The New England Journal of Medicine has published a terrific study on the effectiveness of preventing intraoperative awareness or surgical recall. It is estimated that up to 1% of surgical patients will experience recall of their operation, or 20,000 to 40,000 patients. That is a tremendous number of people who may suffer severe psychological damage and post traumatic stress disorder from a routine operation. There has been no absolute method to prevent recall. The BIS monitor was developed that measures the EEG waves of an anesthetized patient. Through a proprietary algorithm a number between 0 and 100 is determined. A zero indicates a flatline in brain activity while a 100 means a fully awake patient.
The study was conducted as part of a multicenter trial involving over 5,000 patients. Half the patients would have their anesthesia monitored with a BIS monitor. The other half were monitored for awaress with the traditional end tidal anesthetic agent concentration (ETAC) monitor. To prevent surgical recall, the BIS monitor was kept at a reading between 40 and 60. For the ETAC patients, they were kept at 0.7 to 1.3 age adjusted MAC for the inhalational agent. Afterwards, the patients were interviewed at 72 hours and 30 days post surgery for any memory of intraoperative awareness.
To the surprise of the researchers, there were no significant differences in surgical recall between patients monitored with a BIS vs. patients who had an ETAC monitor. In fact, there was a trend for more patients using a BIS to have recall. Of 2,861 patients using a BIS, 7 patients definitely had intraoperative awareness and a total of 19 had definite or probable recall. Of the 2,852 patients who had their ETAC measured intraoperatively, two patients had definite recall while eight had definite or probable recall. Again neither number proved to be significantly different. It's interesting to note that premedicating the patient with a sedative like a benzodiazepine made difference in incidence of recall. Nor did a history of alcohol or drug abuse, ASA classification, or any other criteria looked at by the researchers make a difference.
Surgical recall is one of the most feared complications of surgery for a patient. Questions about it is probably brought up to me during a preop interview more often than almost any other anesthesia subject. The topic is so serious that the American Society of Anesthesiologists has a registry to document all cases of intraoperative awareness in this country.
This study only shows what has long been suspected, that BIS monitoring is not a definitive way to prevent awareness. It probably works better at preventing medical malpractice claims for surgical recall than it is at preventing the recall. You can always tell the patient and his lawyer that you did everything in your power to prevent the awareness, including attaching a BIS monitoring. Thus there really is nothing more that an anesthesiologist can do with current medical technology to prevent this tragic event. Improvements in measuring consciousness in real time is still in the research stages. In the meantime, the best way to prevent awareness is education and studious observation of the patient.