June 8, 2025 • 35 mins
Continue your journey to mastering anaesthesia—one chapter at a time.
In this episode, Dr. J.R. Decker reads and discusses Chapter 6 (Part 3) of Morgan & Mikhail’s Clinical Anesthesiology (7th Edition).
Follow as you read along to strengthen your foundations in anaesthesia, one clear and engaging session at a time.
Perfect for trainees, revision, or daily listening.
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Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
(00:01):
Morgan and Mikhail's Clinical Anesthesiology, 7th Edition,
Chapter 6, Part 3. Cerebral oximetry and other
monitors of the brain. Cerebral oximetry uses near
(00:25):
infrared spectroscopy. NIRS near infrared light is
emitted by a probe on the scalp.Receptors are positioned to
detect the reflected light from both deep and superficial
(00:50):
structures. As with pulse oximetry,
oxygenated haemoglobin and deoxygenated haemoglobin absorb
light at different frequencies. Likewise, cytochrome absorbs
(01:11):
infrared light in the mitochondria.
The the NIRS saturation largely reflects the absorption of
venous haemoglobin as it does not have the ability to identify
the pulsatile arterial component.
(01:37):
Regional saturations of less than 40% on NIRS measures or
changes of greater than 25% of baseline measures may herald
neurological events secondary todecreased cerebral oxygenation.
(02:03):
Next we get to Figure 6-10, explaining the principle of
inverse near infrared spectroscopy technique.
Can it take a while to go through this?
(02:24):
Reduced jugular venous bulb saturation can also provide an
indication of increased cerebraltissue oxygen extraction or
decreased cerebral oxygen delivery.
Direct tissue oxygen monitoring of the brain can be accomplished
(02:48):
by the placement of a probe in or on brain tissue.
Interventions to preserve brain tissue oxygenation are called
for when oxygen tissue tension is less than 20 millimetres of
mercury. Such interventions improve
(03:12):
oxygen delivery by increasing FIO 2, increasing haemoglobin,
improving cardiac outputs, decreasing oxygen demand, for
example, with sedative, stroke, hypnotic drugs or a combination
(03:33):
of these methods. Other monitors.
Temperature indications The temperature of patients
(03:54):
undergoing anaesthesia should bemonitored during all but the
shortest anaesthetics. Post operative temperature is
increasingly used as a measurement of anaesthesia
quality. Hypothermia is associated with
(04:15):
delayed drug metabolism, hyperglycemia, vasoconstriction,
impaired coagulation, post operative shivering accompanied
by tachycardia and hypertension,and increased risk of surgical
(04:39):
site infections. Hypothermia can lead to
tachycardia, vasodilation, and neurological injury.
Consequently, temperature must be measured and recorded
(05:01):
perioperatively. Contraindications.
There are no contraindications, though a particularly a
particular monitoring site may be unsuitable in certain
patients. Techniques and complications.
(05:29):
Intraoperatively, temperature isusually measured using a
thermistor or thermocouple. Thermistors as semiconductors
whose resistance decreases predictably with warming.
A thermocouple is a circuit of two dissimilar metals joined so
(05:56):
that a potential difference is generated when the metals are at
different temperatures. Disposable thermocouple and
thermistor probes are available for monitoring the temperature
of the tympanic membrane, nasopharynx, oesophagus,
(06:20):
bladder, rectum, and skin. Infrared sensors estimate
temperature from the infrared energy that is produced.
Tympanic membrane temperatures reflect core body temperature.
(06:41):
However, the devices used may not reliably measure the
temperature at the tympanic membrane.
Complications of temperature monitoring are usually related
to trauma caused by the probe, that is, for example, rectal or
(07:01):
tympanic membrane perforation. Clinical considerations Each
monitoring site has advantages and disadvantages.
The tympanic membrane theoretically reflects brain
(07:23):
temperature because the auditorycanal's blood supply is the
external carotid artery. Trauma during insertion and
cerumen insulation detract from the routine use of tympanic
probes. Rectal temperature probes have a
(07:46):
slow response to changes in coretemperature.
Nasopharyngeal probes are prone to cause epistaxes, but they
accurately measure core temperature if placed adjacent
to the nasopharyngeal mucosa. The thermistor in a pulmonary
(08:11):
artery catheter also measures core temperature.
There is a variable correlation between auxiliary temperature
and core temperature, depending on skin perfusion.
Liquid crystal adhesive strips placed on the skin are
(08:34):
inadequate indicators of core body temperature during surgery.
