29: Awareness During Anesthesia

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CHAPTER 29 Awareness During Anesthesia

Aaron Murray, MD

1 Review the classifications of memory and awareness

The current classifications of memory include both implicit, or unconscious memory, and explicit, or conscious memory. Explicit memory refers to the conscious recollection of specific intraoperative events that took place during general anesthesia, also called anesthetic awareness. Recall can only be determined after surgery. These memories can occur with or without pain and can range from vivid recollections to vague memories. It was reported in 2008 that subjective auditory perceptions represented 50% of intraoperative awareness experiences. By contrast, implicit memory reflects changes in behavior or performance secondary to an anesthetic experience without the ability to specifically recall the event. The 2006 American Society of Anesthesiologists’ (ASA) practice advisory regarding intraoperative awareness excludes dreaming as anesthetic awareness.

2 What is the incidence of awareness?

Estimates of awareness during anesthesia in nonobstetric and noncardiac surgeries are approximately 0.1% to 0.2%. Whenever lighter anesthesia is provided, the incidence increases. During cardiac surgery the incidence ranges from 1.1% to 1.5%. A higher incidence is also seen in obstetric (0.4%) and trauma cases (11% to 43%). Interestingly, although the use of anesthetic techniques associated with increased awareness (e.g., nitrous-narcotic) has decreased, the claims against anesthesiologists for awareness are increasing, recently accounting for 2% of all claims. This claim rate rivals other common complications such as aspiration pneumonia and myocardial infarction.

3 Are certain techniques and clinical situations more likely to result in awareness?

Recognized risk factors for awareness include the following:

image Light levels of anesthesia, which are common in hypovolemic, injured, and obstetric patients
image Prior history of intraoperative awareness
image Patients undergoing cardiac surgery with cardiopulmonary bypass; a higher incidence of postoperative recall (about 1%) occurs because of the reliance on narcotic-based anesthesia, which minimizes myocardial depression but produces unreliable amnesia
image The use of muscle relaxants
image Machine malfunction such as empty vaporizers, intravenous anesthetic pump malfunction or disconnection, and syringe swaps
image Other patient risks implicated in awareness include: the very young, chronic substance-abusing patients, chronic pain patients, and patients with hypernatremia.

4 Describe clinical signs and symptoms of light anesthesia

Both motor signs and sympathetic activation can indicate lighter levels of anesthesia. Motor signs in response to light anesthesia frequently precede hemodynamic changes or sympathetic activation. Specific motor signs include eyelid or eye motion, swallowing, coughing, grimacing, and movement of the extremities or head. Increased respiratory effort is caused by activity of intercostal and abdominal muscles, which are suppressed at deeper levels of anesthesia. With the use of neuromuscular blockade, motor signs do not provide information about anesthetic depth. Consequently sympathetic activation represents an additional method for assessing light anesthesia. Sympathetic effects associated with light anesthesia include hypertension, tachycardia, mydriasis, tearing, sweating, and salivation. Such findings are nonspecific and are modified by anesthetic agents; thus their presence or absence is also an unreliable indicator of awareness. Other perioperative medications such as β-blockers and sympathetic blockade render these findings unreliable. In fact, signs of light anesthesia (e.g., tachycardia, hypertension) were found in only a minority of patients claiming awareness.

KEY POINTS: Awareness During Anesthesia image

1. Awareness most likely occurs in cases in which minimal anesthetic is administered such as cardiopulmonary bypass, trauma, and obstetric patients.
2. Symptoms of awareness may be very nonspecific, especially when muscle relaxants are used.
3. The small risk of awareness should be discussed during informed consent, especially if patients are at higher risk for recall.

5 What are the ramifications of awareness?

Intraoperative awareness has a powerful association with patient dissatisfaction. Untreated pain is one such disturbing possibility. Other possibilities include the ability to hear operating personnel, sensations of weakness or paralysis, anxiety, helplessness, and fear of death. Subsequent effects can range from sleep disturbances, anxiety, and depression to posttraumatic stress disorder (PTSD). One study indicated late psychological symptoms occurred in 33% of patients with awareness. Symptoms of PTSD can include flashbacks, nightmares, avoidance, emotional numbing, hyperarousal, and preoccupation with death.

6 How should a patient who may have been aware during a recent anesthetic be approached?

First, it is not necessary to ask all patients if they have any recollections because recall is not a common problem. They may volunteer information or may seem angry or sad without a clear reason, as if something is just not right. In instances in which for some reason you have concerns, begin with a structured interview, asking general, open-ended questions. An example would be, “What is the last thing you remember?”

