NEUROMUSCULAR MONITORING
Awati M.N1, Akash.M.Awati,2 Samudyatha TJ3
1-professor& Head of the department , 2-tutor /postgraduate, 3-postgraduate
ABSTRACT : Muscle relaxants are employed in anesthesia to provide muscle relaxation
and/or abolish patient movement.
Residual postoperative neuromuscular blockade
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causes decreased chemoreceptor sensitivity to hypoxia,
functional impairment of the pharyngeal and upper esophageal muscles,
impaired ability to maintain the airway,
an increased risk for the development of postoperative pulmonary complications.
Neuromuscular blockade is monitored during surgery to guide repeated doses of muscle
relaxants and to differentiate between the types of block1.
All techniques for assessing neuromuscular blockade use a peripheral nerve stimulator (PNS)
to stimulate a motor nerve The depth of neuromuscular block (NMB) should be monitored
when muscle relaxants are used to avoid drug overdosage or underdosage and residual NMB
during recovery.
Key words : Twitch , TOF , Fade.
INTRODUCTION
Aims of Neuromuscular Monitoring1
To get precise information regarding the status of neuromuscular functioning
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Detect onset of block
Decide when to intubate
Maintain block
Prevent any sudden movements intra op
Detect when to reverse
Prevent complications of residual blockade
Patterns of Nerve Stimulation2
Single Twitch stimulation
Train of Four stimulation
Tetanic Nerve stimulation
Post Tetanic Count stimulation
Double Burst stimulation
Types of Peripheral Nerve Stimulation
Two types of stimulation can be used:
1.Electrical :
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Electrical nerve stimulation is by far the most commonly used method .
2.Magnetic:
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It is less painful
Does not require physical contact with the body.
It is bulky and heavy,
cannot be used for train-of-four (TOF) stimulation
Difficult to achieve supramaximal stimulation with this method.
Principles of Peripheral Nerve Stimulation 2
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The reaction of a single muscle fiber to a stimulus follows an all-or-none pattern.
After administration of a neuromuscular blocking drug, the response of the muscle decreases
in parallel with the number of fibers blocked.
The reduction in response during constant stimulation reflects the degree of neuromuscular
blockade.
 The force of muscle contraction is proportional to the number of activated muscle fibers.
 If a motor nerve is stimulated with sufficient current, all of the muscle fibers supplied by that
nerve will contract.
 The current required for this is called the maximal current.
Supramaximal Stimulus2
 Strength of current required to stimulate all muscle fibres in a muscle is the maximal stimulus.
 A stimulus of 20%-25 % above the maximal stimulus is the supramaximal stimulus(2.75 X
threshold current).
 Important for accurate response
Patterns of Nerve Stimulation2
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Single Twitch stimulation
Train of Four stimulation
Tetanic Nerve stimulation
Post Tetanic Count stimulation
Double Burst stimulation
1. Single – twitch stimulation2
 This is the simplest form of neurostimulation entailing a single twitch at 0.1 to 1 Hz.
 Single twitch is delivered at a supramaximal current, it induces a single nerve action potential
in each fiber of the nerve bundle
 Single twitch stimulation at 1 Hz is useful during initiation of monitoring as it shortens the
time necessary to determine supramaximal stimulation.
 During nondepolarizing block the response to single twitch stimulation is not reduced until
atleast 75 to 80% of receptors are occupied and therefore does not detect block of less than
70%.3
2. Train – of – four stimulation
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This is a popular mode of stimulation for clinical monitoring of neuromuscular junction
first described by Ali et al in 1970s 4,5.
Four successive stimuli are delivered at 2 Hz (every 0.5sec).
In the presence of non depolarizing relaxants, the margin of safety is decreased such that
some end plates in train of four progressively fade. inversely proportional to the degree of
blockade.
In the absence of non depolarizing block, the T4/T1 ratio is approximately one. For
complete recovery T4/T1 ratio should be more than 0.9
 Pattern of electrical stimulation and evoked muscle responses to TOF nerve stimulation
before and after injection of nondepolarizing (Non-dep.) and depolarizing (Dep.)
neuromuscular blocking drugs
 Disappearance of T4, T3, T2, T1 corresponds to 75%, 80%, 90% and 100% occupancy.
 With recovery of neuromuscular function,the twitches appear in the reverse order.
