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Page 1 Awareness during General Anesthesia Peter S. Sebel, M.B. B.S., Ph.D., M.B.A. Atlanta, Georgia OBJECTIVES
1. Understand the incidence of awareness and factors leading to its occurrence.
2. Understand the concepts of memory function during anesthesia.
The state of general anesthesia implies a lack of consciousness, or awareness. Awareness can
occur with or without conscious recollection of the event. Take, for example, patients who are
intubated yet open their eyes to command at the end of surgery. In terms of responding to
command, they are aware. Yet, frequently, they will not remember commands in the operating
room, nor will they remember extubation. Thus, they are aware without recall. The challenge to
anesthesiologists is to eliminate recall of unpleasant experiences during surgery.
Recent prospective studies suggest that the incidence of awareness varies from 0.0015% (in
11,785 patients)1 to 0.2% (in 1,000 patients)2. To put this in perspective, assuming 20,000,000
general anesthetics are performed in the United States each year, the number of patients suffering
from awareness with recall will be between 30,000 and 40,000. It is noteworthy from the largest
scale study1 that the incidence approaches 0.2% in cases where neuromuscular blocking agents
were used but is approximately half that in the absence of such drugs. Furthermore, although
non-paralyzed patients recalled intraoperative events, none of them had anxiety during the
wakefulness or had delayed psychiatric symptoms. In contrast, when neuromuscular blocking
agents were used, 78% of aware patients had pain, anxiety or post-traumatic stress disorder
Since patients will often not spontaneously report the occurrence of awareness with recall, the
following set of questions has been proposed2 as an instrument to assess the occurrence of
• What is the last thing you remembered before you went to sleep?
• What is the first thing you remembered when you woke up?
• Can you remember anything between these two periods?
• Did you dream during your operation?
• What was the worst thing about your operation?
Cases of awareness represent between 1.9% (ASA Closed Claims Analysis)3 to 12.2% (British
data)4 of claims against anesthesiologists. In the United States, the median payment for such
cases is $18,000 3, although recently, there have been several cases in which much larger claims 225
Page 2 have been settled. The ASA Closed Claims Database3 suggests that the majority of awareness
claims occur in women younger than 60 years of age ASA physical class I-II undergoing elective
surgery. It is noteworthy that what are generally considered to be the classic cues for light
anesthesia were absent in most cases: hypertension occurring in 15% of recall cases and
tachycardia occurring in 7%.
What is it like to be aware?
In a series of patients referred by physicians5, patients frequently describe hearing the sounds of
the operating room. They describe sensations of paralysis and pain and felt anxiety and panic,
helplessness and powerlessness. Sixty nine percent of patients had unpleasant sequelae from this
event in terms of PTSD. The anesthetic records from these patients were reviewed in a doubleblind manner using case-matched controls. The authors were unable to reliably distinguish cases
of awareness on the basis of anesthesia records. So, again, the classic cues of light anesthesia
were absent in these cases. In this study, only 35% of patients informed their anesthesiologists
about what had happened.
Treatment of the Aware Patient
A substantial number of patients who are aware during general anesthesia develop a PTSD. Such
a syndrome may develop after a frightening or unpleasant life experience. Characteristic
symptoms include anxiety, irritability, insomnia, repetitive nightmares, depression and a
preoccupation with death. There may be a fear of doctors, hospitals and, particularly, future
operations. The patients can relive the unpleasant experience in their dreams and the symptoms
can be severe with a prolonged course.
The frequency of persistent PTSD after awareness is not well understood. Sandin and colleagues
were able to follow up 9 of the 18 aware patients from their original study6 and found that 4
(45%) were severely disabled by the psychiatric sequelae of awareness.
If an anesthesiologist suspects that a patient may have had awareness (with or without PTSD), it
is appropriate to treat the patient sympathetically and arrange for appropriate follow up with a
psychiatrist skilled in the treatment of PTSD. Many patients become angry and litigious if the
physician denies the occurrence of an awareness episode or belittles the patient.
Can we monitor for awareness during anesthesia?
It is impossible to study, in an ethical manner, the circumstances under which awareness occurs
during surgical stimulation. Most studies on the occurrence of awareness during anesthesia have,
therefore, concentrated on the period prior to skin incision. In order to determine whether a
monitor is an effective indicator of awareness, it is necessary to have some gold standard by
which awareness can be measured. The isolated forearm technique (IFT) comes closest to
providing such a standard7. The IFT involves placing a tourniquet to above systolic pressure on 225
Page 3 the dominant arm prior to the administration of neuromuscular blocking agents. Then, motor
function in the dominant arm is maintained and the patient can (if aware) respond by moving a
finger or squeezing the investigator′s finger to command. It is generally considered that, if
patients can squeeze fingers in response to command, they are aware. Middle latency auditory
evoked potentials have been evaluated as an index of awareness using the IFT8. However, other
workers have suggested that wide inter-patient variability will limit the practical usefulness of
this technique for assessing intraoperative awareness9. EEG Bispectrum (BIS) has also been
assessed for its relationship to awareness using the IFT. Following induction, patients were
allowed to recover consciousness following a single dose of thiopental or propofol10.