Esophagal temperature sensors, often incorporated into
esophagal stethoscopes, provide the best combination of economy,
performance, and safety. The temperature sensor should be
(08:58):
positioned behind the heart in the lower third of the
oesophagus to avoid measuring the temperature of tracheal
gases. Conveniently, heart sounds are
most prominent at this location.For more on the clinical
(09:21):
considerations of temperature control, see Chapter 52, Urinary
Outputs Indications. Urinary bladder catheterization
(09:42):
is the most reliable method of monitoring urinary outputs.
Catheterization is routine in some complex and prolonged
surgical procedures such as cardiac surgery, aortic or renal
vascular surgery, craniotomy, major abdominal surgery, or
(10:06):
procedures in which large fluid shifts are expected.
Lengthy surgeries and intraoperative diuretic
administration are also are other possible indications.
Occasionally post operative bladder catheterization is
(10:29):
indicated in patients who have difficulty voiding in the
recovery room after general or regional anaesthesia.
Contra indications. Foley Catheters should be
(10:49):
removed as soon as feasible to minimise the risk of catheter
associated urinary tract infections, techniques and
complications. Bladder catheterization is
(11:10):
usually performed by surgical ornursing personnel.
The urologist may be needed to catheterize patients with
strictures and other abnormal urethral anatomy.
A soft rubber follic catheter isinserted into the bladder, trans
(11:34):
urethrally and connected to a disposable, calibrated
collection chamber. To avoid urine reflux and
minimise the risk of infection, the collection chamber should
remain at a level below the bladder.
(11:55):
Complications of catheterizationinclude urethral trauma and
urinary tract infections. Rapid decompression of a
distended bladder can cause hypertension.
Suprapubric drainage of the bladder with tubing inserted
(12:19):
through a large burn needle is an uncommon alternative.
Clinical considerations. An additional advantage of
placing a Foley catheter is the ability to include a thermistor
(12:39):
in the catheter tip so that bladder temperature can be
monitored. As long as urinary output is
high, bladder temperature accurately reflects core
temperature. An added value of the more
widespread use of Uro metres is the ability to electronically
(13:05):
monitor and record urinary outputs and temperature.
Urinary output is an imperfect reflection of kidney profusion
and function and of renal, cardiovascular and fluid volume
status. Non invasive monitors of cardiac
(13:31):
function and output, that is, including echocardiography,
provide more reliable assessments of the adequacy of
intravascular volume. Inadequate urinary outputs, that
is, oligoria is often arbitrarily defined as a urinary
(13:55):
output of less than 0.5 mils perkilogramme per hour, but
actually is a function of the patient's concentrating ability
and osmotic load. Urine electrolyte composition,
osmolality, and specific gravityaid in the differential
(14:20):
diagnosis of oliguria. Peripheral nerve stimulation
indications Because of the variation in patient sensitivity
to neuromuscular blocking agents, the neuromuscular
(14:44):
function of all patients receiving intermediate or long
acting neuromuscular blocking agents must be monitored.
In addition, peripheral nerve stimulation is helpful in
detecting the onset of paralysisduring anaesthesia inductions or
(15:06):
the adequacy of the block duringcontinuous infusions with short
acting agents. Contraindications There are no
contraindications to neuromuscular monitoring, though
(15:26):
setting sites may be precluded by the surgical procedure.
Additionally, atrophied muscles in areas of hemiplasia or nerve
damage may appear refractory to remote neuromuscular blockade
secondary to the proliferation of receptors.
(15:50):
Determining the degree of neuromuscular blockade using
such an extreme meet such an extremity could lead to the
potential overdosing of competitive neuromuscular
blocking agents, techniques and complications.
A peripheral nerve stimulation stimulator delivers currents,
(16:18):
that is, 60 to 80 milliamperes to a pair of either ECG silver
chloride pads or subcutaneous needles placed over a peripheral
motor nerve. The evoked mechanical or
electrical response of the innovative muscle is observed.
(16:43):
Although electromyography provides a fast, accurate and
quantitative measure of neuromuscular transmission,
visual or tactile observation ofmuscle contraction is usually
relied upon in clinical practise.
Ulnar nerve stimulation of the abductor polysis muscle and
(17:08):
facial nerve stimulation of the orbicularis ocular are most
commonly monitored. Because it is the inhibition of
the neuromuscular receptor that needs to be monitored, direct
stimulation of muscle should be avoided by placing electrodes
(17:30):
over the cause of the nerve and not over the muscle itself.
