NEUROMUSCULAR MONITORING

NEUROMUSCULAR
MONITORING
Presenter : DR ASHISH
GUPTA

CONTENTS
INTRODUCTION
CONDITIONS WHERE NEUROMUSCULAR MONITORING IS ESSENTIAL
PRINCIPLES OF PERIPHERAL NERVE STIMULATION
FEATURES OF THE ELECTRICAL IMPULSE
PATTERNS OF NERVE STIMULATION
EQUIPMENT
SITES OF NERVE STIMULATION

CONTD …
METHODS FOR EVALUATING EVOKED RESPONSES
USE OF NERVE STIMULATORS IN DAILY CLINICAL PRACTICE
COMPLICATIONS WHILE NEUROMUSCULAR MONITORING
CLINICAL TESTS OF POSTOPERATIVE NEUROMUSCULAR RECOVERY
SUMMARY

INTRODUCTION
In the mid-1950s, inadequate recovery of neuromuscular function at the end of
surgery was a common occurrence.
In 1958, the use of a peripheral nerve stimulator (PNS) was suggested for the
diagnosis of prolonged apnoea after the use of neuromuscular blockers .
In 1965, Churchill-Davison opined, ‘The only satisfactory method of determining
the degree of neuromuscular block is to stimulate a motor nerve with an electric
current and observe the contraction of the muscles innervated by that nerve.’

CONTD….
In awake patients, muscle power can be evaluated by tests of voluntary muscle
strength, but this is impossible during anesthesia and recovery from anesthesia.
All these tests, however, are influenced by factors other than the degree of
neuromuscular block.
Therefore, whenever more precise information regarding the status of neuromuscular
functioning is desired, the response of muscle to nerve stimulation should be
assessed.

CONDITIONS WHERE NEUROMUSCULAR MONITORING IS
ESSENTIAL
 After prolonged infusions of neuromuscular blocking drugs or when long-
acting drugs are used.
 When surgery or anaesthesia is prolonged.
 When inadequate reversal may have devastating effects, for example, severe
respiratory disease, morbid obesity.
 In conditions where administration of a reversal agent may cause harm, for
example, tachyarrhythmias, cardiac failure

CONTD…
 Liver or renal dysfunction, when pharmacokinetics of muscular relaxants may
be altered.
 Neuromuscular disorders such as myasthenia gravis or Eaton– Lambert
syndrome.
 Surgeries requiring profound neuromuscular blockade e.g. neurosurgery,
vascular surgery in vital areas like thoracic cavity.

Principles of Peripheral Nerve Stimulation
The reaction of a single muscle fiber to a stimulus follows an all-or-none pattern.
 In contrast, the response (the force of contraction) of the whole muscle depends
on the number of muscle fibers activated.
 If a nerve is stimulated with sufficient intensity, all fibers supplied by the nerve
will react, and the maximum response will be triggered.
After administration of a neuromuscular blocking drug, the response of the
muscle decreases in parallel with the number of fibers blocked.

CONTD…
 The reduction in response during constant stimulation reflects the degree of
neuromuscular block.
The difference in the type of blockade produced, by the depolarizing and non
depolarizing relaxants, accounts for their different response to nerve
stimulation.

FEATURES OF THE ELECTRICAL IMPULSE
SUPRAMAXIMAL STIMULUS:
 An action potential can be elicited when an electrical stimulation of sufficient
magnitude is applied to a nerve.
 Threshold stimulus : It is the current amplitude (in milliamperes, mA) required to
depolarize the most sensitive fibres in a given nerve bundle to elicit a detectable
muscle response.
 Maximal stimulus : Current which generate response in all muscle fibre
 For the preceding principles to be in effect, the stimulus must be truly maximal
throughout the period of monitoring. Therefore the electrical stimulus applied is
usually at least 20 to 25% above that necessary for a maximal response. For this
reason, the stimulus is said to be SUPRAMAXIMAL.

CONTD…
SQUARE WAVEFORM-
 The character of the waveform produced by the electrical impulse and the length
of the stimulus are also important.
 The impulse should be monophasic and rectangular, that is, it should be a square
waveform because a biphasic pulse may cause repetitive firing (a burst of action
potentials in the nerve) thus increasing the response to the stimulation.
DURATION OF IMPULSE-
 Optimal duration is 0.2 to 0.3 msec. A pulse exceeding 0.5 msec may stimulate
the muscle directly or cause repetitive firing.

