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The science and use of the peripheral nerve stimulator

Dr David Pescod

A peripheral nerve stimulator can detect both the magnitude and type of neuromuscular blockade.

It should be used to determine:

  • If neuromuscular blockade is adequate or excessive.
  • If neuromuscular blockade can be safely reversed.
  • If neuromuscular blockade reversal is adequate to allow safe extubation.

Nerve stimulation

Nerve stimulation consists of delivering a unipolar square wave of 100 to 200 microseconds in duration. It should be supramaximal, meaning that the current pulse should simultaneously depolarise all of nerve fibres within the nerve. A supramaximal stimulation is 20-50% above a stimulation that causes maximal response, and should ideally be determined in the anaesthetised patient prior to giving a muscle relaxant.

The current pulse should not cause direct stimulation of the muscle. The current required may be between 20 to 60 mA (milliamperes), but not more than 80mA, where direct muscle stimulation becomes more likely.

Muscle groups used for monitoring

Different muscles groups have different sensitivity to neuromuscular blocking agents. The adductor pollicis muscle, supplied by the ulnar nerve, is the most commonly used muscle for monitoring, owing to its ready accessibility in most anaesthetised patients. Importantly, the diaphragm requires 1.4 to 2.0 times as much muscle relaxant dose as the adductor pollicis for the same degree of relaxation (the ‘safety margin’ of the diaphragm). The diaphragm also recovers more rapidly than adductor pollicis, so that if the adductor pollicis muscle has recovered from neuromuscular blockade the anaesthetist can infer that the diaphragm has recovered to the same or a greater degree.

To monitor the adductor pollicis the negative (black) electrode is usually placed about 1 cm proximal to the proximal flexion crease of the wrist lateral to the tendon of flexor carpi ulnaris. The positive (red) electrode is placed 2 to 5 cm proximal (positive = proximal) to the distal electrode, reducing the magnitude of current required for nerve stimulation.

Other muscle groups which can be used for neuromuscular monitoring include:

  • Common peroneal nerve
  • Facial nerve
  • Posterior tibial nerve

Nerve stimulation pulse patterns

Commonly used patterns of peripheral nerve stimulation include single twitch, tetanic stimulation, post-tetanic count (PTC), train of four (TOF) and double burst stimulation (DBS). All patterns involve delivery of monophasic square-wave pulses of 0.1 to 0.2 ms in duration. The frequency and pattern of pulse delivery allow different information to be determined in relation to the degree and type of neuromuscular blockade.

Single twitch

Single twitch stimulation involves a supramaximal stimuli delivered every 1 second (1Hz) or every 10 seconds (0.1Hz). It is not a sensitive measure of the onset or recovery of neuromuscular blockade because single twitch stimulation may appear to be normal even when 75 to 80% of neuromuscular Ach receptors remain blocked. It can be useful as a measure of deep neuromuscular blockade.

Most commonly a single twitch is used to determine the Initial Threshold for Stimulation (ITS) in a patient before muscle relaxants are given. This allows more accurate selection of the stimulus current. The current required to produce supramaximal stimulation can then be estimated as being 2.75 times the current for ITS. This calculated current is then used by the anaesthetist as the selected current for subsequent nerve stimulator pulses.

Train of four

The ‘train of four’ (TOF) pattern is the most commonly used in modern anaesthesia. It is excellent for monitoring if neuromuscular blockade is adequate and may assist in determining if it can be safely reversed. The TOF assesses the degree of fade in a group of muscle twitches, such that each subsequent twitch is of lesser magnitude than the previous. Fade is a property of depolarising muscle blockade, or less commonly, of a stage 2 non-depolarising block with suxamethonium.

The TOF pattern involves delivery of four supramaximal stimulations every 0.5 seconds (2Hz). If used continuously, each train of four group may be repeated every 10 seconds, although it is best to repeat less frequently.

The TOF can be used to assess the degree of blockade in two ways. First, it can be used to determine the Train of Four Ratio (TOFR), the ratio of the height of the fourth twitch to that of the first. A TOFR of more than 0.7 usually indicates adequate reversal. Unfortunately, without special equipment (ie. an accelerometer) it is not physically possible for a clinician to accurately detect a ratio of more than 0.4 by touch alone. Thus without special equipment the TOFR is less useful as a measure of the adequacy of reversal.

