A new study by researchers in the US suggests that a new brain monitoring device that is widely used to check that surgery patients form no memories of their operation was no more effective at lowering the risk of “anesthesia awareness” than older methods. However, the researchers said that the results need to be confirmed with larger studies.
The study was conducted by principal investigator Dr Michael S Avidan, associate professor of anesthesiology and surgery at Washington University School of Medicine in St. Louis, and colleagues, and is published in the March 13 issue of the New England Journal of Medicine (NEJM).
Anesthesia awareness is extremely rare, and occurs in one or two surgical patients per thousand, said Avidan, “but because of the huge number of operations performed annually, the total number of people with awareness during anesthesia is significant,” he explained.
Anesthesia awareness suggests that patients form memories of the operation while they are under anesthesia. The condition is sometimes very distressing and can lead to long term anxiety and in some cases, post traumatic stress disorder.
Avidan and colleagues enrolled nearly 2,000 patients undergoing surgery at Barnes-Jewish Hospital in St. Louis. Half of the patients were monitored using a device that measures the patient’s bispectral index (BIS) using brain wave data taken by electroencephalogram. A person’s BIS is 100 when they are fully awake, and zero indicates no brain wave activity. Recommended practice is to keep BIS between 40 and 60 to stop anesthesia awareness during surgery.
The other patients were managed during surgery using an older method, called end-tidal anesthetic gas (ETAG), that measures the amount of anesthetic gas in the air exhaled by the patient. With ETAG, the anesthesiologist can see when the amount of exhaled anesthetic gas goes below a certain threshold, and can then decide whether to increase the level of anesthetic to keep the patient unconscious and unaware.
Avidan and colleagues also took BIS readings of the ETAG monitored patients.
This study was carried out because another one done four years ago suggested that using a BIS monitor could reduce anesthesia awareness risk, but one of Avidan’s co-investigators, Dr Alex S Evers, was not happy about the design of that study:
“The first study was like having a control group that got no treatment,” said Evers, who is the Henry E. Mallinckrodt Professor and head of the Department of Anesthesiology.
“We thought it made more sense to compare bispectral index to ETAG monitoring to see if we could detect any differences in the incidence of awareness between the two groups,” he explained.
The results showed that two patients in each group had formed memories of their surgical experience, which is 0.2 per cent of the overall number.
The investigators looked at the BIS readings of the ETAG group patients who had formed memories of their operation and found that those patients had been maintained within the recommended range that is supposed to prevent anesthesia awareness.
Avidan explained that:
“Part of the risk of awareness involves the surgical procedure and individual patients.”
“Bigger operations are more likely to produce awareness, so a person having open heart surgery will be at greater risk than one having cartilage removed from a knee,” he said.
He also explained that “patients who are sicker at the time of surgery, and those who take certain drugs or drink alcohol are more likely to have awareness.”
One drawback of this study is that 2,000 patients is not a large enough group to see if there are small differences in the BIS and ETAG methods. Avidan and colleagues plan to continue the study with larger groups.
Even patients with an elevated risk of anesthesia awareness will only experience it about 1 per cent of the time, said the researchers. And for those without a higher risk, the rate of experience is likely to be only one or two in 1,000. So to test the difference between the two methods more sensitively, it is better to use larger groups, they said.
Another potential problem that could be found about this study is that the algorithms in the BIS monitor were developed using intravenous anesthesia, whereas this study compared the methods using only anesthetic gases. But the researchers argued this was still a sensible thing to do because the majority of operations are done with anesthetic gases and not intravenous anasthetics, and thus they have produced a more realistic comparison of the methods.
The researchers concluded there was essentially no difference between the two methods:
“We were able to achieve a very low incidence of anesthesia awareness with both protocols,” said Avidan.
“But neither was able to eliminate the problem completely, and we worry that use of the bispectral index could give anesthesiologists a false sense of security that if they keep the measurement between 40 and 60, they’ll prevent anesthesia awareness. This study has demonstrated that’s not entirely true.”
Avidan said while these findings suggest current widespread use of BIS may not be warranted, larger studies need to confirm these results.
“Anesthesia Awareness and the Bispectral Index.”
Avidan, Michael S., Zhang, Lini, Burnside, Beth A., Finkel, Kevin J., Searleman, Adam C., Selvidge, Jacqueline A., Saager, Leif, Turner, Michelle S., Rao, Srikar, Bottros, Michael, Hantler, Charles, Jacobsohn, Eric, Evers, Alex S.
N Engl J Med 2008 358: 1097-1108
Volume 358, pages 1097-1108, March 13, 2008, Number 11
Sources: Washington University School of Medicine press release.
Written by: Catharine Paddock, PhD