Patterns of Nerve Stimulation
 Single-Twitch Stimulation
 TOF(TRAIN-OF-FOUR)
 Tetanic Stimulation
 Post-tetanic count (PTC)
 Double-burst stimulation (DBS).

Single-Twitch Stimulation
 A Single supramaximal electrical stimuli are applied to a peripheral motor nerve
at frequencies ranging from 1.0 Hz (once every second) to 0.1 Hz (once every 10
seconds) .
 Because 1-Hz stimulation shortens the time necessary to determine
supramaximal stimulation, this frequency is sometimes used during induction of
anesthesia.

Interpretation
S.No. Total receptors blocked
( in % )
Twitch height
(as % of normal)
1 100 0
2 90-95 0
3 85-90 0
4 80-85 25-95
5 75 100
6 50 100
7 25 100

CONTD…
ADVANTAGES-
 Useful in establishing a supramaximal stimulus and for identifying when conditions
satisfactory for intubation have been achieved.
 Can be used in conjunction with Tetanic stimulus to monitor deep levels of NMB.
DISADVANTAGES-
 Cannot differentiate between depolarizing and non-depolarising neuromuscular
blockade.
 Requires standardization and calibration of the ST amplitude.
 The response’s return to control level does not guarantee that full recovery from NMB
occurred.

Train-of-Four Stimulation
 Train-of-four nerve stimulation pattern was introduced by Ali and associates
during the early 1970s.
 It consists of four supramaximal stimuli given every 0.5 seconds (2 Hz).
 When used continuously, each set (train) of stimuli is normally repeated
every 10th to 20th second.
 Each stimulus in the train causes the muscle to contract, and “fade” in the
response provides the basis for evaluation.
 The Train-of-four ratio(T4:T1, TOFR) is the ratio of amplitude of the fourth
response to that of the first, expressed as percentage or fraction.

CONTD…
 In the control response (the response obtained before the administration of a muscle
relaxant), all four responses are ideally the same: the TOF ratio is 1.0.
 During a partial nondepolarizing block, the ratio decreases (fades) and is inversely
proportional to the degree of block.
 During a partial depolarizing block, reduction in twitch height occurs; ideally, the TOF
ratio is approximately 1.0.
 Fade in the TOF response after injection of succinylcholine signifies the development
of a phase II block.

CORRELATION BETWEEN CLINICAL SIGNS & SYMPTOMS AND TRAIN OF
FOUR RATIOTRAIN OF
FOUR RATIO
SIGNS AND SYMPTOMS
LESS THAN
OR EQUAL TO
0.4
Unable to lift the head or arm. Tidal volume may be normal, but
Vital capacity and inspiratory force is reduced.
MORE THAN
0.4 AND
UPTO 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.
0.70-0.75 Diplopia and visual disturbances
Decreased handgrip strength
Inability to maintain apposition of the
incisor teeth
“Tongue depressor test” negative
Inability to sit up without assistance
Severe facial weakness
Speaking a major effort
Overall weakness and tiredness
0.80-0.90 Vital capacity and inspiratory force are normal
Diplopia and visual disturbances
Generalized fatigue

The TOF count (TOFC) is
defined as the number of
evoked responses that can
be detected.
It permits the quantitative
assessment of non
depolarising block.
With recovery or reversal of
a non depolarizing block,
the TOFC increases until
there are four responses,
then fade decreases.

(TOFC )
TRAIN OF FOUR
COUNT
PERCENTAGE(%) OF NEUROMUSCULAR BLOCKADE
4 0-75%
3 75%
2 80%
1 90%
0 100%
Train of four count and physiological
correlation

CONTD…
ADVANTAGES
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.
Can distinguish between depolarizing and non-depolarizing block.
It is of value in detecting and following of a phase II block after succinylcholine
administration.
It is less painful and, unlike tetanic stimulation, does not generally influence
subsequent monitoring of the degree of neuromuscular block.

CONTD…
DISADVANTAGES-
 Poor performance at both extremes of NMB, deep relaxation or near
complete recovery.
 Tactile observation or visual observation of TOFR is of little value above a
ratio of 0.4-0.5 .

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 clinical practice is 50-Hz stimulation given for 5
seconds, although some investigators have advocated the use of 50-, 100-, and
even 200-Hz stimulation for 1 second.
During normal neuromuscular transmission and a pure depolarizing block, the
muscle response to 50-Hz tetanic stimulation for 5 seconds is sustained.

