A few years ago, a new anesthesia gadget called the BIS monitor, short for bi-spectral index, made the headlines. The claim was that this new monitor would ensure that the patient was truly “asleep” and unaware of the surgery…but not getting too much medication either. It works by calculating a score from EEG (brain-wave) patterns. The BIS monitor divided anesthesiologists – some felt it was the latest and greatest thing, others felt it was not particularly accurate.
This week, in the New England Journal of Medicine, a randomized study compared awareness after anesthesia in patients treated with standard monitoring versus the BIS monitor.
More than 6,000 patients were randomized to receive intra-operative monitoring by either BIS or End-Tidal Anesthesia Concentrations (the standard). Forty-nine patients reported intra-operative memories. Expert review determined that 9 of these patients (0.15%) had truly experienced definite awareness.
The group with standard (End-Tidal monitoring) had a lower incidence of awareness: compared to 7 out of 2,861 patients in the BIS group, only 2 out of 2,852 patients in the standard group experienced definite awareness (p=0.98).
There was no statistically significant difference in the rate of intra-operative awareness between the two monitoring systems.
Editor’s note: This is a good study that gives clinically helpful information. The BIS monitor isn’t magic – there’s still a lot about the state of consciousness (or lack thereof) that we don’t fully understand. So, it’s not surprising to me that the BIS monitoring system isn’t foolproof.