Neuro muscular Monitoring

Dr. Anup Kumar Harichandan PG 2nd yr

Objectives of NM Monitoring

• Monitoring onset of NM Blockade.

• To determine level of muscle relaxation during surgery.

• Assessing patients recovery from blockade to minimize risk of residual paralysis.

• Neuromuscular monitoring permit optimal surgical relaxation and reverses the block spontaneously or revesed quickly with antagonists.

• There is no detectable block until 75 to 85% of Ach receptors are occupied and paralysis is complete at 90 to 95% Ach receptor occupancy.

Need of Monitoring

To prevent residual post-op NM blockade

• Decrease chemo receptor sensitivity to hypoxia

• Functional impairment of pharyngeal and upper esophageal muscles

• Impaired ability to maintain the airway • Increased risk for post-op pulmonary

complications

Difficult to exclude clinically significant residual paralysis by clinical evaluation

Absence of tactile fade to TOF , tetanic stimulation, DBS does not exclude significant residual blockade

Variable individual response to muscle relaxants

Narrow therapeutic window

Principles of Peripheral Nerve Stimulation

• Each muscle fiber to a stimulus follows an all-or-none pattern

• In contrast, response of the whole muscle depends on the number of muscle fibers activated

• Response of the muscle decreases in parallel with the numbers of fibers blocked

• Reduction in response during constant stimulation reflects degree of NM Blockade

• For this reason stimulus is supramaximal

Features of Neurostimulation

• Nerve stimulator- device that delivers depolarizing current via electrodes

• Essential Features• Monophasic, Square-wave impulse, 0.1-0.5msec

• Constant current variable voltage

• Battery powered

• Multiple patterns of stimulation

• Stimulus strength- it is the depolarizing intensity of stimulating current

• Pulse width-duration of the individual impulse delivered by nerve stimulator

• Threshold current –lowest current required to depolarize a nerve fiber

• Supramaximal current-it is 10 -20% higher intensity than the current required to depolarize all fibers in a nerve bundle

• Stimulus Frequency- rate at which each impulse is repeated in cycles per sec(Hz)

Electrodes

• Surface electrodes• More commonly used than needle electrodes

• Pregelled silver chloride surface electrodes for transmission of impulses to the nerves through the skin

• Transcutaneous impedance reduced by rubbing

• Conducting area should be small(7-11mm)

• Needle electrodes• Subcutaneous needles deliver impulse near the nerve

Patterns of nerve stimulation

For evaluation of neuromuscular function, the most commonly used patterns of electrical nerve stimulation are•Single-twitch stimulation•Train-of-Four stimulation (TOF) •Tetanic stimulation •Post-tetanic count stimulation (PTC) •Double-burst stimulation (DBS)

Single-twitch stimulation

• This is the simplest form of neuro stimulation,in which single supra-maximal electrical stimuli are applied to a peripheral motor nerve at frequencies ranging from 1.0 Hz to 0.1 Hz

• Height of response depends on the number of unblocked junctions

• Does not detect receptor block of <70% • Used to assess potency of drugs

• Pattern of electrical stimulation and evoked muscle responses to single-twitch nerve stimulation (at frequencies of 0.1 to 1.0 Hz) after injection of nondepolarizing (Non-dep.) and depolarizing (Dep.) neuromuscular blocking drugs (arrows).

Pattern of electrical stimulation and evoked muscle responses to single-twitch nerve stimulation

Train-of-Four stimulation (TOF)

• This is a popular mode of stimulation for clinical monitoring of neuromuscular junction first described by Ali et al.

• Four supra maximal stimuli are given every 0.5 sec (2 Hz)

• When used continuously, each set (train) of stimuli is normally repeated every 10th to 20th second.

• “Fade” in the response provides the basis for evaluation

• The ratio of the height of the 4th response(T4) to the 1st response(T1) is TOF ratio

• In partial non- depolarizing block TOF ratio (T4/T1) is inversely proportional to degree of blockade

• In partial depolarizing block, no fade occurs in TOF ratio ,ideally it is approx. one.

• Fade, in depolarizing block signifies the development of phase II block .