CONTD…
 During a nondepolarizing block and a phase II block after the injection of
succinylcholine, the response will not be sustained (i.e., fade occurs).
Fade in response to tetanic stimulation is normally considered a presynaptic event;
the traditional explanation is that at the start of tetanic stimulation, large amounts
of acetylcholine are released from immediately available stores in the nerve
terminal.
As these stores become depleted, the rate of acetylcholine release decreases until
equilibrium between mobilization and synthesis of acetylcholine is achieved.

CONTD…
When the “margin of safety” at the postsynaptic membrane (i.e., the number
of free cholinergic receptors) is reduced by nondepolarizing neuromuscular
blocking drugs, a typical reduction in twitch height is seen with a fade during,
for instance, repetitive stimulation.
In addition to this postsynaptic block, nondepolarizing neuromuscular
blocking drugs may also block presynaptic neuronal-type acetylcholine
receptors, thereby leading to impaired mobilization of acetylcholine within
the nerve terminal. This effect substantially contributes to fade in the
response to tetanic (and TOF) stimulation.

CONTD…
 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.
Tetanic stimulation is extremely painful, which limits its use in unanesthetized
patients.

Post-Tetanic Count Stimulation
 During intense block no response to TOF and single-twitch stimulation occurs.
 Therefore, these modes of stimulation cannot be used to determine the degree of
blockade.
It is possible, however, to quantify intense neuromuscular blockade of the peripheral
muscles by applying tetanic stimulation (50 Hz for 5 seconds) and observing the post-
tetanic response to single-twitch stimulation given at 1 Hz starting 3 seconds after
the end of tetanic stimulation.

CONTD…
 As the intense block dissipates, more and more responses to POSTTETANIC
TWITCH STIMULATION appear.
 For a given neuromuscular blocking drug, the time until return of the first
response to TOF stimulation is related to the number of post-tetanic twitch
responses present at a given time (i.e., the PTC).

Double-Burst Stimulation
Double burst stimulation has been introduced as an alternative to TOF
stimulation in an attempt to improve the ability to detect residual
neuromuscular blockade by subjective means.
DBS consists of two short bursts of 50-Hz tetanic stimulation separated by 750
msec.
The duration of each square wave impulse in the burst is 0.2 msec.
The two commonly used patterns are DBS3,3 and DBS3,2.

CONTD…
 The pattern DBS3,3 consists of a mini-tetanic sequence of three stimuli at 50
Hz, followed 750 ms later by an identical sequence.
The pattern DBS3,2 consists of brief three 50 Hz tetanic stimuli, followed 750
ms later by two short 50 Hz stimuli.
 In nonparalyzed muscle, the response to DBS3,3 is two short muscle
contractions of equal strength.
In a partly paralyzed muscle, the second response is weaker than the first (i.e.,
the response fades

EQUIPMENT
The neuromuscular monitoring equipment consists of:
1. Nerve Stimulator
2. Stimulating electrodes

THE NERVE STIMULATOR
 The stimulus should produce a monophasic and rectangular waveform, and the
length of the pulse should not exceed 0.2 to 0.3 msec.
 A pulse exceeding 0.5 msec may stimulate the muscle directly or cause repetitive
firing.
 Stimulation at a constant current is preferable to stimulation at a constant voltage
because current is the determinant of nerve stimulation.
 Furthermore, for safety reasons, the nerve stimulator should be battery operated,
include a battery check, and be able to generate 60 to 70 mA, but not higher than 80
mA.

CONTD…
The Ideal Nerve Stimulator should have other features as well:
The polarity of the electrodes should be indicated, and the apparatus should be
capable of delivering the following modes of stimulation:
 TOF (as both a single train and in a repetitive mode, with TOF stimulation being
given every 10 to 20 seconds)
 Single-twitch stimulation at 0.1 and 1.0 Hz
 Tetanic stimulation at 50 Hz.
 Post Tetanic count
 If the nerve stimulator does not allow objective measurement of the response to
TOF stimulation, at least one DBS mode should be available, preferably DBS3,3.

Electrodes
Surface electrodes :
 They contain gel covered conducting surfaces for transmission of impulses to the
nerves through the skin.
 The transcutaneous impedance can be reduced by rubbing an electrolye solution or
conducting gel, decornifying or degreasing the skin.
 With careful skin preparation the threshold for twitch response is generally <15 mA.
 It may be noted that the regular ECG electrodes i.e. silver-silver chloride electrodes
offer greater impedance than those recommended for NMJ monitoring.

