NEUROMUSCULAR MONITORING

Dr.Malini Joshi

Dr.Deepak Chavan

NEUROMUSCULAR MONITORING

• Onset of NM Blockade.• To determine level of muscle relaxation during

surgery.• Assessing patients recovery from blockade to

minimize risk of residual paralysis.

Objectives of NM Monitoring

Why do we Monitor?

Residual post-op NM Blockade – 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 curarization by clinical evaluation

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 • Surgeries requiring profound NM blockade

Various ways of nerve stimulation

• Electrical: Most commonly used in clinical practice.

• Magnetic: Less painful and does not require physical contact with body.

Principles of Peripheral Nerve Stimulation

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

• 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

Essential features of PNS

• Shape of stimulus should be monophasic and rectangular i.e Square-wave stimulus.

• 0.2- 0.3 msec duration so it falls within absolute refractory period of motor unit in the nerve.

• Constant current variable voltage• Battery powered.• Digital display of delivered current.• Audible signal on delivery of stimulus.• Audible alarm for poor electrode contact.• Multiple patterns of stimulation (single

twitch,train-of-four, double-burst, post-tetanic count).

• Current intensity : It is the amperage (mA) of the current delivered by the nerve stimulator(0-80 mA). The intensity reaching the nerve is determined by the voltage generated by the stimulator and resistance and impedance of the electrodes, skin and underlying tissues.

• Nerve stimulators are constant current and variable voltage delivery devices.

• Reduction of temperature increases the tissue resistance (increased impedance) and may cause reduction in the current delivered to fall below the supramaximal level

• Threshold current : It is the lowest current required to depolarize the most sensitive fibres in a given nerve bundle to elicit a detectable muscle response.

• Maximal current:current which generate response in all muscle fibre

• Supramaximal current : • It is approximately 25% higher intensity than the

current required to depolarize all fibres in a particular nerve bundle. This is generally attained at current intensity 2-3 times higher than threshold current.

• Submaximal current : A current intensity that induces firing of only a fraction fibres in a given nerve bundle. A potential advantage of submaximal current is that it is less painful than supramaximal current.

• Stimulus frequency : The rate (Hz) at which each impulse is repeated in cycles per second (Hz).

Electrodes • Surface 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• Require less current

POLARITY

• Stimulators produce a direct current by using one negative and one positive electrode

• Should be indicated on the stimulator• Maximal effect is achieved when the negative

electrode is placed directly over the most superficial part of the nerve being stimulated

• The positive electrode should be placed along the course of the nerve, usually proximally to avoid direct muscle stimulation

• Ulnar nerve: MOST COMMON SITE

• place negative electrode (black) on wrist in line with the smallest digit 1-2cm below skin crease

• positive electrode (red) 2-3cms proximal to the negative electrode

• • Response: Adductor pollicis muscle – thumb adduction

VARIOUS SITE USED FOR NM MONITORING

• Facial nerve: place negative electrode (black) by ear lobe and the positive (red) 2cms from the eyebrow (along facial nerve inferior and lateral to eye)

• • Response: Orbicularis occuli muscle – eyelid twitching

• Posterior tibial nerve:

place the negative electrode (black) over inferolateral aspect of medial malleolus (palpate posterior tibial pulse and place electrode there) and positive electrode (red) 2-3cm proximal to the negative electrode

• • Response: Flexor hallucis brevis muscle – plantar flexion of big toe

Patterns of Stimulation

• Single-Twitch Stimulation• Train-of-Four Stimulation• Tetanic Stimulation• Post-Tetanic Count Stimulation• Double-Burst Stimulation

Single-Twitch Stimulation• Single supramaximal stimuli applied to a nerve

at frequencies from 1.0Hz-0.1Hz• Height of response depends on the number of

unblocked junctions• Prerelaxant control response is noted &

compared with subsequent responses• Response will only be depressed when NM

blocker occupies 75% receptor

• Used to assess potency of drugs• Useful before induction to determine level at

which supramaximal stimulus obtained• Useful to determine onset of NM block• In both depolarising & non depolarising blocks

there is progressive decrease in twitch height• So can not differentiate between depolarising &

non depolarising NM blocker• Major limitation is need to measure control

twitch before NM blocker i.e. prerelaxant control response is necessary

Single-Twitch Stimulation

Train-of-Four Stimulation• Four supramaximal stimuli are given every 0.5 sec• “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 T4/T1 ratio is inversely proportional to degree of blockade

• In Depolarizing block, no fade occurs in TOF ratio so equal depression in twitch height

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

Train-of-Four Stimulation

Tetanic Stimulation• Tetanic Stimulation is 50-Hz stimulation given for 5 sec• During tetanus, progressive depletionof acetylcholine output

is balanced by increased synthesis and transfer of transmitter from it’s mobilization stores.