Pattern of electrical stimulation and evoked muscle responses to TOF nerve stimulation

ADVANTAGES OF TOF STIMULATION

• This pattern of stimulation can be applied at anytime during the neuromuscular block and can provide quantification of depth of block without the need for control measurement before relaxant administration.

• It is more sensitive to lesser degree of receptor occupancy than single twitch.

The relatively low frequency allows response to be evaluated manually or visibly.

There is no post tetanic facilitation therefore can be repeated every 10 to 12 sec.

It may be delivered at sub maximal current which is less painful and is associated with same degree of fade.

Tetanic stimulation

• Tetanic stimulation consists of very rapid (e.g., 30-, 50-, or 100-Hz) delivery of electrical stimuli.

• The most commonly used pattern in cinical practice is 50-Hz stimulation given for 5 sec

• During normal NM transmission and pure depolarizing block the response is sustained

• During non- depolarizing block & phase II block the response fades

During tetanus, progressive depletion of acetylcholine output is balanced by increased synthesis and transfer of transmitter from its mobilization stores.

High frequency stimulation (50Hz or more) results in sustained or tetanic contraction of the muscle during normal neuromuscular transmission despite decrement in acetylcholine release.

• During partial non- depolarizing block, tetanic stimulation is followed by post-tetanic facilitation

• It occurs because the increase in mobilization and synthesis of acetylcholine caused by tetanic stimulation continues for some time after discontinuation of stimulation.

• The degree and duration of post-tetanic facilitation depend on the degree of neuromuscular blockade, with post-tetanic facilitation usually disappearing within 60 seconds of tetanic stimulation

The presence of nondepolarizing muscle relaxants reducing the number of free cholinergic receptors and also by impairing the mobilization of acetylcholine within the nerve terminal contributes to the fade in the response to tetanic and TOF stimulation.

A frequency of 50Hz is physiological as it is similar to that generated during maximal voluntary effort. Fade is first noted at 70% receptor occupancy.

Pattern of stimulation and evoked muscle responses to tetanic (50-Hz) nerve stimulation for 5 seconds (Te) and post-tetanic twitch stimulation (1.0-Hz)

DISADVANTAGES

• The post tetanic facilitation depends on frequency and duration of block

• It is very painful and therefore not suitable for un anaesthetised patients.

Post-tetanic count stimulation (PTC)

Tetanus at 50 Hz for five seconds is applied followed 3 sec later by single twitch stimulation at 1 Hz.

The number of evoked post-tetanic twitches detected is called the post-tetanic count (PTC).

PTC is a prejunctional event, the response can vary with the nondepolarising muscle relaxant used.

A PTC of 8 to 9 indicated imminent return of TOF.

Pattern of electrical stimulation and evoked muscle responses to train-of-four (TOF) nerve stimulation, 50-Hz tetanic nerve stimulation for 5 seconds (TE), and 1.0-Hz post-tetanic twitch stimulation (PTS) during four different levels of nondepolarizing neuromuscular blockade

• Used to assess degree of NM Blockade when there is no reaction single-twitch or TOF

• Number of post-tetanic twitch correlates inversely with time for spontaneous recovery

• Return of 1st response to TOF related to PTC

APPLICATION OF PTC

• The PTC method is mainly used to assess the degree of neuromuscular blockade when there is no reaction to single-twitch or TOF nerve stimulation, as may be the case after injection of a large dose of a nondepolarizing neuromuscular blocking drug.

• Used during surgery where sudden movement must be eliminated(e.g., ophthalmic surgery)

Double-burst stimulation (DBS)

• TOF ratio of less than 0.2 to 0.3 is difficult to detect even by trained observers.

• To improve the detection rate, a new mode of stimulation which consist of two short tetani, separated by a interval long enough to allow relaxation, evaluating the ratio of second to first response has been proposed.