CONTD..
Needle electrodes:
 Subcutaneous needles deliver the impulse in the immediate vicinity of the nerve.
 Useful when supramaximal stimulation cannot be achieved using surface electrodes
which usually occurs when the skin is thickened, cold, or edematous and in obese,
hypothyroid, diabetic, or renal failure patients.
 These are highly effective because they bypass the tissue impedence so that the
tissue impedance is typically < 2000 Ohms.
 Disadvantages include local irritation, infection, nerve damage especially if placed
intra-neurally, diathermy burns and delivery of excessive amounts that may induce
repeatitive nerve firing or direct muscle stimulation.

Sites of Nerve Stimulation
 In principle, any superficially located peripheral motor nerve may be
stimulated.
 In clinical anesthesia, the ulnar nerve is the most popular site.
 The median, posterior tibial, common peroneal, and facial nerves are also
sometimes used.
 For stimulation of the ulnar nerve, the electrodes are best applied to the volar
side of the wrist.
 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.

CONTD…
The proximal electrode should
preferably be placed so that
the distance between the
centers of the two electrodes is
3 to 6 cm .
With this placement of the
electrodes, electrical
stimulation normally elicits
only finger flexion and thumb
adduction.

CONTD…
 Nerve: Facial nerve
 Muscle: Orbicularis oculi
And Corrugator Supercilii
 Action: Twitching of eyelid
And eyebrow

CONTD…
 Nerve: Posterior tibial Nerve (sural
nerve)
 Muscle: Flexor hallicus brevis
 Action: Plantar flexion of great toe
 Black: Over posterior aspect Of
medial malleolus, over posterior tibial
artery
 Red: 2-‐3cm Proximal to black

CONTD…
The diaphragm is among the most resistant of all muscles to both depolarizing
and nondepolarizing neuromuscular blocking drugs.
In general, the diaphragm requires 1.4 to 2.0 times as much muscle relaxant as
the adductor pollicis muscle for an identical degree of blockade .
Also of clinical significance is that onset time is normally shorter for the
diaphragm than for the adductor pollicis muscle and the diaphragm recovers
from paralysis more quickly than the peripheral muscles do .
 The other respiratory muscles are less resistant than the diaphragm, as are
the larynx and the corrugator supercilia muscles.

METHODS FOR EVALUATING EVOKED RESPONSES
 SUBJECTIVE METHODS
a) VISUAL-
 Visual assessment can be used to count the number of responses present with a TOF
stimulus, to determine the PTC, and to detect the presence of fade with TOF or DBS.
 For visual assessment the observer should be at an angle of 90 degrees to the motion.
 It is difficult to determine the TOFR or to compare a single twitch height to its control
visually.
 Visually assessing fade with Tetanic stimulus or DBS appears to be fairly accurate.

CONTD…
b)TACTILE-
 It is accomplished by placing the evaluator’s fingertips lightly over the muscle to
be stimulated and feeling the strength of contraction.
 More sensitive than visual assessment.
 Can be used to evaluate the presence or absence of responses and/or fade with
TOF, DBS, Tetanic stimulation.
 However it is difficult for trained observers to detect TOF fade manually unless
the TOF ratio is below 40%

CONTD…
 OBJECTIVE METHODS
a) Measurement of the evoked mechanical response of the muscle
(mechanomyography [MMG])
b) Measurement of the evoked electrical response of the muscle (electromyography
[EMG])
c) Measurement of acceleration of the muscle response(acceleromyography[AMG])
d) Measurement of the evoked electrical response in a piezoelectric film sensor attached
to the muscle (piezoelectric neuromuscular monitor [PZEMG]
e) Phonomyography [PMG]).

Mechanomyography
 The mechanomyogram (MMG) is the mechanical signal observable from the surface
of a muscle when the muscle is contracted.
 At the onset of muscle contraction, gross changes in the muscle shape cause a large
peak in the MMG.
 The force of contraction is then converted into an electrical signal, which is
amplified, displayed, and recorded.
 The arm and hand should be rigidly fixed, and care should be taken to prevent
overloading of the transducer.
 In addition, the transducer should be placed in correct relation to the thumb (i.e.,
the thumb should always apply tension precisely along the length of the transducer).