Once a case is identified, a detailed note in the chart is appropriate. Listen to the patient’s story and explain the circumstances surrounding the situation (cardiovascular instability, trauma). What trauma patients may remember is their time in the emergency room or intensive care, not necessarily intraoperative events. Reassure the patient, apologize, offer psychologic support, and continue to stay in touch with the patient, even after discharge. Notify the surgeons, nursing staff, and hospital legal counsel. The ASA Task Force on Intraoperative Awareness disagrees if all patients should be advised of intraoperative awareness during informed consent but agrees that those at increased risk should be informed before surgery.

7 What are methods to avoid awareness?

image Premedication with amnestic drugs such as benzodiazepines or scopolamine is a standard method of potentially reducing the likelihood of intraoperative awareness, especially in higher-risk patients, surgeries, or anesthetic techniques.
image Administer appropriate doses of induction agents and supplement them if airway management is difficult and the opportunity to administer volatile anesthetics is truncated.
image Avoid muscle relaxants unless necessary.
image Supplement nitrous-narcotic techniques with volatile anesthetics.
image Attempt to use 0.8 to 1 minimum alveolar concentration of anesthetic or more if feasible.
image Maintain anesthetic machines appropriately, ensure proper intravenous pump function with pressure/volume alarms, and administer anesthetic agents via a dedicated and functioning intravenous line.
image Consider using neurophysiologic monitoring to assess depth of anesthesia.

8 Are monitors available to assess the depth of anesthesia?

Brain electrical activity monitoring can be used to assess depth of anesthesia and includes two categories: processed electroencephalogram (pEEG) and evoked responses (e.g., auditory). No single monitor can provide a fail-safe answer to the question of depth of anesthesia. pEEG waveform technology of differing formats can be used and has entered the market to a greater extent then evoked responses. The most familiar technology is known as the bispectral index (BIS). The purpose of any of these forms of EEG analysis is to estimate the degree of hypnosis that the patient is experiencing. Raw data, collected by electrodes placed on the forehead and temporal area, are processed by the computer module to create a dimensionless numeric representation of the degree of sedation. Lower numbers correspond to a greater depth, whereas higher numbers are found in awake or lightly sedated patients. The incidence of awareness in high-risk cases was reduced by approximately 82% (0.91% to 0.17%) when BIS-guided anesthesia was provided with a BIS goal of 40 to 60 during anesthesia. Using BIS during general anesthesia with concurrent muscle relaxation resulted in a similar reduction of 77% (0.18% to 0.04%) in the incidence of awareness. In addition to the Food and Drug Administration–approved use of the BIS monitor for depth of anesthesia, using the BIS may help the anesthetist to better titrate the anesthesia to the individual. A deeper anesthetic has been associated with higher 1-year mortality in older adults, leading investigators to the conclusion that titrating BIS values may improve outcomes other than awareness. Although a BIS score less than 60 may reduce probability of awareness, the risk is not eliminated. A case report highlighted a patient with chronic pain and intraoperative hemodynamic lability who had vivid and painful explicit recall of surgery despite an average BIS score of 44. The ASA Task Force suggests that brain electrical activity monitoring is not routinely indicated but the use of brain monitoring should be considered on a case-by-case basis for selected patients (high-risk patients, light general anesthesia).

SUGGESTED READINGS

1. ASA Task Force on Intraoperative Awareness. Practice advisory for intraoperative awareness and brain function monitoring. Anesthesiology. 2006;104:847-864.

2. Bowdle T.A. Depth of anesthesia monitoring. Anesthesiol Clin. 2006;24:793-822.

3. Domino K.B., Posner K.L., Cheney F.W. Awareness during anesthesia: a closed claims analysis. Anesthesiology. 1999;90:1053-1061.

4. Errando C.L., Sigl J.C., Robles M., et al. Awareness with recall during general anaesthesia: a prospective observational study evaluation of 4001 patients. Br J Anaesth. 2008;101:178-185.

5. Mashour G.A. Integrating the science of consciousness and anesthesia. Anesth Analg. 2006;103:975-982.

6. Samuelsson P., Brudin L., Sandin R.H. Late psychological symptoms after awareness among consecutively included surgical patients. Anesthesiology. 2007;106:26-32.

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