 Accepted values for TOF count are:
1 twitch for tracheal intubation
1–2 twitches during established anaesthesia
3–4 twitches before reversal of neuromuscular blockade is attempte
Advantages of TOF stimulation6
 Can be applied at any time during the neuromuscular block .
 Can provide quantification of depth of block without the need for control measurement before
relaxant administration.
 More sensitive to lesser degree of receptor occupancy than single twitch.
 Relatively low frequency allows response to be evaluated manually or visibly.
 No post tetanic facilitation. Can be repeated every 10-12s.
 It may be delivered at sub maximal current which is less painful and is associated with same
degree of fade.
Use
 The advantages of TOF stimulation are greatest during nondepolarizing blockade, because the
degree of block can be read directly from the TOF response even though a preoperative value
is lacking.
 It is less painful than titanic stimulation.
 Generally does not affect the degree of neuromuscular blockade like tetanic stimulation.
1. 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.
 During normal neuromuscular transmission and a pure depolarizing block, the muscle
response to 50-Hz tetanic stimulation for 5 seconds is sustained.
 During a nondepolarizing block and a phase II block after the injection of succinylcholine, the
response will not be sustained (i.e., fade occurs).
 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 tetanus, progressive depletion of acetylcholine output is balanced by increased
synthesis and transfer of transmitter from its mobilization stores.
 The presence of nondepolarizing muscle relaxants 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 6 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. Fade is first noted at 70% receptor occupancy.
 It has been shown that tetanic response to 50 Hz for five sec is sustained when TOF ratio is
greater than 0.7.
Disadvantages
 Post tetanic facilitation which depends on frequency and duration of block.
 It is very painful and therefore not suitable for unanaesthetised patients.
 In late phase of neuromuscular recovery,it may produce lasting antagonism of the NM
blockade.
FADE
 A presynaptic event during repetitive stimulation
 Non depolarising NMBs reduce the no. of free cholinergic receptors
 Also impair mobilisation of Ach from blockade of presynaptic neuronal type cholinergic
receptors
POST – TETANIC FACILITATION7
 Increase in mobilisation & synthesis of Ach continues for some time after discontinuation of
tetanic stimulation.
 Usually disappears within 60 sec.
 Event occurs because the increase in mobilization and synthesis of acetylcholine caused by
tetanic stimulation continues for some time after discontinuation of stimulation.
4. Post – tetanic count stimulation
 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.
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 Because no response to TOF and single-twitch stimulation occurs under these conditions,
these modes of stimulation cannot be used to determine the degree of blockade
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The response to PTC stimulation depends primarily on the degree of neuromuscular blockade.
It also depends 
on the frequency and duration of tetanic stimulation,
the length of time between the end of tetanic stimulation and the first post-tetanic stimulus,
the frequency of the single-twitch stimulation
APPLICATION OF PTC8
 Evaluating the degree of neuromuscular blockade when there is no reaction to single twich or
TOF as after administration of large dose of nondepolarizing muscle relaxant.
 PTC can also be used whenever sudden movement must be eliminated (Ophthalmic Surgery).
 Elimination of responses to tracheobronchial stimulation requires intense neuromuscular
blockade of zero PTC.
 PTC can be used during continuous infusion of intermediate nondepolarizing muscle relaxant
as a guidance to intensity of neuromuscular blockade.
 PTC predicts time to reappearance of first response to TOF stimulation
5. Double-Burst Stimulation2
 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 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 DBS3,3 ratio is the amplitude of the second response to DBS3,3 divided by the amplitude of
the first response.
 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).
 DBS was developed with the specific aim of allowing manual (tactile) detection of small
amounts of residual blockade under clinical conditions
 The primary use of DBS has been to detect residual NMB.
 Studies show that fade (response to the second burst weaker than that to the first) is more
readily detected with DBS than TOF using visual or tactile monitoring
 Absence of fade in manually evaluated response to DBS and TOF does not exclude residual
NM blockade 9
NERVE STIMULATOR10
 The stimulus produced should be
a. monophasic.
b. with a rectangular waveform.
c. length of the pulse generated should not exceed 0.2 – 0.3 msec.
 Stimulus at a constant current is preferable and generate 60-70 mA.
 Warning system or a current level display.
 Battery operated & include a battery check.
 Polarity of the electrodes should be indicated.
 Capable of delivering the following modes- TOF,single twitch stimulation & tetanic
stimulation.
 Built-in time constant system to facilitate PTC.