Electromyography
 (EMG) is a technique for evaluating and recording the electrical activity
produced by skeletal muscles.
Evoked EMG records the compound action potentials produced by
stimulation of a peripheral nerve.
The compound action potential is a high-speed event that for many years
could be picked up only by means of a preamplifier and a storage
oscilloscope.
The evoked EMG response is most often obtained from muscles innervated
by the ulnar or the median nerves.

CONTD…
Most often, the evoked EMG
response is obtained from the thenar
or hypothenar eminence of the hand
or from the first dorsal interosseous
muscle of the hand, preferably with
the active electrode over the motor
point of the muscle .
The signal picked up by the analyzer is
processed by an amplifier, a rectifier,
and an electronic integrator. The
results are displayed either as a
percentage of control or as a TOF
ratio.

Acceleromyograph
It is a piezoelectric myograph, used to measure the force produced by a muscle
after it has undergone nerve stimulation.
Acceleromyographs measure muscle activity using a miniature piezoelectric
transducer that is attached to the stimulated muscle.
 A voltage is created when the muscle accelerates and that acceleration is
proportion to force of contraction.
When an accelerometer is fixed to the thumb and the ulnar nerve is stimulated,
an electrical signal is produced whenever the thumb moves.
This signal can be analyzed in a specially designed analyzer.

CONTD…
When AMG is used with a free-
moving thumb, as originally
suggested, wide limits of agreements
in twitch height (T1) and TOF ratio
and differences in the onset and
recovery course of blockade between
AMG and MMG have been found.
Moreover, the AMG control TOF ratio
is consistently higher than when
measured with a force-displacement
transducer.

CONTD…
Originally claimed advantages of the
method, that fixation of the hand
could be reduced to a minimum as
long as the thumb could move freely.
 In daily clinical practice it is often
not possible to ensure that the
thumb can move freely and that the
position of the hand does not
change during a surgical procedure.
 Use of an elastic preload on the
thumb may improve the agreement
between results obtained with AMG
and MMG.

Piezoelectric Neuromuscular
Monitor
 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.

Phonomyography
 Contraction of skeletal muscles
generates intrinsic low frequency
sounds, which can be recorded
with special microphones.
 In theory the method can be
applied not only to the adductor
pollicis muscle but also to other
muscles of interest such as the
diaphragm, larynx, and eye
muscles.

USE OF NERVE STIMULATORS IN DAILY CLINICAL PRACTICE
 During Induction.
 During Surgery.
 During Reversal of Neuromuscular Block.
 Postoperative period
 Long term muscle relaxant infusions
 Nerve Location

COMPLICATIONS WHILE NEUROMUSCULAR MONITORING
Underestimating the extent of blockade
Unable to elicit a twitch by electrical stimulation
 Chemical burn from electrolysis caused by direct current via surface
electrodes.
Burn with needle electrodes .
Pacemaker suppression with a nerve stimulator

Clinical Tests of Postoperative
Neuromuscular Recovery
Unreliable
• Sustained eye opening
• Protrusion of the tongue
• Arm lift to the opposite shoulder
• Normal tidal volume
• Normal or nearly normal vital capacity
• Maximum inspiratory pressure less than 40 to 50 cm H2O
Most Reliable
• Sustained head lift for 5 seconds
• Sustained leg lift for 5 seconds
• Sustained handgrip for 5 seconds
• Sustained “tongue depressor test”
• Maximum inspiratory pressure 40 to 50 cm H2O or greater

Summary
There are important reasons for monitoring the neuromuscular
function:
There is a wide variability in onset and recovery times with various relaxant
regimens.
In clinical setting, infusion of both depolarizing and nondepolarizing agents is
difficult to titrate accurately.
The development of phase 2 block following succinylcholine requires the use of a
nerve stimulator for its diagnosis.
 The newer non-depolarizing agents with short half-lives (especially mivacurium)
may have such rapid offset that monitoring is needed to assure steady
intraoperative relaxation.

CONTD…
 Patient response and adequate recovery is difficult to predict accurately in certain
disease states (myasthenia gravis, Eaton– Lambert Syndrome, hypothermia, and
hypokalemia ).
 The pharmacokinetic parameters of relaxants maybe altered in the elderly, those
with renal or hepatic impairment, and in patients with atypical or reduced
pseudocholinesterase levels.
 Objective assessment of neuromuscular function using quantitative nerve
stimulators complements clinical assessment and improves detection of residual
paralysis.
 Better understanding and wider perioperative application of neuromuscular
monitoring techniques will reduce patient morbidity and provide better anaesthetic
care.

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