• DBS consist of two train of three impulses at 50Hz tetanic stimulation separated by 750msec

• Duration of each impulse is 0.2msec

• DBS allow manual detection of residual blockade under clinical conditions

• Tactile evaluation of fade in DBS 3,3 is superior to TOF

• However, absence of fade by tactile evaluation to DBS does not exclude residual NM Blockade

Pattern of electrical stimulation and evoked muscle responses to train-of-four (TOF) nerve stimulation and double-burst nerve stimulation

Sites of nerve stimulation

• Ulnar nerve is most commonly used for NM monitoring

• Adductor pollicis muscle is a useful clinical tool because of its accessibility for visual, tactile & mechanographic assessment is very good.

• Diaphragm is most resistant of all muscles & requires 1.4-2times as much MR as adductor pollicis

• Other sites• Facial nerve - orbicularis oculi

• Posterior tibial nerve - flexion of great toe

Relative sensitivities of muscle groups to non depolarizing

muscle relaxants

• In clinical anesthesia, the ulnar nerve is the most popular site

• The distal electrode should be placed about 1 cm proximal to the point at which the proximal flexion crease of the wrist crosses the radial side of the tendon to the flexor carpi ulnaris muscle.

• The proximal electrode should preferably be placed so that the distance between the centers of the two electrodes is 3 to 6 cm

Electrode placement for stimulation of the ulnar nerve and for recording of the compound action potential from three sites of the hand. A, Abductor digiti minimi muscle (in the hypothenar eminence). B, Adductor pollicis muscle (in the thenar eminence). C, First dorsal interosseus muscle

Recording of Evoked Responses

1. Measurement of the evoked mechanical response of the muscle (mechanomyography [MMG])

2. Measurement of the evoked electrical response of the muscle (electromyography [EMG])

3. Measurement of acceleration of the muscle response (acceleromyography [AMG])

4. Measurement of the evoked electrical response in a piezoelectric film sensor attached to the muscle (piezoelectric neuromuscular monitor [PZEMG]

5. Phonomyography [PMG]).

Mechanomyography

• This device objectively quantifies the force of isometric contraction of muscle in response to nerve stimulation

• The force is translated into an electrical signal

• A preload of 200-300gm can be used to align contractile elements

Electromyography

• It records the compound muscle action potential (it is cumulative electrical signal generated by individual APs of individual muscle fibers)

• EMG records the compound MAP via recording electrodes

• The amplitude of compound MAP is proportional to number of muscle units that generate MAP

Electrosensors

Acceleromyography

• It uses miniature piezoelectric transducer to determine the rate of angular acceleration

• The piezoelectric crystal is distorted by the movement and is translated into an electrical signal

• This is a nonisometric measurement & no preload is necessary

Piezoelectric Neuromuscular Monitors

• The technique of the piezoelectric monitor is based on the principle that stretching or bending a flexible piezoelectric film (e.g., one attached to the thumb) in response to nerve stimulation generates a voltage that is proportional to the amount of stretching or bending

Piezoelectric Neuromuscular Monitors

Phonomyography

• Contraction of skeletal muscle generates

intrinsic low frequency sounds

• This low frequency sounds can be recorded

with special microphones

• Good correlation exists between PMG &

EMG,MMG,AMG

Evaluation of recorded evoked responses

• Non-depolarizing NM Blockade• Intense NM Blockade

• Deep NM Blockade

• Moderate or Surgical blockade

• Recovery

• Depolarizing NM Blockade• Phase I blockade

• Phase II blockade

Levels of block after a normal intubating dose of a nondepolarizing neuromuscular blocking agent (NMBA) as classified by post-tetanic count (PTC) and train-of-four (TOF) stimulation.

Non-depolarising blockade

• Intense NM Blockade• Intense neuromuscular blockade occurs within 3 to 6

minutes of injection of an intubating dose of a non depolarizing muscle relaxant, depending on the drug and the dose given.

• This phase is called “Period of no response”

• Deep NM Blockade • Deep block characterized by absence of TOF response

but presence of post-tetanic twitches

•

• Surgical blockade • Begins when the 1st response to TOF stimulation

appears

• Presence of 1 or 2 responses to TOF indicates sufficient relaxation

• This phase is characterized by a gradual return of the four responses to TOF stimulation

• The presence of one or two responses in the TOF pattern normally indicates sufficient relaxation for most surgical procedures.