 If it does not allow permit objective measurement of response to TOF,atleast one DBF should
be available.
ELECTRODES
1. Surface Electrodes11
 disposable pregelled silver or silver chloride.
 conducting area 7-11 mm diameter.
 skin preparation prior to application of these electrodes.
2. Metal Electrodes
 Some stimulators are supplied with two metal balls or plates spaced about 1 inch apart, which
attach directly to the stimulator
3. Needle Electrodes
 Subcutaneous needles deliver the impulse in the immediate vicinity of the nerve.
 These are highly effective because they bypass the tissue impedance so that the tissue
impedance is typically <2000 Ohms.
 Ordinary steel needles can be used.
MONITORING SITES
 The specific nerve-muscle site utilized for monitoring has drawn interest in the recent years
because of the variability among muscle groups in sensitivity and onset time.12
 Relative sensitivities of muscle groups to nondepolarizing muscle relaxants
MUSCLES
SENSITIVITY
Vocal cord
Most Resistant
Diaphragm
Orbicularis oculi
Abdominal rectus
Adductor pollicis
Masseter
Pharyngeal
Extraocular
Most sensitive
1. ULNAR NERVE
 The nerve is most commonly used for neuromuscular monitoring in the perioperative period.
 The ulnar nerve innervates the adductor pollicis, abductor digiti mimimi, abductor pollicis
brevis and dorsal interosseous muscles.
 One stimulating electrode is typically placed more than 2cm proximally on the volar forearm
or over the olecranon groove.
 The recording electrodes are placed over the appropriate muscle.
ADVANTAGES
 Risk of NMB overdosage is reduced if a sensitive muscle is used for monitoring.
 When adductor pollicis recovers sufficiently,it can be assumed that no residual blockade
exists in the diaphragm or the other resistant muscles.
DISADVANTAGE
 Even total elimination of response to single twitch & TOF of adductor pollicis does not
exclude the possibility of movement of diaphragm.
 PTC can be used to overcome this drawback.
2. FACIAL NERVE
 The response to the stimulation is monitored commonly at the orbicularis oculi (contraction of
eyebrow) and orbicularis oris(contraction of the lip).
3. NERVES OF THE FOOT
i. .The posterior tibial nerve may be stimulated as it comes behind the medial malleolus,
causing plantar flexion of the great toe and foot.
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The peroneal nerve and lateral popliteal nerve elicit dorsi flexion of the foot.
 Results obtained from one muscle cannot be extrapolated to other muscles due to their
different sensitivities to NMBs.13
 The diaphragm requires 1.4 – 2 times as much muscle relaxant as the adductor pollicis for an
identical degree of blockade.
 Onset time for diaphragm is shorter than that for adductor pollicis & it recovers more quickly
from the blockade than peripheral muscles.
 Response of corrugator supercilii to facial nerve stimulation reflects the extent of NMB of
laryngeal adductors & abdominal muscles better than adductor pollicis to ulnar nerve
stimulation.14
METHODS FOR EVALUATING EVOKED RESPONSES
1. 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.
2. Tactile
 placing the evaluator's fingertips lightly over the muscle to be stimulated and feeling the
strength of contraction.
 It is more sensitive than visual monitoring for assessing NMB using TOF
 It can be used to evaluate the presence or absence of responses and/or fade with train-of-four,
double burst, and tetanic stimulation,PTC .
3. Mechanomyography15
(MMG) utilizes a force-displacement transducer, such as a strain gauge, attached to a finger
or other part of the body that can be restrained by a preload and will move when stimulated.
The transducer converts the contractile force into an electrical signal, which is amplified and
displayed on a monitor .
Single-twitch height, response to tetanic stimulation, and the T4 ratio can be accurately
measured by using an MMG.
rarely used clinically but is regarded as the gold standard for scientific measurement of
neuromuscular response
4. Acceleromyography
(ACG, AMG) a thin piezoelectric transducer or a small aluminum rod with electrodes on
both sides is fixed to the moving part
When the part moves, a voltage which is proportional to the acceleration of the moving part is
generated.
This method requires unrestricted movement of the muscle being stimulated.
Accelerometry is easy and convenient to use, relative inexpensive, and can be interfaced with
a computer
5.Kinemyography
Kinemyography (KMG) utilizes a bending sensor that is placed between the thumb and
forefinger
The core of the sensor is a piezoelectric material .