Peripheral nerve stimulators must be capable of generating at
least 50 milliampers current across A-1000 ohm load to
(17:51):
deliver a supramaximal stimulation to the underlying
nerve. This current is uncomfortable
for a conscious patient. Complications of nerve
stimulation are limited to skin irritation and abrasion at the
(18:11):
site of electrode attachment. Next we get to Figure 6-11A and
B talking about stimulation of the ulnar nerve which causes
contraction of the abductor polysis muscle, as stimulation
of the facial nerve, which leadsto obicularis ocular muscle
(18:35):
contraction. Take you a while to study this
figure because of concerns of residual neuromuscular blockade,
Increased attention has been focused on providing
(18:55):
quantitative measures of the degree of neuromuscular blockade
post perioperatively. Acceleromyography uses a
piezoelectric transducer on the muscle to be stimulated.
(19:16):
The movement of the muscle generates an electrical current
that can be quantified and displayed.
Indeed, aceleromyography can better predict residual
paralysis compared with routine tactile train of four monitoring
(19:38):
used in most operating rooms if it is calibrated from the
beginning of the operative period to establish baselines
before administration of neuromuscular blocking agents.
Clinical Considerations The degree of neuromuscular blockade
(20:04):
is monitored by applying variouspatterns of electrical
stimulation. All stimuli are 20.
Microseconds in duration and of square wave pattern and equal
current intensity. A twitch is a single pulse that
(20:29):
is delivered from everyone to every 10 seconds that is 1 to
0.1 Hertz. Increasing block results in
decreased evoke response to stimulation.
Next we get to Figure 6-12, describing peripheral nerve
(20:53):
stimulators which can generate various patterns of electrical
impulses. Kindly pause this recording to
go through Figure 6-12. Train of four stimulation
denotes 4 successive 200 microseconds stimuli in two
(21:20):
seconds that is 2 Hertz. The twitches in the train of
four pattern progressively feed as non depolarizing muscle
relaxant block increases. The ratio of the responses to
the 1st and 4th twitches is a sensitive indicator of non
(21:45):
depolarizing muscle paralysis. Because it is difficult to
estimate the train of four ratio, it is more convenient to
visually observe the sequential disappearance of the twitches,
as this also correlates with theextent of blockade.
(22:11):
The disappearance of the 4th twitch represents a 75% block,
the third twitch an 80% block, and the second twitch in 90%
block. Clinical relaxation usually
requires 75 to 95% neuromuscularblockade.
(22:40):
Tetani at 50 or 100 Hertz is a sensitive test of neuromuscular
function. Sustained contraction for five
seconds indicates adequate, but not necessarily complete
reversal from neuromuscular blockade.
(23:04):
Double burst stimulation, that is DBS represents 2 variations
of tetani that are less painful to the patient.
The DBS 33 pattern of nerve stimulation consists of three
(23:25):
shot that is 20 microsecond, 200microsecond high frequency
bursts separated by 20 millisecond intervals, that is
50 Hertz followed by followed 750 milliseconds later by
(23:49):
another three birds. DBS 32 consists of 3200
microsecond impulses at 50 Hertz, followed 750 milliseconds
later by two such impulses. DBS is more sensitive than train
(24:17):
or force stimulation for the clinical, that is, visual
evaluation of fate 2. Because muscle groups differ in
their sensitivity to neuromuscular blocking agents,
use of the peripheral nerve stimulator cannot replace direct
(24:41):
observation of the muscles, for example, the diaphragm, that
need to be relaxed for a specific surgical procedure.
Furthermore, recovery of abductor policies function does
not exactly parallel recovery ofmuscles required to maintain an
(25:05):
airway. The diaphragm, rectus abdominis,
laryngeal abductors and herbicularis ocular muscles
recover from neuromuscular blockade sooner than the
abductor policies. Other indicators of adequate
(25:28):
recovery include sustained that is greater than or equals to 5
second head lift, the ability togenerate an inspiratory pressure
of at least -25 centimetres of water, and a forceful hand grip.
(25:50):
Twitch tension is reduced by hypothermia of the monitored
muscle group, that is 60% per degrees centigrade.
Decisions regarding the adequacyof reversal of neuromuscular
blockade as well as the timing of extubation should be made
(26:15):
only by considering the patient's clinical presentation
and assessments determined by peripheral nerve stimulation.
Post operative residual paralysis remains a problem in
post anaesthesia care, producingpotentially injurious airway and
(26:39):
respiratory function compromise and increasing length of stay
and cost in the post anaesthesiacare unit.
PACU reversal of neuromuscular blocking agents is warranted, as
is the use of intermediate acting neuromuscular blocking
(27:01):
agents instead of longer acting drugs.
Quantitative monitors of neuromuscular blockade are
recommended to reduce the incidence of patients admitted
to the PACU with residual paralysis.
