Anesth-aware - 225 Page 1 Awareness during General...

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Unformatted text preview: 225 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. CASE PRESENTATION Introduction 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. Incidence 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 (PTSD). 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 awareness: • 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? Legal Implications 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). Memory Function 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 of awareness. 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 BIS/AEP 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 them. Consider the use of amnestic agents (especially when anesthesia is “light”). Ear plugs: Reducing auditory input to the patient may decrease memory function. References 1. Sandin RH, Enlund G, Samuelsson P, Lennmarken C: Awareness during anaesthesia: a prospective case study. Lancet 2000; 355: 707-11 2. Liu WHD, Thorp TA, Graham SG, Aitkenhead AR: Incidence of awareness with recall during general anaesthesia. Anaesthesia 1991; 46: 435-7 3. Domino KB, Posner KL, Caplan RA, Cheney FW: Awareness during anesthesia. Anesthesiology 1999; 90: 1053-61 4. Aitkenhead AR: The pattern of litigation against anaesthetists. British Journal of Anaesthesia 1994; 73: 10-21 225 Page 5 5. Moerman N, Bonke B, Oosting J: Awareness and recall during general anesthesia. Anesthesiology 1993; 79: 454-64 6. Lennmarken C, Bildfors K, Enlund G, Samuelsson P, Sandin R: Victims of awareness. Acta Anaesthesiologica Scandinavica 2002; 46: 229-31 7. Tunstall ME: Detecting wakefullness during general anesthesia for caesarian section. British Medical Journal 1977; 1: 1321 8. Thornton C, Barrowcliffe MP, Konieczko KM, Ventham P, Doré CJ, Newton DEF, Jones JG: The auditory evoked response as an indicator of awareness. British Journal of Anaesthesia 1989; 63: 113-5 9. Loveman E, van Hooff JC, Smith DC: The auditory evoked response as an awareness monitor during anaesthesia. British Journal of Anaesthesia 2001; 86: 513-8 10. Flaishon R, Windsor A, Sigl J, Sebel PS: Recovery of consciousness after thiopental or propofol: Bispectral index and the isolated forearm technique. Anesthesiology 1997; 86: 613-9 11. Mychaskiw GI, Horowitz M, Sachdev V, Heath BJ: Explicit intraoperative recall at a Bispectral index of 47. Anesthesia and Analgesia 2001; 92: 808-9 12. Rampil I, Mychaskiw GI, Horowitz M: False negative BIS? Maybe, maybe not! Anesthesia and Analgesia 20021; 93: 798-9 13. Drummond JC: Monitoring depth of anesthesia with emphasis on the application of the Bispectral index and the middle latency auditory evoked response to the prevention of recall. Anesthesiology 2000; 93: 876-82 14. O′Connor MF, Daves SM, Tung A, Cook RI, Thisted R, Apfelbaum J: BIS monitoring to prevent awareness during general anesthesia. Anesthesiology 2001; 94: 520-2 15. Ghoneim MM, Block RI: Learning and consciousness during general anesthesia. Anesthesiology 1992; 76: 279-305 16. Lubke GH, Kerssens C, Phaf RH, Sebel PS: Dependence of explicit and implicit memory on hypnotic state in trauma patients. Anesthesiology 1999; 90: 670-80 17. Lubke GH, Kerssens C, Gershon RY, Sebel PS: Memory function during general anesthesia for emergency Cesarean sections. Anesthesiology 2001; 92: 1029-34 18. Kerssens C, Klein J, van der Woerd A, Bonke B: Auditory information processing during adequate propofol anesthesia monitored by electroencephalogram bispectral index. Anesthesia and Analgesia 2001; 92: 1210-1215 ...
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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.

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