Consciousness did not occur in any patient with a BIS value of less than 58. A BIS value of less
than 65 signified a less than 5% probability of return of consciousness within 50 seconds. It
should be emphasized that these studies were conducted in the absence of surgical stimulation
and we cannot be certain that these same values will hold true during periods of stimulation.
Of the potential technologies that may monitor for the risk of awareness, BIS is the only one that
has been used in a substantial number of patients. Although not a prospective study, there have
been more than 5,000,000 general anesthetics using BIS monitoring. Of those cases, a total of 83
have been referred to the manufacturers as cases of possible awareness; an approximate
incidence of 1 in 100,000 uses (Manberg, P., Aspect Medical, personal communication). In 48
cases (59%), BIS was determined to be greater than 65 at the time of awareness. In 10 cases
(12%), BIS was not being used at the time of awareness. The remaining cases were judged to be
inconclusive or are still under investigation. Thus, BIS monitoring appears to be generally
associated with a low incidence of awareness. There is one case report of patient awareness
where BIS was reported to be 47 11, although subsequent examination of the stored BIS record
demonstrated that BIS was above 70 at the time of awareness12. It has been suggested that it is
unrealistic to expect any monitor to be proof positive against the occurrence of awareness13 and
that an effective anesthesia monitor must have 100% specificity (no false negatives)13. No
anesthesia monitor reaches this unrealistic standard. It has also been suggested that wide-spread
indiscriminate use of BIS monitoring may lead to an increase in intraoperative awareness14. On
the basis of the data presented above, the incidence does not appear to be increased. Several
studies are ongoing in an attempt to clarify this issue. In the USA, a multicenter surveillance
study on the incidence of awareness has recruited over 15,000 patients (www.aimtrial.com). In
Australia, a trial of awareness in 2,500 high-risk patients randomized to either BIS monitoring or
no monitoring is nearing completion (www.b-aware-trial.org). In Sweden, Sandin et al. are
replicating their large-scale awareness-incidence monitoring study using BIS monitoring. To
date, over 5,000 patients have been entered (Sandin R., personal communication).
So far, we have been discussing the occurrence of awareness with recall. This is a form of
explicit or direct memory, in that the patient can spontaneously and directly recall the events.
There is another form of memory, implicit memory, which may also function during anesthesia.
Implicit memory is characterized by a change in task performance where the original stimulus 225
Page 4 cannot be remembered directly. Examples of implicit memory occurring during anesthesia
include: reduced PCA usage when patients have heard tapes (during anesthesia) suggesting that
they will feel more comfortable after surgery; completing word stems with words that they have
“been primed with” during their anesthetic. The characteristic of implicit memory is that the
patients cannot remember hearing the tapes. The circumstances under which implicit memory
occurs have been generally unclear15. However, recent data suggest that the occurrence of
implicit memory is directly related to the degree of hypnosis16;17. Indirect memory does not
appear to occur at BIS values between 40 and 60 18 whereas at higher BIS values, implicit
memory function can be detected16 and at BIS values greater than 70, some weak evidence of
explicit memory can be detected17.
Prevention of Awareness
The following suggestions may be useful in preventing cases of awareness with recall.
• Inform patients, particularly in circumstances where anesthesia may be deliberately “light”
(such as open-heart surgery, trauma surgery, Cesarean section, etc.), about the potential risk
Avoid complete neuromuscular blockade: Unless necessitated by surgical requirements,
always keep at least one “twitch” in a train-of-four.
Use adequate concentrations of inhalation or intravenous anesthetic at all times.
Anesthetic agent monitoring
Remember to re-dose the induction agent during difficult intubations.
Reverse neuromuscular blockade before discontinuing nitrous oxide.
Talk to the patients in circumstances where anesthetic delivery is reduced (e.g., acute
hypovolemia): When anesthetic delivery is suspended, speak to the patients and reassure
Consider the use of amnestic agents (especially when anesthesia is “light”).
Ear plugs: Reducing auditory input to the patient may decrease memory function. References
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prospective case study. Lancet 2000; 355: 707-11
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1994; 73: 10-21 225
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Medical Journal 1977; 1: 1321
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This note was uploaded on 11/03/2010 for the course MEL 4011 taught by Professor Gold during the Fall '08 term at University of Florida.
- Fall '08