(27:24):
Case Discussion. Monitoring during magnetic
resonance imaging. A 50 year old patient with
recent onset of seizures is scheduled for magnetic resonance
(27:44):
imaging MRI. A prior MRI attempt was
unsuccessful because of the patient's severe claustrophobic
reaction. The radiologist requests your
help in providing either sedation or general anaesthesia.
(28:07):
Question Why does the MRI suit pose special problems for the
patient and the anesthesiologist?
Answer MRI studies tend to be long, that is often more than
one hour, and many scanners totally surround the body
(28:29):
causing a high incidence of claustrophobia in patients
already anxious about their health.
Patient discomfort may also be amplified by pre-existing pain.
Good imaging requires immobility, something that is
difficult to achieve in many patients without sedation or
(28:54):
general anaesthesia. Because the MRI device uses a
powerful magnet, no ferromagnetic objects can be
placed near the scanner. This includes implanted
prosthetic joints, artificial pacemakers, surgical clips,
(29:16):
batteries, ordinary anaesthesia machines, watches, pens, credit
cards, intravenous poles, conventional oxygen cylinders,
housekeeping buckets, and lead ankle weights, nearly all of
(29:38):
which we have seen drawn in and captured by MRI devices.
Ordinary metal LED wires for pulse oximeters or
electrocardiography act as antennas and may attract enough
radio frequency energy to distort the MRI or even cause
(30:03):
patient burns. In addition, the scanner's
magnetic field causes severe monitor artefacts.
The more powerful the scanner's magnet is, as measured in Tesla
units, that is 1 Tesla is equalsto 10,000 ghosts, the greater
(30:30):
the potential problem. Other problems include poor
access to the patient during theimaging, particularly the
patient's airway, hypothermia inpaediatric patients, dim
lighting within the patient tunnel, and very loud noise that
(30:53):
is up to 100 decibels. Question How have these
monitoring and anaesthesia machine problems been addressed?
Answer Equipment manufacturers have modified monitors so that
(31:17):
they are compatible with the MRIenvironment.
These modifications include non ferromagnetic
electrocardiographic electrodes,graphite and copper cables,
extensive filtering and gating of signals, extra long blood
(31:39):
pressure cuff tubing and the useof fibre optic technologies.
Anaesthesia machines with no ferromagnetic components, that
is, for example aluminium gas cylinders, have been fitted with
MRI compatible ventilators and long circle systems on Mepos and
(32:07):
deep breathing circuits. Question What factors influence
the choice between general anaesthesia and intravenous
sedation? Answer Although most patients
(32:28):
will tolerate an MRI study with sedation, head injured and
paediatric patients presents special challenges and often
require general anaesthesia because of machine and
monitoring limitations. An argument could be made that
(32:49):
sedation, when possible, would be a safer choice.
On the other hand, loss of airway control from deep
sedation could prove catastrophic because of poor
patient access and delayed detection.
(33:11):
Other important considerations include the monitoring
modalities available at a particular facility and the
General Medical condition of thepatient.
Question Which monitors should be considered mandatory in this
(33:32):
case? Answer The patient should
receive at least the same level of monitoring and care in the Mr
suite as in the operating room for a similarly non invasive
procedure. Thus, the American Society of
(33:53):
Anesthesiologists standards for basic anaesthetic monitoring see
the following guidelines section.
Apply as they would to a patientundergoing sedation or general
anaesthesia in order anesthesizing locations.
(34:16):
Continuous auscultation of breath sounds with a plastic,
not metal, precordial stethoscope can help identify
airway obstruction caused by excessive sedation.
Palpation is a peripheral pulse.Sorry, palpation of a peripheral
(34:38):
pulse or listening for corrupt cough sounds is impractical in
these settings. Ensuring adequacy of circulation
depends on electrocardiographic and oscillometric blood pressure
monitoring and tidal carbon dioxide analysis can be adapted
(35:02):
to sedation cases by connecting the sampling line to a site near
the patient's mouth or nose. If a nasal cannula with a carbon
dioxide sampling channel is not available because room air
entrainment precludes exact measurements, this technique
(35:27):
provides a qualitative indicatorof ventilation whenever sedation
is planned. Equipment for emergency
conversion to general anaesthesia, for example,
tracheal tubes. Resuscitation bag must be
immediately available. Although MRI suits are often
(35:50):
located in remote areas far fromthe operating room suite,
appropriate equipment and medication must still be
immediately available for anaesthesia related emergencies
such as a difficult airway or malignant hypothermia.
(36:10):
Question Is the continuous presence of qualified
anaesthesia personnel required during these cases?