Movement is determined by the change in shape of the material when it is bent by adductor
pollicis muscle contraction.
The hand need not be immobilized
This technology can measure TOF, double burst, and single twitch.
Kinemyography
6. Piezoelectric Film
This method uses a disposable piezoelectric film
Muscle movement from evoked stimulation bends the film and generates a voltage that is
proportional to the amount of bending 13/14.
It has been used on the thumb, fifth digit, and the great to
7. Electromyography
Electromyography (EMG) is the process of recording the electrical activity of a muscle.
When a motor nerve is stimulated
Five electrodes are used
Two stimulating electrodes are placed over the nerve to be stimulated.
Three electrodes, two receiving (sensing, recording) and one ground, are used for recording
 Ulnar or median nerve most commonly used.
 Active electrode is placed over the motor point of the muscle
 Analyser picks up the signal & processes it with an amplifier,rectifier & an electronic
integrator.
 Results displayed as % of control or as TOF ratio.
 Response reflects only factors influencing NM transmission.
 Records even from inaccessible nerves
EMG has several advantages
• Less immobilization is required.
• It does not require bulky apparatus.
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It can be used to monitor muscles not available to the MMG such as the diaphragm and the
laryngeal muscles.
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It can be used to assess motor nerve block induced by regional anesthesia
Disadvantages of EMG.
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It is sensitive to electrical interference.
• The response may vary according to the muscle used.
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The equipment is expensive and takes some time and effort to set up.
• . Since the site is not immobilized, changes in the relative position of the recording electrodes
cause variation in EMG response.
• amplitude increasing with decreasing muscle temperature
8. Phonomyography
 Skeletal muscle contraction generates intrinsic low – frequency sounds which are recorded
with special microphones.
 Ease of application even to muscles of diaphragm,larynx,eye
Evaluation of Recorded Evoked Responses.
Nondepolarizing Neuromuscular Blockade
1. Intense neuromuscular blockade:
• occurs within 3 to 6 minutes of injection of an intubating dose of a NDMR
• also called the “period of no response.
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The length of this period varies, depending primarily on the duration of action of the muscle
relaxant and the dose.
2.Deep Neuromuscular Blockade:
4. characterized by absence of response to TOF stimulation.
5. presence of post-tetanic twitches.
6. correlation does exist between PTC stimulation and the time until reappearance of the first
response to TOF stimulation
3.Moderate or Surgical Blockade
7. Moderate or surgical blockade begins when the first response to TOF stimulation appears.
8. This phase is characterized by a gradual return of the four responses to TOF stimulation.
9. good correlation exists between the degree of neuromuscular blockade and the number of
responses to TOF stimulation.
10. only one response is detectable the degree of neuromuscular blockade is 90% to 95%.
11. When the fourth response reappears, neuromuscular blockade is usually 60% to 85% 15,16.
12. when elimination of sudden movements is crucial, a deeper block (or a deeper level of
anesthesia) may be necessary.
13. The deep block can then be evaluated by PTC.
 Antagonism with cholinesterase inhibitors should not be initiated before at least 2 or
preferably 3 or 4 responses are observed.
RECOVERY
 Return of 4th response in TOF begins the recovery phase.
 TOF 0.4 – tidal volume may be normal
 TOF 0.6 – head lift for 3s,eye opening widely, sticking out of the tongue
 TOF 0.7 to 0.75 –
cough, head lift for 5sec.
decreased handgrip strength.
inability to maintain apposition of incisor teeth.
severe facial weakness.
 TOF 0.8 –
vital capacity & inspiratory force are normal.
diplopia & visual disturbances.
generalised fatigue.
Depolarizing Neuromuscular Blockade (Phase I and II Blocks)
phase I block the response to TOF or tetanic stimulation does not
fade, and no post-tetanic facilitation of transmission
occurs)
phase II block (dual, mixed, or desensitizing block)
In patients with genetically determined abnormal plasma cholinesterase activity who are
given the same dose of succinylcholine
characterized by fade in the response to TOF and tetanic stimulation and the occurrence of
post-tetanic facilitation of transmission.
 Phase II block in normal patients can be antagonised by cholinesterase inhibitor a few minutes
after discontinuation of succinylcholine.
 In patients with abnormal cholinesterase,response to cholinesterase inhibitor is unpredictible.
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
Use of a Peripheral Nerve Stimulator during Induction of Anesthesia
Use Single-twitch stimulation at 1 Hz to seek supramaximal stimulation.
