Awareness and recall

Incidence of and risk factors for awareness during anaesthesia

Explicit recall of events during general anaesthesia is detected by direct questioning, as patients may not report awareness spontaneously or if they are questioned non-specifically. More than one interview is needed and credibility of reports should always be verified. The overall incidence of awareness has decreased over the last 40 years and is now 0.1-0.2%. Prospective study of patients who undergo general anaesthesia is the only valid method for determining the incidence of awareness. Studies of patients recruited through referrals by colleagues or advertisements, studies of compensation claims and those carried out through quality improvement systems are inadequate. Several factors increase the risk of awareness, including light anaesthesia, some types of surgery, a history of awareness, chronic use of central nervous system depressants, younger age, obesity, inadequate or misused anaesthesia delivery systems, insufficient knowledge about awareness, and ignoring the use of electroencephalographic monitors when the risk is otherwise increased.

Minimizing intraoperative recall

Intraoperative recall has plagued the administration of general anesthesia since the technique was first described, but the media's recent attention to the issue has heightened public awareness and brought it to the forefront of patient concerns. Now, more than ever, patients are asking about the possibility of 'waking up during the operation'. The following review of the literature reveals potential strategies for reducing the risk and managing the sequelae of intraoperative recall.

Auditory evoked potentials

This chapter will focus on the two auditory evoked potentials (AEP) most commonly used to assess the effects of general anesthetics on the brain, the auditory middle latency response (AMLR) and the 40 Hz auditory steady-state response (40 Hz-ASSR). We will review their physiological basis, the recording methodology, the effects of general anesthetics, their ability to track changes in level of consciousness and their clinical applications. Because of space constraints, this review will be limited to human studies.

Unintentional intraoperative awareness during sufentanil anaesthesia for cardiac surgery

PURPOSE

The aim of this clinical report is to describe a case of unintentional intraoperative awareness during sufentanil anaesthesia in a patient undergoing elective aortocoronary bypass grafting.

CLINICAL FINDINGS

After premedication with morphine (5 mg) and scopolamine (0.2 mg), this 51-yr-old woman received sufentanil (10 micrograms.kg-1), midazolam (4 mg) and isoflurane (0.3-0.4% end-tidal). The patient recalled specific events and discussions which took place in the operating room during surgery. This patient's report was clear and corroborated by operating room personnel. The patient denied having felt pain, anxiety or emotional distress.

CONCLUSION

Although awareness during opioid anaesthesia has been previously described with morphine and fentanyl, as far as we know this is the first clinical report of awareness with sufentanil. Given that recent efforts of early extubation in cardiac surgery patients may involve a reduction in the amount of opioid administered, this report serves as a reminder of the ever present potential for this disturbing complication.

Comparison of the effects of enflurane/N2O on the 40-Hz auditory steady-state response versus the auditory middle-latency response

The auditory middle-latency response (AMLR) is a sequence of negative-positive waves occurring 12-50 ms after the onset of an auditory stimulus presented at rates of 10/s or less. When the rate of stimulus presentation is increased to approximately 40/s, overlapping of the AM-LRs results in a sustained, nearly sinusoidal wave, called the "40-Hz auditory steady-state response" (40-Hz ASSR). The AMLR and 40-Hz ASSR have been used to study the effects of general anesthetics on the brain. The primary aim of this investigation was to determine whether the effects of a general anesthetic, namely enflurane, on the 40-Hz ASSR can be predicted from its effects on the AMLR. A secondary aim was to examine the relationship between the level of consciousness and the 40-Hz ASSR during emergence from anesthesia. Twelve ASA class I-II women undergoing reduction mammoplasty were tested. Anesthesia was induced with fentanyl (3 micrograms/kg) and thiopental (3-5 mg/kg) intravenously and was maintained with enflurane (0.5%, 0.8%, or 1.1% end-tidal; four patients per concentration; random assignment) in N2O (66% end-tidal), along with fentanyl (1 microgram/kg as needed). The 40-Hz ASSR and AMLR were recorded before induction and during anesthesia and surgery. The 40-Hz ASSR was also recorded during emergence. The amplitude of the 40-Hz ASSR was reduced profoundly during anesthesia and surgery (P < 0.001). The attenuation was not dose-dependent, and was much more pronounced than predicted by the effects of enflurane on the AMLR. The 40-Hz ASSR during anesthesia was surprisingly large (0.09 and 0.11 microV) in two patients, both of the 1.1% enflurane group. The regaining of the ability to follow verbal commands was associated with a significant (P < 0.001) increase in the amplitude of the 40-Hz ASSR. We conclude that, although auditory neurons remain capable of responding at a slow stimulus rate during enflurane-N2O anesthesia, their ability to be driven at a faster stimulus rate is markedly curtailed. The 40-Hz ASSR may be useful for detecting unintentional intraoperative awareness because the return of consciousness is associated with a clear increase in amplitude.

Awareness and general anaesthesia

A state exists after the induction of anaesthesia in which patients may be aware of their surroundings yet unable to communicate. This problem of awareness and recall during general anaesthesia is a recent one in the relatively short history of anaesthesia. Prior to the introduction of muscle relaxants in 1942 by Griffith and Johnson, it was felt that "light anaesthesia" would be signified by violent movements. Today, the concepts of anaesthetic depth, awareness, and recall have become more complicated with the addition of numerous newer, shorter-acting, intravenous anaesthetic agents with varying effects on the conscious state. Several methods have been described to detect awareness. None has yet been found to be totally reliable and numerous reports of awareness can be found in the literature. Light inhalation and total intravenous anaesthesia have been blamed for the majority of these case reports. However, awareness during total intravenous anaesthesia is avoidable with the proper use of a combination of a hypnotic and an analgesic such as midazolam and alfentanil for general anaesthesia.

Intraoperative awareness due to malfunction of a Siemens 900B ventilator

A case of intraoperative awareness during a thoracotomy is described. The patient's recall coincided with an intraoperative period during which a Siemens 900B ventilator and a Siemens 952 isoflurane vaporiser were used. Subsequent assessment of this equipment with an anaesthetic agent analyzer revealed that, at the ventilator settings which had been used, the delivered anaesthetic vapour concentration varied greatly from the vaporizer settings. This problem eventually was traced to a malfunctioning inlet control valve on the ventilator. This complication may have been prevented if the end-tidal anaesthetic concentration had been monitored intraoperatively.

Effects of a subanesthetic concentration of nitrous oxide on overt and covert assessments of memory and associative processes

Drug effects on human memory are usually assessed by overt recall or recognition tests. Covert tests which do not explicitly assess memory but which indirectly elicit previously presented information may be more sensitive to low levels of learning than overt tests. Three covert tests and corresponding overt recall tests were given to 16 men and 16 women breathing 30% nitrous oxide in oxygen or 100% oxygen to see if the covert tests resisted the memory-impairing effects of nitrous oxide. Nitrous oxide impaired performance on the overt tests. Performance in two covert tests, Constrained Associations and Word Completion, showed resistance to memory impairment. In the least resistant covert test, Free Associations, nitrous oxide altered associative processes. Performance in an additional test involving recognition and preferences for nonsense words repeated with varying frequencies also showed some resistance to memory impairment. The results support the distinction between declarative and procedural memory. Constrained Associations, Word Completion, and Nonsense Words tests may be useful for assessing low levels of learning during drug states.