| "Awareness During Cardiac Surgery" |
| Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (2) 214-9 |
| Article type: Review |
| Authors: Deepak K. Tempe et al, (G.B. Pant Hospital, New Delhi) |
| WHAT THE ARTICLE SAYS | Skip to References |
| Awareness during general anaesthesia can be defined as "the
undesired, unanticipated, patient wakefulness during surgery, or recall afterward”.
The degree of awareness may be gauged by the presence of explicit (conscious, deliberate
recollection of events) or implicit (where conscious recognition is not required) memory
of intraoperative events. The latter may be manifested by changes in task performances or
behaviour and require psychological testing for diagnosis. Patients’ dreams during
recovery, and recall of events in the recovery area may be mistaken for intraoperative
awareness.
Intraoperative awareness can be a horrifying experience resulting in
psychological trauma. Patients may present with anxiety, irritability, nightmares, sleep
disturbances, preoccupation with death, rage, panic and depression. They may require
psychotherapy. Case reports also suggest that conversation in the operating room may
adversely affect the postoperative course of a patient.
The overall incidence of intraoperative awareness is unknown, but it is most often stated to be 1%. Difficulties arise with the definition and criteria for intraoperative awareness. Certain procedures mandate light anaesthesia and therefore have higher incidences of awareness, e.g. obstetric procedures- 28% awareness major trauma surgery- 11-43% awareness cardiac surgery- up to 23%. The largest of 6 studies on awareness during cardiac surgery, is by Phillips et al, and involves 700 patients who all received benzodiazepines, low dose fentanyl and a volatile anaesthetic agent intraoperatively. Only 1.1% had explicit awareness on postoperative assessment. REASONS FOR INTRAOPERATIVE AWARENESS DURING CARDIAC SURGERY
ANAESTHETIC AGENTS AND AWARENESS Since 1969, high dose morphine anaesthesia has been used for valvular surgery in patients with no cardiac reserve. Indeed, a high dose opioid technique has been the mainstay of cardiac anaesthesia, however, this technique may only provide adequate analgesia and expose patients to awareness. Morphine has been replaced by more potent and cardiovascularly stable opioids, but once again these do not reliably produce amnesia. Opioids have been combined with nitrous oxide, benzodiazepines and inhalational anaesthetic agents with better results, as Phillips' study showed (1.1% incidence of awareness). Volatiles may increase anaesthetic depth temporarily in anticipation of a strong noxious stimulus, there have been few reports of awareness where a volatile is used in inspired concentrations of >1%. It has been shown that volatiles produce amnesia at concentrations of <1 MAC. There are, however, drawbacks with volatiles: cardiovascular depression, lack of analgesia (especially at subanaesthetic doses during recovery) and postoperative shivering (increased oxygen demand). Nitrous oxide's use has declined due to the fear of expansion of air emboli. Fast track cardiac anaesthesia developed because of cost and resource factors and has led to a decreased amount of opioids given intraoperatively, in order to allow postsurgical extubation within a few hours. This approach may be associated with a higher incidence of intraoperative awareness. Propofol and alfentanil have been used for TIVA, but in one study, 1 out of 10 patients developed hypertension on sternotomy and had awareness. There is no consensus regarding the choice of agents to ensure adequate depth of anaesthesia during the high risk periods in cardiac surgery. Anaesthesiologists tend to use agents of their choice, as boluses, or infusion step ups. Feedback information to anaesthesiologists post surgery, may decrease the incidence of awareness, as it leads to the use of higher doses of the primary anaesthetic agent, decreased use of pancuronium, and the favoured use of continuous infusions in contrast to intermittent bolus techniques. A report by Ranta et al, recommended the following: the continuous use of inhalational/ intravenous anaesthetic agents, monitoring of end tidal concentrations of inhalational agents, minimum possible neuromuscular blockade and monitoring for clinical signs of anaesthetic depth. NORMOTHERMIC BYPASS The minimum alveolar concentration of all volatile anaesthetic agents decreases with decreasing body temperature. In 1989, Lichtenstein et al, introduced “warm heart surgery”, which has been shown to provide superior myocardial protection, and is growing in popularity. As yet no relationship between temperature and awareness has been established in these cases, in the mean time, the liberal use of benzodiazepines and/or inhalational agents seems justified. DEPTH OF ANAESTHESIA This depends on the anaesthetic agent being used and the degree of surgical stimulation. Clinically it is judged by the presence of somatic and autonomic reflexes, and in the presence of paralysis, by autonomic reflexes only. Other methods of assessing anaesthetic depth include:
With auditory evoked potentials, an increase in the middle latency auditory evoked potential (MLAEP), reflects the hypnotic component of anaesthesia as opposed to the analgesic component. As latency increases, amnesia is followed by loss of consciousness and then implicit memory. Surgical stimulation may reverse some of the MLAEP prolongation. PREVENTION OF AWARENESS Currently there is no totally reliable monitor for consciousness, therefore, some incidents of awareness may have to be accepted to avoid greater morbidity and mortality from deeper levels of anaesthesia. Awareness is a distinct possibility in some individuals and is a potentially disabling experience. A pre surgical visit by the anaesthetist may relieve the patient's anxiety, thereby decreasing the patient's sympathetic activity, which may in turn decrease the risk of awareness. Light anaesthesia may be planned for sick patients, and informing such a patient of the possibilty of awareness might increase the level of anxiety and predispose the patient to the risk of developing myocardial ischaemia. However, this is often necessary to meet the guidelines of informed consent! PREMEDICATION Agents which decrease apprehension, fear and produce amnesia should be used, benzodiazepines and scopolamine are often preferred. Supplemental intravenous drugs may be given if the patient has inadequate sedation in the pre surgical area. All equipment used for delivery of anaesthetic agents must be checked thoroughly pre operatively. ASSESSMENT OF ANAESTHETIC DEPTH Clinical judgement has to be relied on, keeping in mind that reflexes may be obtunded due to beta blocker or calcium channel blocker therapy and poor myocardial function in a sick patient. In cardiac patients anaesthetised with opioids, a dose of a hypnotic agent, e.g. diazepam, may lead to severe hypotension due to vasodilation, which does not respond well to a vasopressor or noxious stimulation. It is suggested that the administration of a benzodiazepine could be delayed until a hypertensive response to intubation, etc, is observed, the amnestic qualities will likely prevent patient awareness. It is also not advisable to rely on anaesthetic agents associated with preservation of the middle latency auditory evoked potential (MLAEP) i.e. receptor based anaesthetics- opioids, benzodiazepines, nitrous oxide, but rather agents which suppress the MLAEP should be included in the technique, i.e. volatiles and propofol. Top up doses of benzodiazepines and volatiles should be considered during specific events, e.g. sternotomy, DC shock, rewarming. Patient related factors, e.g. age/ smoking/ alcohol and drug use may increase dose requirements. The incidence of intraoperative awareness can be decreased by educational measures and vigilance, e.g. feedback information to the anaesthesiologist. I S THE ANAESTHESIOLOGIST LIABLE? Awareness of any sort during cardiac surgery, is potentially a failing of the anaesthesiologist. However, with the currently available techniques, awareness during cardiac surgery may not be totally avoidable. Indeed, with the increasing number of patients with compromised cardiac function presenting for surgery, the incidence is likely to increase. There are differing schools of thought on the matter, but it may well be prudent to engage in some form of quality control to reduce the incidence of awareness during cardiac surgery. |
|
Up
| Date of First Publication : 2000-01-22 | Date of Last Update : 2006/10/23 | Web page author: Click here |