|Year : 2007 | Volume
| Issue : 6 | Page : 461-463
Depth of anaesthesia: clinical applications
Editor, IJA, India
|Date of Web Publication||20-Mar-2010|
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Bajaj P. Depth of anaesthesia: clinical applications. Indian J Anaesth 2007;51:461-3
A fundamental component of general anaesthesia is unconsciousness. Patients consenting to general anaesthesia do so with the expectation that they will not see, hear, feel, or remember intraoperative events. Recently, there has been increased public concern regarding intraoperative awareness, and studies show that a large percentage of patients who undergo general anaesthesia report preoperative fears of awareness or recall  .
In the past, conventional monitoring of anaesthetic depth has included rudimentary signs such as patient movement, autonomic changes, and subjective clinical instinct. A considerable effort has been devoted to establishing a monitor that will reliably determine a patient's depth of anaesthesia. Several different methods have been evaluated, yet none are 100% effective. At present there are at least two inherent obstacles in the development of a "foolproof" monitor of anaesthetic depth. The first is that at present we have not yet comprehensively validated a unitary mechanism of general anaesthesia. The second concerns the fact that general anaesthesia occurs on a continuum without a quantitative dimension, and there is considerable interpatient and interanaesthetic variability. Attempting to translate a conscious or unconscious state into a quantitative number can at best be limited to the practice of probability.
Depth of anaesthesia is dependent on the balance between two antagonistic factors : the anaesthetic dose and surgical stimulation. Optimal depth of anaesthesia requires a sufficient amount of anaesthetic to achieve and maintain unconsciousness without compromising vital organ function. It is a tenet of anaesthesiology dogma that the quantitative pharmacodynamic effect of a given dose of an anaesthetic cannot be absolutely predicted in a specific patient. Accordingly, the dilemma for the anaesthesiologist is - give too small an anaesthetic dose and the patient may experience intraoperative recall, while too large an anaesthetic dose may convey risk to the patient (e.g., decrease organ perfusion) and increase the incidence of troublesome side effects (e.g., delayed awakening). The optimal depth of anaesthesia depends on balancing multiple anaesthetic goals in the best interests of the patient. These goals include:
Strategies utilized are part of the overall medical plan that balances risk of awareness against the risks of physiologic instability and postoperative complications.
- Avoid intraoperative awareness ,
- Optimize quality of recovery ,
- Maintain optimal haemodynamics
- Avoid postoperative neurocognitive dysfunction
- Avoid postoperative mortality ,,
The incidence of awareness is greater than most practitioners believe as the incidence is best estimated by formally interviewing patients postoperatively. Patients may not voluntarily report awareness if they were not disturbed by it. In addition, memory for awareness may be delayed. Only one-third of cases of awareness were identified before they left the postanaesthesia care unit  . One-third of the cases of awareness were not reported until 1-2 weeks postoperatively. A structured interview is therefore used to evaluate the incidence of awareness.
Anaesthetic technique is important in the pathogenesis of awareness during anaesthesia. Several case reports and small clinical studies have suggested that intraoperative awareness is more likely to occur during anaesthetics based on nitrous oxide and IV agents, and is less likely to occur when potent volatile anaesthetics are used  . Isoflurane in concentractions of ≥ 0.6 MAC prevented conscious recall and unconscious learning in anaesthetized patients , .
The most common causes of intraoperative awareness include light anaesthesia, increased anaesthetic requirement, or machine malfunction or misuse resulting in an inadequate anaesthesia delivery  . Light anaesthesia may be necessary for physiologic stability in hypovolemic patients or those with limited cardiac reserve. ASA 3-5 patients undergoing major surgery had a higher incidence of awareness  . Patients with awareness were more likely to have impaired cardiovascular status, undergo emergency surgery, receive lower doses of volatile anaesthetics, and have more technical difficulties with anaesthesia  . Neuromuscular blockade prevents the most common sign of light anaesthesia, patient movement. An inadequately anaesthetized, nonparalyzed patient usually moves before experiencing recall, as lower anaesthetic concentrations are needed to prevent awareness than to render immobility. Some patients, such as those using alcohol, opiates, amphetamines, and cocaine may require an increase in anaesthetic dose. In addition, equipment problems with the vaporizer or IV infusion devices may lead to awareness, although these are less common causes of awareness, especially with use of end-tidal anaesthetic gas analysis. In contrast to conventional clinical wisdom, most cases of awareness are not associated with hypertension and tachycardia , . In fact, patients with awareness were more likely to have intraoperative hypotension requiring vasopressors  .