• When only one response is detectable, the degree of neuromuscular blockade is 90% to 95%.

• When the fourth response reappears, neuromuscular blockade is usually 60% to 85%

• Antagonism of neuromuscular blockade with a cholinesterase inhibitor should not normally be attempted when the blockade is intense or deep because reversal will often be inadequate, regardless of the dose of antagonist administered

• Recovery • Return of 4th response to TOF heralds recovery

phase

• T4/T1 ratio > 0.9 exclude clinically important residual NM Blockade

• Antagonism of NM Blockade should not be initiated before at least two TOF responses are observed

• When the TOF ratio is 0.4 or less, the patient is generally unable to lift the head or arm.Tidal volume may be normal, but vital capacity and inspiratory force will be reduced.

• When the ratio is 0.6, most patients are able to lift their head for 3 seconds, open their eyes widely, and stick out their tongue, but vital capacity and inspiratory force are often still reduced.

• At a TOF ratio of 0.7 to 0.75, the patient can normally cough sufficiently and lift the head for at least 5 seconds, but grip and higher, vital capacity and inspiratory force are normal.The patient may, however, still have diplopia and facial weakness

• In clinical anesthesia, the TOF ratio, whether recorded mechanically or by EMG, must exceed 0.80 or even 0.90 to exclude clinically important residual neuromuscular blockade.

Depolarising NM blockade

• Phase I block

• Response to TOF or tetanic stimulation does not fade, and no

post-tetanic facilitation

• Phase II block

• “Fade” in response to TOF in depolarizing NM Blockade

indicates phase II block

• Occurs in pts with abnormal cholinesterase activity and

prolonged infusion of succinylcholine

Difference between depolarising and non

depolarising blockDEPOLARISING BLOCK NON-DEPOLARISING BLOCK

Fasiculation No fasiculation

No tetanic fade Tetanic fade

No post-tetanic potentiation Post-tetanic facilitation

Potentiated by anticholinesterases Antagonised by anticholinesterases

Antagonism by non-depolarisers Potentiation by non-depolarisers

Potentiation by other depolarisers Antagonism by other depolarisers

May develop Phase 2 block No change in character of block

Clinical Vs TOF evoked stimulation

Clinical vs TOF evoked stimulation

Clinical tests of Postoperative Neuromuscular Recovery

Clinical application

To differentiate depolarising block and Non-depolarising block

To see efficacy of depolarising block after administration of drug and to judge whether the patient is fully relaxed

To see whether the pt is out of effect of block of depolarising muscle relaxed

As a guide to administer first dose of non-depolarising muscle relaxant

As a guide to see whether completeness of non-depolarising neuromuscular block

As a guide to for starting of reversal of non-depolarising block

To see a completeness of recoveryAs a guide for incremental doses administration of non-

depolarising muscle relaxantTo differentiate respiratory paralysis i.e. central or

peripheral due to neuro muscular blockTo diagnose overdose of sedatives cerebral depressants or

muscle relaxants

To diagnose phase II block after suxamethoniumTo diagnose various neuromuscular disordersTo diagnose site of nerve injuryTo diagnose electrolyte imbalance or disturbances

affecting NM TransmissionAs a guide in diagnosis of prolonged apnea or recovery

after balanced anaesthesia

Who should be Monitored ?

• Patients with severe renal, liver disease• Neuromuscular disorders like myasthenia gravis,

myopathies, UMN and LMN lesions• Patients with severe pulmonary disease or

marked obesity• Continuous infusion of NMBs or long acting

NMBs• Long surgeries or surgeries requiring elimination

of sudden movement

Limitations of NM Monitoring

• Neuromuscular responses may appear normal despite persistence of receptor occupancy by NMBs.

• T4:T1 ratios is one even when 40-50% receptors are occupied

• Patients may have weakness even at TOF ratio as high as 0.8 to 0.9

• Adequate recovery do not guarantee ventilatory function or airway protection

• Hypothermia limits interpretation of responses

References

1. Millers text book of anesthesia,7th ed.

2. Morgan`s principles of anesthesia