Before muscle relaxant is injected, the mode of stimulation should be changed to TOF (or 0.1-Hz
twitch stimulation).
Then, after the response to this stimulation has been observed (the control response), the
neuromuscular blocking agent is injected.
Intubate, when the response to TOF stimulation disappears
Use of a Peripheral Nerve Stimulator During Surgery
For most surgical procedures requiring muscle relaxation, twitch depression of approximately
90% will be sufficient, provided that the patient is adequately anesthetized.
To ensure paralysis of the diaphragm, neuromuscular blockade of the peripheral muscles must be
so intense that the PTC is zero in the thumb.
An added advantage of keeping the neuromuscular blockade at a level of one or two responses to
TOF stimulation is that antagonism of the block is facilitated at the end of surgery.
Use of a Peripheral Nerve Stimulator during Reversal of Neuromuscular Blockade
Initiate neostigmine after 2 responses are seen
During recovery of neuromuscular function, when all four responses to TOF stimulation can be
felt, an estimation of the TOF ratio may be attempted.
Manual evaluation of muscle strength after TOF and even DB3,3 is not sensitive enough to rule out
neuromuscular blockade.
Therefore manual evaluation of responses to nerve stimulation should be combined with clinical
symptoms and signs
References:
1. Millers anaesthesia 7th edtion
2. Dorsch and Dorsch Understanding Anesthesia Equipment.5th.Edition.
3. Curran MJ, Donati F, Bevan DR: Onset and recovery of atracurium and suxamethonium-induced
neuromuscular blockade with simultaneous train-of-four and single twitch stimulation. Br J Anaesth
59:989,1987.
4. Ali HH, Utting JE, Gray C: Stimulus frequency in the detection of neuromuscular blocks in humans. Br J
Anaesth 42:967, 1970.
5. Ali HH, Utting JE, Gray C: Quantitative assessment of residual antidepolarizing block (part II). Br J
Anaesth 43:478, 1971.
6. Jonsson M,Gurley D, Dabrowski M, et al: Distinct pharmacologic properties of neuromuscular blocking
agents on human neuronal nicotinic acetylcholine receptors. A possible explanation for the train-of-four
fade. Anesthesiology 105:521, 2006.
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response during intense neuromuscular blockade caused by atracurium. Br J Anaesth 59:1089, 1987.
8. Muchhal KK, Viby-Mogensen J, Fernando PUE, et al: Evaluation of intense neuromuscular blockade
caused by vecuronium using post-tetanic count (PTC). Anesthesiology 66:846, 1987.
9. Fernando PUE, Viby-Mogensen J, Bonsu AK, et al: Relationship between post-tetanic count and
response to carinal stimulation during vecuronium-induced neuromuscular blockade. Acta Anaesthesiol
Scand 31:593, 1987.
10. Schultz P, Ibsen D, Østergaard D, et al: Onset and duration of action of rocuronium: From tracheal
intubation, through intense block to complete recovery. Acta Anaesthesiol Scand 45:612, 2001.
11. El-Orbany MI, Joseph JN, Salem MR: The relationship of post-tetanic count and train-of-four
responses during recovery from intense cisatracurium-induced neuromuscular blockade. Anesth Analg
97:80, 2003.
12. Fruergaard K, Viby-Mogensen J, Berg H, et al: Tactile evaluation of the response to double burst
stimulation decreases, but does not eliminate the problem of postoperative residual paralysis. Acta
Anaesthesiol Scand 42:1168, 1998.
13. Kern SE, Johnson JO, Westenkow DR, et al: An effectiveness study of a new piezoelectric sensor for
train of-four measurement. Anesth Analg 78:978, 1994.
14. Pelgrims K, Vanacker B: Comparative study of the TOF-ratio measured by the ParaGraph versus the
TOF-Guard, with and without thumb repositioning. Acta Anaesthesiol Belg 52:297, 2001.
15. Gibson FM, Mirakhur RK, Clarke RSJ, et al: Quantification of train-of-four responses during recovery
of block from non-depolarizing muscle relaxants. Acta Anaesthesiol Scand 31:655, 1987.
16. O’Hara DA, Fragen RJ, Shanks CA: Comparison of visual and measured train-of-four recovery after
vecuronium-induced neuromuscular blockade using two anaesthetic techniques. Br J Anaesth 58:1300,
1986.