Awareness during general anaesthesia is a frightening experience, which may result in serious emotional injury and posttraumatic stress disorder  . Patients who experienced awareness and recall during anaesthesia most commonly described auditory perceptions, the sensation of paralysis, anxiety, helplessness and panic. The sensation of pain occurs less frequently. Up to 70% of patients who had intraoperative awareness experience unpleasant aftereffects, including sleep disturbances, dreams and nightmares, and flashbacks and anxiety during the day. Some patients develop posttraumatic stress disorder associated with repetitive nightmares, anxiety, irritability, and preoccupation with death. The predisposing factors for development of posttraumatic stress disorder are unknown, although many patients have underlying psychological disorders, especially depression. Most patients fail to inform their anaesthesiologist that they experienced intraoperative recall. This is unfortunate because acknowledgment of what happened and prompt referral to psychological therapy may reduce the likelihood of long-term emotional sequelae.
Published suggestions for the prevention of awareness include premedication with an amnesic agent, giving adequate doses of induction agents, avoiding muscle paralysis unless totally necessary, supplementing opioid and N 2 O anaesthesia with ≥0.6 MAC of a volatile agent, administering 0.8-1.0 MAC when only volatile agents are used, adding amnesic agents when light anaesthesia may be employed, and confirming the delivery of anaesthetic agents to the patient  . Monitoring of end-tidal levels of volatile anaesthetics has been suggested to insure delivery of adequate levels of volatile anaesthetics. Hypertension and tachycardia do not reliably predict awareness , .
Depth of anaesthesia may affect the quality and side effects encountered in the recovery period. It has been shown that the use of BIS monitoring leads to less anaesthetic drug utilization and faster recovery times , . In a metaanalysis of randomized controlled trials of 1380 ambulatory anaesthesia patients, Liu reported that BIS monitoring reduced anaesthetic use by 19%, reduced the risk of postoperative nausea and vomiting by 6%, but only reduced the time that patients spent in the PACU by 4 min.
Chan et al reported at the 2005 ASA meeting the findings of a study in which they assessed the effect of a device in which anaesthetic depth was controlled by use of an auditory evoked potential monitor (AEP) in over 1000 patients. In the AEP group, sevoflurane and propofol doses were reduced by 29% and 16% respectively; emergence was faster and patients were discharged home earlier (6.9±5.2 vs 9.1±8.1 days); and the incidence of postoperative nausea and vomiting was reduced from 48% to 20%. Even more remarkable was the observation that 30 days following the surgical procedure, AEP patients rated their quality of recovery higher than the controls.
Depth of anaesthesia is an important factor in the anaesthetic management of patients. When considering depth of anaesthesia as it relates to the risk of intraoperative awareness, the following points are key:
When considering other outcome measures which depth of anaesthesia may affect, the data is not as compelling. Monitoring depth of anaesthesia may decrease anaesthetic dose, enhance "fast-tracking" of patients, decrease side effects from anaesthesia (e.g.,PONV), and enhance the patient's quality of recovery.
- Incidence of 1-2/1000
- There is the potential for serious psychological / medicolegal sequelae
- Equipment check is paramount in the prevention of intraoperative awareness
- Amnestic agents - although the evidence is lacking, the clinician may consider an amnestic as a premedicant in patients at risk for intraoperative awareness, and as a treatment when patients are lightly anaesthetized
- Re-dose hypnotics in clinical situations that are at risk for intraoperative awareness(e.g., difficult airway)
- Haemodynamics are unreliable as a predictor of inadequate anaesthesia
- There is no proven awareness monitor that has 100% sensitivity and specificity. Current monitors have low positive and negative predictive power for awareness.
- Monitor the end-tidal anaesthetic level
- Consider at least a 0.6 MAC level of a volatile anaesthetic
- Neuromuscular blockers will mask an important indicator of inadequate anaesthesia
- Consider a brain function monitor as an adjunct to other available indicators of anaesthetic depth
The effect of avoiding deep levels of anaesthesia on other outcome measures such as neurocognitive dysfunction and mortality is less clear, but preliminary data suggests that avoiding deep levels of anaesthesia may be a useful goal of anaesthetic management.
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