Answer Absolutely yes. Sedated patients must have
continuously monitored anaesthesia care to prevent a
(36:31):
multitude of unforeseen complications such as apnea or M
SS. We come to the end of Chapter 6.
Morgan and Mikhail's Clinical Anesthesiology, 7th Edition,
Chapter 6, Part 3. Cerebral oximetry and other
monitors of the brain. Cerebral oximetry uses near
(00:25):
infrared spectroscopy. NIRS near infrared light is
emitted by a probe on the scalp.Receptors are positioned to
detect the reflected light from both deep and superficial
(00:50):
structures. As with pulse oximetry,
oxygenated haemoglobin and deoxygenated haemoglobin absorb
light at different frequencies. Likewise, cytochrome absorbs
(01:11):
infrared light in the mitochondria.
The the NIRS saturation largely reflects the absorption of
venous haemoglobin as it does not have the ability to identify
the pulsatile arterial component.
(01:37):
Regional saturations of less than 40% on NIRS measures or
changes of greater than 25% of baseline measures may herald
neurological events secondary todecreased cerebral oxygenation.
(02:03):
Next we get to Figure 6-10, explaining the principle of
inverse near infrared spectroscopy technique.
Can it take a while to go through this?
(02:24):
Reduced jugular venous bulb saturation can also provide an
indication of increased cerebraltissue oxygen extraction or
decreased cerebral oxygen delivery.
Direct tissue oxygen monitoring of the brain can be accomplished
(02:48):
by the placement of a probe in or on brain tissue.
Interventions to preserve brain tissue oxygenation are called
for when oxygen tissue tension is less than 20 millimetres of
mercury. Such interventions improve
(03:12):
oxygen delivery by increasing FIO 2, increasing haemoglobin,
improving cardiac outputs, decreasing oxygen demand, for
example, with sedative, stroke, hypnotic drugs or a combination
(03:33):
of these methods. Other monitors.
Temperature indications The temperature of patients
(03:54):
undergoing anaesthesia should bemonitored during all but the
shortest anaesthetics. Post operative temperature is
increasingly used as a measurement of anaesthesia
quality. Hypothermia is associated with
(04:15):
delayed drug metabolism, hyperglycemia, vasoconstriction,
impaired coagulation, post operative shivering accompanied
by tachycardia and hypertension,and increased risk of surgical
(04:39):
site infections. Hypothermia can lead to
tachycardia, vasodilation, and neurological injury.
Consequently, temperature must be measured and recorded
(05:01):
perioperatively. Contraindications.
There are no contraindications, though a particularly a
particular monitoring site may be unsuitable in certain
patients. Techniques and complications.
(05:29):
Intraoperatively, temperature isusually measured using a
thermistor or thermocouple. Thermistors as semiconductors
whose resistance decreases predictably with warming.
A thermocouple is a circuit of two dissimilar metals joined so
(05:56):
that a potential difference is generated when the metals are at
different temperatures. Disposable thermocouple and
thermistor probes are available for monitoring the temperature
of the tympanic membrane, nasopharynx, oesophagus,
(06:20):
bladder, rectum, and skin. Infrared sensors estimate
temperature from the infrared energy that is produced.
Tympanic membrane temperatures reflect core body temperature.
(06:41):
However, the devices used may not reliably measure the
temperature at the tympanic membrane.
Complications of temperature monitoring are usually related
to trauma caused by the probe, that is, for example, rectal or
(07:01):
tympanic membrane perforation. Clinical considerations Each
monitoring site has advantages and disadvantages.
The tympanic membrane theoretically reflects brain
(07:23):
temperature because the auditorycanal's blood supply is the
external carotid artery. Trauma during insertion and
cerumen insulation detract from the routine use of tympanic
probes. Rectal temperature probes have a
(07:46):
slow response to changes in coretemperature.
Nasopharyngeal probes are prone to cause epistaxes, but they
accurately measure core temperature if placed adjacent
to the nasopharyngeal mucosa. The thermistor in a pulmonary
(08:11):
artery catheter also measures core temperature.
There is a variable correlation between auxiliary temperature
and core temperature, depending on skin perfusion.
Liquid crystal adhesive strips placed on the skin are
(08:34):
inadequate indicators of core body temperature during surgery.
Esophagal temperature sensors, often incorporated into
esophagal stethoscopes, provide the best combination of economy,
performance, and safety. The temperature sensor should be
(08:58):
positioned behind the heart in the lower third of the
oesophagus to avoid measuring the temperature of tracheal
gases. Conveniently, heart sounds are
most prominent at this location.For more on the clinical
(09:21):
considerations of temperature control, see Chapter 52, Urinary
Outputs Indications. Urinary bladder catheterization
(09:42):
is the most reliable method of monitoring urinary outputs.