Alternatively, the Train of Four Count (TOFC) can be used. This is simply the number of twitches that a clinican can feel during a TOF stimulation. It does not rely on the clinician’s assessment of the magnitude of the twitches. The TOFC is very useful for determining:

  • If muscle relaxation is adequate or excessive during surgery. Generally a TOFC of 1-2 is considered as indicating adequacy of relaxation for surgery requiring paralysis. During neurosurgery a TOFC of 1 may be more appropriate, where inadvertent patient movement carries a high cost.
  • When non-depolarising relaxation has resolved sufficiently in order for the patient to be ‘reversible’. Generally a TOFC of 3 is accepted as indicating that an anaesthetised patient’s paralysis is able to be reversed by a cholinesterase inhibitor, such as neostigmine.

(NB: a TOFC less than 4 is by definition a TOFR of 0.)

After the injection of an intubating dose of a non-depolarising muscle relaxant, train of four stimulation will demonstrate intense blockade within a few minutes followed by gradual recovery from neuromuscular blockade. The duration of intense blockade and recovery will depend primarily on which muscle relaxant is used and the dose given.

During the period of intense blockade there will be no response to train of four stimulation. Gradually there will be recovery with first one then two, three and four twitches. When only one twitch is detectable 90 to 95% of receptors are blocked. A TOFC=2 represents 80% of receptors blocked and TOFC=3 75% receptor blockade. Most surgery can be adequately performed with one or two twitches present though if prevention of movement is essential, a more intense block may be needed. Reversal of muscle blockade should not be attempted until at least three twitches have returned.

The advantages of the TOF pattern for assessment of neuromuscular blockade includes:

  • It does not alter the degree of block, or the perceived degree of block (unlike a tetanic stimulation).
  • A pre-paralysis ‘control’ is not required as the first twitch in the TOF is compared against the subsequent twitches.
  • Although uncomfortable in the awake patient it is not as painful as a tetanic stimulation.
  • It can allow accurate determination of the degree of blockade without further complex monitoring technology.
Tetanic stimulation

Tetanic stimulation involves delivering a train of pulses at a frequency between 50 and 100Hz. Most commonly 50Hz pulses are delivered for 5 seconds. A sustained response to this pattern corresponds with adequate reversal however it cannot be reliably evaluated visually or by touch and is very painful. It should not be performed in the awake patient.

Tetanic stimulation is now rarely used alone to assess neuromuscular blockade. Usually it is used as a component of a further pulse pattern, such as the post tetanic count (PTC).

Post-tetanic count

With intense blockade by a non-depolarising muscle relaxant there may often no response to single twitch or TOF stimulation. In this situation the PTC is used to assess the degree of blockade.

PTC involves a 5 second 50 Hz tetanic stimulation, a 3 second pause and then single twitch stimulation every second (1 Hz). The PTC is then a count of the number of twitches which follow the tetanic stimulation. Tetanic stimulation results in a large release of ACh at the motor end-plate which competes with the muscle relaxant, temporarily overcoming the blockade at this muscle. It is useful to determine the degree of an intense blockade and to estimate the time until return of a single twitch.

Time from PTC=1 to TOFC=1 is 8 minutes for atracurium and vecuronium and almost 40 minutes for pancuronium. These figures will vary between patients and in certain disease states, but are a useful predictor of the progression of a resolving block.

Double-burst stimulation

Double stimulation is generally used to assess the adequacy of reversal. It involves two sets of pulse bursts, chosen because they enhance the degree of fade between the two sets. The decrease in magnitude between the first and second bursts (’fade’) is readily detectable by touch alone if the patient has residual paralysis.

If no fade is detected with DBS then there is little risk of residual muscle relaxation. Nevertheless, an anaesthetist must always be mindful of assessing the clinical signs of adequate reversal before a patient returns to a regular ward.

DBS uses a ‘3:3’ pattern, where 3 pulses at 50 Hz are delivered, followed by a 750 ms pause, then a further 3 pulses at 50 Hz.

Putting it all together - using the pulse patterns

Each of the four types of pulse patterns deliverable by the jfish PNS has its advantages and disadvantages in the assessment of neuromuscular blockade. It is by using the patterns appropriately that the most accurate information can be determined.

  1. Single twitch should ideally be used at the start of a case, after anaesthetising the patient, but before paralysing, in order to determine the initial threshold for stimulation and then calculate the current required for supramaximal stimulation.
  2. Train of four is sometimes used at induction to indicate when sufficient relaxation has developed in order for the trachea to be safely intubated. It is most commonly used throughout the cause to indicate the adequacy of relaxation for surgery and at the end of the case to assess the adequacy of resolution of the block prior to reversal.
  3. Post tetanic count is used during the case when there is intense muscle blockade such that the TOFC=0. It is used to predict when the TOF will return.
  4. Double burst stimulation is most commonly used at the end of the case before and after reversal of paralysis in order to indicate the adequacy of reversal and to detect the presence of residual blockade.
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