• NDMR reduces the margin of safety by reducing the number of free cholinergic receptors and also by impairing the mobilization of acetylcholine within the nerve terminal there by contributing 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.

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

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

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

Tetanic Stimulation

Post-Tetanic Count Stimulation

• Mobilization and enhanced synthesis of acetylcholine continue during and after cessation of tetanic stimulation.

• 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

• Tetanic stimulation(50Hz for 5sec.) and observing post-tetanic response to single twitch stimulation at 1Hz,3sec after end of tetanic stimulation

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

• Return of 1st response to TOF related to PTC

Post-Tetanic Count Stimulation

Double-Burst Stimulation

• 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 as human senses DBS fade better.

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

Double-Burst Stimulation

Non-depolarizing blockade • Intense NM Blockade• 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

Contd…

• Recovery

• Return of 4th response to TOF heralds recovery phase • presence of spontaneous respiration is not a sign of

• adequate neuromuscular recovery.

• 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

Depolarizing 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

Visual or tactile:Not sensitive enough to exclude

possibility of residual neuromuscular blockade. Fade is usually undetected until

TOF ratio values are <0.5.

Recording devices for measuring NM Function

• Compound muscle action potential: It is the cumulative electrical signal generated by the individual action potentials of the individual muscle fibres.

Electromyogram (EMG)

• It records the compound MAP via recording electrodes placed near the mid portion or motor point of the muscle and a slightly remote indifferent side.

• The latency of the compound MAP is the interval between stimulus artifact and evolved muscle response.

• The amplitude of the compound MAP is proportional to the number of muscle units that generate a MAP within the designated time interval (epoch) and this correlates with the evoked mechanical responses.

• For experimental studies The best signal is usually obtained by placing the active receiving

electrode over the belly of the muscle with the reference electrode over the tendon insertion site

The ground electrode is placed between the stimulating and recording electrodes.

Mechanomyographic device(isometric)

(Adductor pollicis force translation monitor)• Quantifies the force of isometric contraction• The force electrical signal pressure monitor and recorded.• Key features :a. Alignment of the direction of thumb movement with that of the pressure

transducer.b. Application of consistent amount of baseline muscle tension (preload

200-300 gms)c. Transducer and monitor with adequate monitoring range and zeroing of

the monitor before stimulation.DISADV: These devices are difficult to set up for stable and accurate

measurements Proper transducer orientation, isometric conditions, and application of a

stable preload are required Maintenance of muscle temperature within limits is important

Accelerography (non isometric)

• This technique uses a miniature piezoelectric transducer to determine the rate of angular acceleration.

• Newton’s second law, F=m*a• Muscle must be able to move freely.• The piezoelectric crystal is distorted by the movement of the

crystal inlaid transducer which is applied to the finger and an electric current is produced with an output voltage proportional to the deformation of the crystal.

• This is a non-isometric measurement and there are less stringent requirements for immobilization of arm, fingers and thumb and also no preload is necessary.

• TOFguard,• TOF–watch (Organon Teknika),• Para Graph Neuromuscular Blockade Monitor (Vital signs), • Part of Datex AS/3 monitoring system (M-NMT)

Reliable Unreliable

Sustained head lift for 5 sec Sustained eye opening

Sustained leg lift for 5 sec Protrusion of tongue

Sustained handgrip for 5 sec Arm lifted to the opposite shoulder

Sustained “tongue depressor test” Normal tidal volume

Maximum inspiratory pressure 40 to 50 cm H2O or greater

Normal or nearly normal vital capacity

Maximum inspiratory pressure less than 40 to 50 cm H2O

Clinical tests of Postoperative Neuromuscular Recovery

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

THANK YOU !