Catheterization is routine in some complex and prolonged
surgical procedures such as cardiac surgery, aortic or renal
vascular surgery, craniotomy, major abdominal surgery, or
(10:06):
procedures in which large fluid shifts are expected.
Lengthy surgeries and intraoperative diuretic
administration are also are other possible indications.
Occasionally post operative bladder catheterization is
(10:29):
indicated in patients who have difficulty voiding in the
recovery room after general or regional anaesthesia.
Contra indications. Foley Catheters should be
(10:49):
removed as soon as feasible to minimise the risk of catheter
associated urinary tract infections, techniques and
complications. Bladder catheterization is
(11:10):
usually performed by surgical ornursing personnel.
The urologist may be needed to catheterize patients with
strictures and other abnormal urethral anatomy.
A soft rubber follic catheter isinserted into the bladder, trans
(11:34):
urethrally and connected to a disposable, calibrated
collection chamber. To avoid urine reflux and
minimise the risk of infection, the collection chamber should
remain at a level below the bladder.
(11:55):
Complications of catheterizationinclude urethral trauma and
urinary tract infections. Rapid decompression of a
distended bladder can cause hypertension.
Suprapubric drainage of the bladder with tubing inserted
(12:19):
through a large burn needle is an uncommon alternative.
Clinical considerations. An additional advantage of
placing a Foley catheter is the ability to include a thermistor
(12:39):
in the catheter tip so that bladder temperature can be
monitored. As long as urinary output is
high, bladder temperature accurately reflects core
temperature. An added value of the more
widespread use of Uro metres is the ability to electronically
(13:05):
monitor and record urinary outputs and temperature.
Urinary output is an imperfect reflection of kidney profusion
and function and of renal, cardiovascular and fluid volume
status. Non invasive monitors of cardiac
(13:31):
function and output, that is, including echocardiography,
provide more reliable assessments of the adequacy of
intravascular volume. Inadequate urinary outputs, that
is, oligoria is often arbitrarily defined as a urinary
(13:55):
output of less than 0.5 mils perkilogramme per hour, but
actually is a function of the patient's concentrating ability
and osmotic load. Urine electrolyte composition,
osmolality, and specific gravityaid in the differential
(14:20):
diagnosis of oliguria. Peripheral nerve stimulation
indications Because of the variation in patient sensitivity
to neuromuscular blocking agents, the neuromuscular
(14:44):
function of all patients receiving intermediate or long
acting neuromuscular blocking agents must be monitored.
In addition, peripheral nerve stimulation is helpful in
detecting the onset of paralysisduring anaesthesia inductions or
(15:06):
the adequacy of the block duringcontinuous infusions with short
acting agents. Contraindications There are no
contraindications to neuromuscular monitoring, though
(15:26):
setting sites may be precluded by the surgical procedure.
Additionally, atrophied muscles in areas of hemiplasia or nerve
damage may appear refractory to remote neuromuscular blockade
secondary to the proliferation of receptors.
(15:50):
Determining the degree of neuromuscular blockade using
such an extreme meet such an extremity could lead to the
potential overdosing of competitive neuromuscular
blocking agents, techniques and complications.
A peripheral nerve stimulation stimulator delivers currents,
(16:18):
that is, 60 to 80 milliamperes to a pair of either ECG silver
chloride pads or subcutaneous needles placed over a peripheral
motor nerve. The evoked mechanical or
electrical response of the innovative muscle is observed.
(16:43):
Although electromyography provides a fast, accurate and
quantitative measure of neuromuscular transmission,
visual or tactile observation ofmuscle contraction is usually
relied upon in clinical practise.
Ulnar nerve stimulation of the abductor polysis muscle and
(17:08):
facial nerve stimulation of the orbicularis ocular are most
commonly monitored. Because it is the inhibition of
the neuromuscular receptor that needs to be monitored, direct
stimulation of muscle should be avoided by placing electrodes
(17:30):
over the cause of the nerve and not over the muscle itself.
Peripheral nerve stimulators must be capable of generating at
least 50 milliampers current across A-1000 ohm load to
(17:51):
deliver a supramaximal stimulation to the underlying
nerve. This current is uncomfortable
for a conscious patient. Complications of nerve
stimulation are limited to skin irritation and abrasion at the
(18:11):
site of electrode attachment. Next we get to Figure 6-11A and
B talking about stimulation of the ulnar nerve which causes
contraction of the abductor polysis muscle, as stimulation
of the facial nerve, which leadsto obicularis ocular muscle
(18:35):
contraction. Take you a while to study this
figure because of concerns of residual neuromuscular blockade,
Increased attention has been focused on providing
(18:55):
quantitative measures of the degree of neuromuscular blockade
post perioperatively. Acceleromyography uses a
piezoelectric transducer on the muscle to be stimulated.
(19:16):
The movement of the muscle generates an electrical current
that can be quantified and displayed.
Indeed, aceleromyography can better predict residual
paralysis compared with routine tactile train of four monitoring
(19:38):
used in most operating rooms if it is calibrated from the
beginning of the operative period to establish baselines
before administration of neuromuscular blocking agents.
Clinical Considerations The degree of neuromuscular blockade
(20:04):
is monitored by applying variouspatterns of electrical
stimulation. All stimuli are 20.
Microseconds in duration and of square wave pattern and equal
current intensity. A twitch is a single pulse that
(20:29):
is delivered from everyone to every 10 seconds that is 1 to
0.1 Hertz. Increasing block results in
decreased evoke response to stimulation.
Next we get to Figure 6-12, describing peripheral nerve
(20:53):
stimulators which can generate various patterns of electrical
impulses. Kindly pause this recording to
go through Figure 6-12. Train of four stimulation
denotes 4 successive 200 microseconds stimuli in two
(21:20):
seconds that is 2 Hertz. The twitches in the train of
four pattern progressively feed as non depolarizing muscle
relaxant block increases. The ratio of the responses to
the 1st and 4th twitches is a sensitive indicator of non
(21:45):
depolarizing muscle paralysis. Because it is difficult to
estimate the train of four ratio, it is more convenient to
visually observe the sequential disappearance of the twitches,
as this also correlates with theextent of blockade.
(22:11):
The disappearance of the 4th twitch represents a 75% block,
the third twitch an 80% block, and the second twitch in 90%
block. Clinical relaxation usually
requires 75 to 95% neuromuscularblockade.
(22:40):
Tetani at 50 or 100 Hertz is a sensitive test of neuromuscular
function. Sustained contraction for five
seconds indicates adequate, but not necessarily complete
reversal from neuromuscular blockade.
(23:04):
Double burst stimulation, that is DBS represents 2 variations
of tetani that are less painful to the patient.
The DBS 33 pattern of nerve stimulation consists of three
(23:25):
shot that is 20 microsecond, 200microsecond high frequency
bursts separated by 20 millisecond intervals, that is
50 Hertz followed by followed 750 milliseconds later by
(23:49):
another three birds. DBS 32 consists of 3200
microsecond impulses at 50 Hertz, followed 750 milliseconds
later by two such impulses. DBS is more sensitive than train
(24:17):
or force stimulation for the clinical, that is, visual
evaluation of fate 2. Because muscle groups differ in
their sensitivity to neuromuscular blocking agents,
use of the peripheral nerve stimulator cannot replace direct
(24:41):
observation of the muscles, for example, the diaphragm, that
need to be relaxed for a specific surgical procedure.
Furthermore, recovery of abductor policies function does
not exactly parallel recovery ofmuscles required to maintain an
(25:05):
airway. The diaphragm, rectus abdominis,
laryngeal abductors and herbicularis ocular muscles
recover from neuromuscular blockade sooner than the
abductor policies. Other indicators of adequate
(25:28):
recovery include sustained that is greater than or equals to 5
second head lift, the ability togenerate an inspiratory pressure
of at least -25 centimetres of water, and a forceful hand grip.
(25:50):
Twitch tension is reduced by hypothermia of the monitored
muscle group, that is 60% per degrees centigrade.
Decisions regarding the adequacyof reversal of neuromuscular
blockade as well as the timing of extubation should be made
(26:15):
only by considering the patient's clinical presentation
and assessments determined by peripheral nerve stimulation.
Post operative residual paralysis remains a problem in
post anaesthesia care, producingpotentially injurious airway and
(26:39):
respiratory function compromise and increasing length of stay
and cost in the post anaesthesiacare unit.
PACU reversal of neuromuscular blocking agents is warranted, as
is the use of intermediate acting neuromuscular blocking
(27:01):
agents instead of longer acting drugs.
Quantitative monitors of neuromuscular blockade are
recommended to reduce the incidence of patients admitted
to the PACU with residual paralysis.
(27:24):
Case Discussion. Monitoring during magnetic
resonance imaging. A 50 year old patient with
recent onset of seizures is scheduled for magnetic resonance
(27:44):
imaging MRI. A prior MRI attempt was
unsuccessful because of the patient's severe claustrophobic
reaction. The radiologist requests your
help in providing either sedation or general anaesthesia.
(28:07):
Question Why does the MRI suit pose special problems for the
patient and the anesthesiologist?
Answer MRI studies tend to be long, that is often more than
one hour, and many scanners totally surround the body
(28:29):
causing a high incidence of claustrophobia in patients
already anxious about their health.
Patient discomfort may also be amplified by pre-existing pain.
Good imaging requires immobility, something that is
difficult to achieve in many patients without sedation or
(28:54):
general anaesthesia. Because the MRI device uses a
powerful magnet, no ferromagnetic objects can be
placed near the scanner. This includes implanted
prosthetic joints, artificial pacemakers, surgical clips,
(29:16):
batteries, ordinary anaesthesia machines, watches, pens, credit
cards, intravenous poles, conventional oxygen cylinders,
housekeeping buckets, and lead ankle weights, nearly all of
(29:38):
which we have seen drawn in and captured by MRI devices.
Ordinary metal LED wires for pulse oximeters or
electrocardiography act as antennas and may attract enough
radio frequency energy to distort the MRI or even cause
(30:03):
patient burns. In addition, the scanner's
magnetic field causes severe monitor artefacts.
The more powerful the scanner's magnet is, as measured in Tesla
units, that is 1 Tesla is equalsto 10,000 ghosts, the greater
(30:30):
the potential problem. Other problems include poor
access to the patient during theimaging, particularly the
patient's airway, hypothermia inpaediatric patients, dim
lighting within the patient tunnel, and very loud noise that
(30:53):
is up to 100 decibels. Question How have these
monitoring and anaesthesia machine problems been addressed?
Answer Equipment manufacturers have modified monitors so that
(31:17):
they are compatible with the MRIenvironment.
These modifications include non ferromagnetic
electrocardiographic electrodes,graphite and copper cables,
extensive filtering and gating of signals, extra long blood
(31:39):
pressure cuff tubing and the useof fibre optic technologies.
Anaesthesia machines with no ferromagnetic components, that
is, for example aluminium gas cylinders, have been fitted with
MRI compatible ventilators and long circle systems on Mepos and
(32:07):
deep breathing circuits. Question What factors influence
the choice between general anaesthesia and intravenous
sedation? Answer Although most patients
(32:28):
will tolerate an MRI study with sedation, head injured and
paediatric patients presents special challenges and often
require general anaesthesia because of machine and
monitoring limitations. An argument could be made that
(32:49):
sedation, when possible, would be a safer choice.
On the other hand, loss of airway control from deep
sedation could prove catastrophic because of poor
patient access and delayed detection.
(33:11):
Other important considerations include the monitoring
modalities available at a particular facility and the
General Medical condition of thepatient.
Question Which monitors should be considered mandatory in this
(33:32):
case? Answer The patient should
receive at least the same level of monitoring and care in the Mr
suite as in the operating room for a similarly non invasive
procedure. Thus, the American Society of
(33:53):
Anesthesiologists standards for basic anaesthetic monitoring see
the following guidelines section.
Apply as they would to a patientundergoing sedation or general
anaesthesia in order anesthesizing locations.
(34:16):
Continuous auscultation of breath sounds with a plastic,
not metal, precordial stethoscope can help identify
airway obstruction caused by excessive sedation.
Palpation is a peripheral pulse.Sorry, palpation of a peripheral
(34:38):
pulse or listening for corrupt cough sounds is impractical in
these settings. Ensuring adequacy of circulation
depends on electrocardiographic and oscillometric blood pressure
monitoring and tidal carbon dioxide analysis can be adapted
(35:02):
to sedation cases by connecting the sampling line to a site near
the patient's mouth or nose. If a nasal cannula with a carbon
dioxide sampling channel is not available because room air
entrainment precludes exact measurements, this technique
(35:27):
provides a qualitative indicatorof ventilation whenever sedation
is planned. Equipment for emergency
conversion to general anaesthesia, for example,
tracheal tubes. Resuscitation bag must be
immediately available. Although MRI suits are often
(35:50):
located in remote areas far fromthe operating room suite,
appropriate equipment and medication must still be
immediately available for anaesthesia related emergencies
such as a difficult airway or malignant hypothermia.
(36:10):
Question Is the continuous presence of qualified
anaesthesia personnel required during these cases?
Answer Absolutely yes. Sedated patients must have
continuously monitored anaesthesia care to prevent a
(36:31):
multitude of unforeseen complications such as apnea or M
SS. We come to the end of Chapter 6.
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