whilst aware of their surroundings which would cause distress not only to the patient but also to their families. (Loyola and Dreher, 2003; Murray, et al, 2002).

Rationale 3: There is debate surrounding minimum age for TOF monitoring as the neuromuscular system is immature at birth and the neuromuscular junction is still developing between 4 months to 1 year (Corso, 2008). The response therefore to TOF monitoring in children would be variable. The size of the child will also be a factor in accurate monitoring (Luckett and Hays, 2013b). After discussion with a Consultant on the unit for the purpose of this guideline, it was agreed that the monitoring of babies below two months of age or weighing less that 5kg, would not be undertaken. However, it may be the case that it is not appropriate for an older/heavier child due to pharmacological distribution differences (Banner, 2013) – clinical judgement of the bedside nurse as well as the multidisciplinary must also have a role in the decision to carry out a TOF.

Rationale 4: Due to the stimulus of the nerve along the ulnar, if there was a fracture, it would be inappropriate to carry out the TOF in this limb as it would cause unnecessary pain due to the muscle contraction caused by the stimulus and may impede healing – use the other arm if possible (Kudchadkar, et al, 2016). 

Rationale 5: If the skin were damaged due to burns, it would not be possible to apply the ECG electrode to the skin – note that conduction of the electrodes may also be affected if patient has wet or oily skin, be aware of the potential impact of topical medications (Foster, et al, 2002).

Rationale 6: If the patient has existing damage to peripheral nerves, it may not be possible to use the ulnar nerve as it could lead to false readings and over/under dosing of patient (Loyola and Dreher, 2003; Luckett and Hays, 2013b).

Rationale 7: This is in order for them to be prepared and also to consent to the procedure, effective communication to provide all information is necessary (Nursing and Midwifery Council, 2015).

Rationale 8: In order to carry out the procedure quickly and effectively with minimal disruption to the patient and family.

Rationale 9: The black lead is negative, the red lead is positive in order to complete the circuit when the electrodes are placed on the patient and the stimulus is applied. If the equipment appears to not be working, check that these leads are correctly situated (Rutter, 2016).

Rationale 10: When selecting ECG electrodes, ensure that they are the appropriate size for the patient so there is not a large gap in between as this may skew the results. Ensure that there is enough gel and the electrodes are not dry as this may cause the skin to burn (Foster, et al, 2002; McGrath and Hunter, 2006).

Rationale 11: Skin is naturally oily due to production of sebum, cleaning the skin pre-ECG electrode placement with an alcohol based skin wipe and allowing to air dry will ensure that the electrodes will adhere correctly (Foster, et al, 2002; McGrath and Hunter, 2006).

Rationale 12: If the patient requires sustained neuromuscular blockade, they will be unable to move and reposition themselves rendering them extremely vulnerable – ensure that steps have been taken to ensure that skin integrity is maintained. Use aderma if available. Ensure that the ECG electrodes are changed at least 24 hourly – more regularly if required. Your patient will be relying on good nursing care and management (Crawford and Dixon, 2012).

Rationale 13: The TOF machine should be tested to see if operational prior to use to ensure that incorrect administration of neuromuscular blockade is avoided due to inappropriate interpretation of results

Rationale 14: If they are not correctly positioned it could lead to incorrect interpretation of TOF (Crawford and Dixon, 2012).

Rationale 15: The ulnar nerve is to be used as it is easily accessible – it is more responsive than the diaphragm and vocal cords to NMBAs (Yentis, et al, 2009). Other nerves can be used such as the facial but this is more difficult to interpret and more distressing for families.

Rationale 16: Ensure there are no breaks/burns/previous nerve injury to arm. Also ensure that there is no equipment in the way meaning that you have to compromise you own safety to reach over the patient risking injury to your back.

Rationale 17: With the palm facing upwards, you can use your hand to place under the thumb to feel as well as observe the adduction of the thumb due to stimulus of the adductor pollicis brevis (Ferguson, 2008).

Rationale 18: In order to prevent spread of infection (See Hand Hygiene clinical practice guideline). 

Rationale 19: The first electrode is placed along the ulnar nerve, under the crease of the wrist as this is proximal to the adductor pollicis brevis (Luckett and Hays, 2013b).

Rationale 20: The second electrode is placed distally along the ulnar nerve (Luckett and Hays, 2013b).

Rationale 21: It may be easier to remember that the red electrode is attached here and is closer to the heart – red for heart/or red for head, whichever is easiest to recall.

Rationale 22: If there is twitching in the fingers, this does not indicate a positive Train of Four, it must be the thumb that moves due to the muscle that is being moved through stimulation of the adductor pollicis brevis. The position of the electrodes may require changing if there is no thumb twitching (Jones, 2003; Kudchadkar, et al, 2016).

Rationale 23: The baseline will not be the same for everybody. It is important that a baseline is found and documented as at higher mA, there might be pain at the site (Kudchadkar, et al, 2016). 

Rationale 24: Documentation is vital so that the supramaximal stimulation point can be used and so that TOF can be interpreted correctly. All relevant information pertaining to the treatment of a patient should be documented so that it can be shared with the relevant people in order to maintain consistency in care provided (Nursing and Midwifery Council, 2015).

Rationale 25: By placing your hand underneath the thumb during the TOF measurement, it will be easier to ascertain the results as it can be easier to feel when TOF is returning and the thumb is adducting if it were previously 0/4 (Jones, 2003).

Rationale 26: If a patient is being sedated and muscle relaxed electively, if planned with the medical team, it would be ideal to factor this measurement in when sedated. However, if the patient requires emergency endotracheal intubation, this would not be appropriate. It would, however, be pertinent post muscle relaxation to start with low mA and increase in order to try assessing where TOF is measured (Kudchadkar, et al, 2016). 

Rationale 27: Assessing if patient is breathing above the ventilator, moving extremities, shivering if cooling to normothermia, coughing during suctioning – use your clinical judgement (Corso, 2008). 

Rationale 28: Currently in use on the unit, however, due to research obtained by author, the frequency of monitoring has been amended. It was previously documented as 4 hourly, however, if after initial infusion commenced, it is carried out more regularly, it will be easier to assess the ideal infusion rate. Caution should also be noted as if waiting 4 hours in between monitoring, there may be an accumulation of the drug due to renal failure leading to prolonged clearance. Due to differences between extracellular fluid compartments between infants and older children/adults, volume of drug distribution may be variable and also metabolic rates are different as children mature. Close monitoring would be preferable to avoid critical illness polyneuropathy and myopathy (Banner, 2013; Collins, 2011).

Rationale 29: It is important that this is noted as if not monitored until the whole TOF stimulus is complete could lead to failure to recognise that the neuromuscular blockade is reducing and the patient is starting to recover 

Rationale 30: It is important that as the bedside nurse you are aware of the parameters to aim for as it is you who will be regularly assessing the depth of neuromuscular blockade and responding accordingly. The team as a whole should be aware of the plan in order to ensure that communication and team work is effective (Nursing and Midwifery Council, 2015).

Rationale 31: According to manufacturer instructions (Rutter, 2016).



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Document control information

Lead Author(s)

Helen O'Boyle, PICU Staff Nurse

Document owner(s)

Helen O'Boyle, PICU Staff Nurse

Approved by

Guideline Approval Group

Reviewing and Versioning

First introduced: 
02 March 2017
Date approved: 
02 March 2017
Review schedule: 
Three years
Next review: 
02 March 2019
Document version: