Transcript Measuring Depth of Anesthesia

Measuring Depth of
Anesthesia
 The word "anesthesia" was first used by the Greek
philosopher Dioscorides in the first century of the
current era to describe the narcotic effect of the
plant mandragora.
 The word reappeared in the 1771 Encyclopaedia
Britannica, where it was defined as a "privation of
the senses.
 Plomley, in 1847, was the first to attempt to
define depth of anesthesia. He described
three stages:
intoxication, excitement (both conscious
and unconscious), and the deeper levels of
narcosis.
 In that same year, John Snow described
"five degrees of narcotism" for ether
anesthesia.
 In 1937, Guedel published his classic
description of the clinical signs of ether
anesthesia. He used clear physical signs
involving somatic muscle tone, respiratory
patterns, and ocular signs to define four
stages.
What Is Anesthesia?
 The sine qua non of the anesthetized
state is unconsciousness, the lack of
conscious processing of thoughts.
 The crux of the difficulty in defining
anesthetic depth is that
unconsciousness cannot be measured
directly.
 The most important is that for any
stimulus response pair, depth of
anesthesia is the probability of
nonresponse.
 More generally, depth of anesthesia is the
probability of nonresponse to stimulation,
calibrated against the strength of the
stimulus, the diffiCulty of suppressing the
response, and the drug-induced probability
of nonresponsiveness.
 Anesthetic depth ranges from a 100%
probability of an easily suppressed
response (verbal answer) to a mild
stimulus (e.g., calling one's name) and
readily suppressed responses (e.g.,
verbal answer) to a 100%probability of
nonresponse to profoundly noxious
stimuli (e.g., intubation) and responses
that are difficult to suppress
(e.g.,tachycardia).
response surfaces for a profound
stimulus movement response to intubation,
MEMORY AND AWARENESS
Recall, Conscious or Explicit Memory
 Detection of Auditory Input, Unconscious
or Implicit Memory
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Recall, Conscious or Explicit Memory
 Ghoneim recently reviewed cases of recall in
different anesthetic situations.
 The incidence of awareness in a non obstetric and
noncardiac surgical population approximates
0.2%.
 A higher incidence is reported for obstetric
general anesthesia, 0.4%.
 The incidence in cardiac surgery ranges from
1.1%to 1.5%.
 major trauma cases can have a range of awareness
from 11% to 43%.
 Intraoperative awareness or recall has
occurred with high-dose opioid anesthesia.
 Two clinical signs possibly predicting the
occurrence of recall are movement and
autonomic response.
 The use of muscle relaxants can eliminate
the movement response, which leaves only
autonomic activity as a measure of
intraoperative awareness.
Detection of Auditory Input,
Unconscious or Implicit Memory
 Although the patient may not overtly recall
a stimulus or an event, auditory input can
register in the brain during apparently
adequate surgical anesthesia.
 Auditory and verbal input must be
"meaningful" for it to register in the
patient's memory.
 Frequently, hypnosis or other cues may be
needed to elicit recall.
 Ten volunteers undergoing dental surgery
were given thiopental followed by nitrous
oxide and diethyl ether. Monitoring the
EEG for an irregular slow-wave-highvoltage pattern allowed the anesthetist to
maintain a similar depth of anesthesia in all
patients. This EEGpattern was considered
equivalent to moderate to deep ether
anesthesia.
 During surgery, the anesthetist
provided verbal stimulation to the
patient in the form of an intraoperative
crisis by verbally stating that cyanosis
was present and then treated
appropriately.
 All 10 patients had no spontaneous recall of the
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simulated intraoperative crisis.
Under hypnosis, however,four patients could
remember the frightening words in exact detail.
An additional four remembered someone
speaking to them.
All eight became anxious and either emerged
spontaneously from their hypnotic trance or
refused to continue exploring the event.
One subject had activation of the EEGpattern
when the intraoperative crisis occurred, but no
recall of the event.
 Large effects on memory were produced
only by propofol and midazolam.
 Thiopental had mild memory effects,
 whereas fentanyl had none.
Implications of Explicit, Intraoperative
 Blacher described a traumatic post-
cardiac surgery neurosis involving
anxiety and irritability, repeated
nightmares, preoccupation with death,
and a reluctance to discuss these
symptoms.
 He attributed this postoperative state to
patients' being awake and paralyzed
during open heart surgery.
HYPNOTICS, ANALGESICS, AND
ANESTHETIC DEPTH
Inhaled Anesthetics
 Movement Response and the MAC Concept:
 MAC is the minimum alveolar
concentration of inhaled anesthetic required
to prevent 50% of subjects from responding
to a painful stimulus with "gross purposeful
movement.“
 For determination of MAC in humans, the
standard noxious stimulus has been the
initial surgical skin incision.
 The MAC concept has been expanded by
evaluating other clinical end points and
defined stimuli.
 the MAC of anesthetic that would allow
opening of the eyes on verbal command
during emergence from anesthesia
("MACawake)
 Generally, MACawake values are a third
to a fourth the MAC values for surgical
incision.
 the MAC of inhaled anesthetic that
would inhibit movement and coughing
during endotracheal intubation
("MACintubation").
 Intubation is significantly more
stimulating than skin incision, and
higher concentrations of inhaled
anesthetic are required to eliminate the
movement response.
 MACBAR: The MAC of anesthetic
necessary to prevent an adrenergic
response to skin incision,as measured
by the concentration of
catecholamine in venous blood.
Because cerebral blood perfusion is large, it
is possible to achieve an equilibration among
end-tidal, alveolar, arterial, and brain
anesthetic partial pressures within 15
minutes of exposure to a constant end-tidal
anesthetic concentration.
If the difference between the inspired and
end-tidal partial pressures was less than 10%
difference between end-tidal and arterial
concentrations would be minimal.
Eger and associates proposed that
volatile anesthetics cause a lack of
movement response to noxious
stimuli by action in the spinal cord
and create a hypnotic/amnestic
loss of consciousness at a
supraspinal, cortical site of action.
Other Clinical Responses:
 Other Clinical Responses Responses other than
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purposeful movement have been investigated as
possible clinical measures of the depth of
anesthesia:
the rate and volume of ventilation in
spontaneously breathing subjects,
eye movement,
the diameter and reactivity of pupils to light,
heart rate,
arterialblood pressure,
and autonomic signs such as sweating.
It has not been possible, however, to use these clinical
signs to generate uniform measures of depth of
anesthesia for inhaled anesthetics.
Although some clinical signs do correlate with depth
of anesthesia for certain inhaled anesthetics, the same
cannot be said for other inhaled anesthetics.
 Zbinden and colleagues
systematically examined the
interaction of isoflurane
concentrations with the
hemodynamic response to different
noxious stimuli.
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When used as a sole agent, even at high
concentrations isoflurane is unable to
suppress hemodynamic responses to
noxious stimuli.
the addition of analgesic components, such
as nitrous oxide and fentanyl, can prevent
the sympathetic stimulation and
hemodynamic responses seen with noxious
surgical stimuli when inhaled anesthetics
are used.
Intravenously Administered (Nonopioid)
Anesthetics
 Assessing Depth during Induction of Anesthesia
 Assessing Depth during Maintenance of Anesthes
Assessing Depth during
Induction of Anesthesia
Induction of anesthesia often consists
of a rapid intravenous bolus injection
of a hypnotic (e.g.,
propofol,thiopental, etomidate).
Plasma concentrations peak within a
half to 1 minute and decline rapidly on
redistribution of the drug.
 Clinical end points useful in assessing the
depth of anesthesia during induction
include loss of verbal responsiveness, loss
of eyelid reflex, and loss of corneal reflex.
 Typical stimulation occurring during
induction of anesthesia includes
laryngoscopy and intubation, which
constitute profoundly noxious stimuli.
 Frequently, response to these two
procedures cannot be eliminated
completely with just the intravenously
administered hypnotic.
Thus, assessment of the depth of anesthesia
with the use of clinically relevant noxious
stimuli such as laryngoscopy and
intubation requires the concurrent
administration of other analgesic drugs
(opioids or nitrous oxide) to provide
reasonable and clinically acceptable
hemodynamic control.
Assessing Depth during
Maintenance of Anesthesia
For the patients given 67%nitrous
oxide, the plasma concentrations of
thiopental necessary to achieve the
same surgical end points were as
much as 71%lower than those in
patients given only thiopental.
Kazama and coworkers found propofol CPso values for
the following defined stimuli:
loss of verbal responsiveness, electrical tetanus,
laryngoscopy, skin incision, and intubation.
With the addition of a steady-state fentanyl , there was
only a minimal decrease in the propofol CPso for loss of
verbal responsiveness.
For the other, more intense noxious stimuli (tetanus,
laryngoscopy, skin incision, and intubation), a much
greater decrease in propofol CPso occurred.
In clinical practice, intravenously
administered anesthetic drugs are
frequently combined with other
drugs that provide additional
analgesia (opioids, nitrous oxide,
potent inhaled anesthetics).
large intravenously administered doses of
thiopental or propofol are less than effective in
eliminating the hemodynamic response to relevant
clinical stimuli such as laryngoscopy and
intubation.
Fentanyl decreases the anesthetic requirement for
thiopental or propofol by providing anti
nociceptive effects that the intravenous hypnotics
do not provide.
Clinically, the hemodynamic response
to laryngoscopy, intubation, or skin
incision is most commonly used to
assess depth of anesthesia.
The use of muscle relaxants to ease
endotracheal intubation precludes use
of the movement response.
Because laryngoscopy and intubation
are single events, if clinical depth is
inadequate (e.g., in the event of a
profound hemodynamic response),
additional intravenous anesthetics,
opioids, or maintenance anesthetic
drugs are rapidly administered.
When precise hemodynamic control
becomes important (as in coronary
artery disease), larger doses of opioids
are used instead of intravenously
administered anesthetics.
Opioids
The analgesia produced by these drugs through
specific receptor systems within the CNS
decreases autonomic, endocrine, and somatic
responses to noxious stimulation.
Although opioids have been used as sole
anesthetics, they create incomplete hypnotic
effects at very large doses.
Opioids need to be combined with hypnotic drugs
to induce the anesthetic state.
Murphy and Hug found that even high plasma
concentrations of fentanyl (20 ng/mL) did not
decrease enflurane MAC beyond 60% to 70% of its
initial value (Fig. 31-17).
That is, there was a ceiling to the enflurane-sparing
effect.
Morphine, sufentanil, and alfentanil also decrease
enflurane MAC and have a similar ceiling effect in
dogs.
 McEwan and coworkers found very similar
results in humans.
 The maximal MAC reduction was 82% at a
steady-state fentanyl plasma concentratio of 10.6
ng/mL.
 Similar MAC reduction results have been
obtained with other inhaled anesthetics
(desflurane, sevoflurane) and other opioids
(alfentanil, sufentanil, remifentanil).
ElECTROPHYSIOlOGIC MONITORING
Bispectral Electroencephalographic
Monitoring
In the 1990s, Aspect Medical Systems, a
medical device company in Natick,
Massachusetts, undertook an integrated
research effort to develop the EEG as a
measure of anesthetic depth.
The Aspect EEG monitor quantitates the
anesthetic effects on the brain, specifically, the
hypnotic component of anesthesia.
The device presents a continuous EEG
parameter, the bispectral (BIS) index,
which ranges from an awake, no-drugeffect value of 95 to 100 to zero with no
detectable EEG activity.
• Successful development of the Aspect BIS EEG
monitoring system can be identified with the
following concepts:
1. The
simultaneous use of multiple
EEGsignal-processing approaches captured
incremental information that was not
captured with traditional approaches based
on a single signal-processing approach.
2. Multiple clinically relevant measures
(movement, hemodynamics, drug
concentrations, consciousness, recall) in
patients and volunteers were gathered with
concurrent EEG data.
3. Advanced multivariate statistical data
analysis was used to correlate the components
of the multiple EEG signal-processing
approaches with the clinical data to create the
univariate BIS parameter.
4. Prospective clinical evaluation of the BIS
index was performed at multiple institutions
under varying anesthetic and surgical
conditions.
5. The BIS index was recognized as measuring
the hypnotic components of the anesthetic
and was relatively insensitive to the analgesic
(e.g., opioid) components of an anesthetic.
6. Prospective clinical trials demonstrated that BIS
monitoringcould improve the outcome of an
anesthetic regimen.
7. Simple hardware and sensors were developed
and are commercially available to facilitate highquality signal capture despite the noisy electrical
environment of the operating room.
 Several factors that will be encountered in clinical
anesthesia care can interact with the BIS index:
 hypothermia decreased the BIS index by 1.12 units per
degree Celsius decline in temperature.
 Infusion of esmolol can also alter the BIS index.
 Epidural anesthesia can also decrease the amount of
hypnotic anesthetic needed for sedation.
 Ketamine doses that create unresponsiveness (0.25 to
0.5 mg/kg) did not change the BIS index.
Bispectral lndex and Clinical
Utility/Outcome
 The use of BIS monitoring was associated with
a significantly reduced incidence of awareness
(78%reduction) when compared with
historical controls from the same hospitals and
investigators.
 Mortality was significantly higher if the BIS
index was less than 40 in patients older than
40 years.
 Increasing age and lower BIS values were
both independently associated with higher
mortality rates.
Clinical Use of the Bispectral Index
 Clinical use of BIS monitoring involves
separating the hypnotic and analgesic
components of an anesthetic regimen.
 The concept entails titration of the hypnotic
drug (e.g., isoflurane, desflurane, sevoflurane,
propofol, midazolam) to lower the BISvalue to
40 to 60.
 This range appears to be the therapeutic
window associated with a high probability of
unconsciousness.