BIS: RELAX DON’T DO IT?

frankie saysRecently saw BIS discussion re-surface. NICE recommends it, yet very few use it. We could make a long discussion about this, but the short version is we sometimes use it, despite its shortcomings. Its best use would presumably be in patients with neuromuscular blockers onboard. Problem is BIS value seems to be affected by neuromuscular block alone. BIS
Bispectral Index or BIS is supposed to be a fancy, condensed EEG for dummies, letting the anaesthesiologist know if the patient is awake or not by a simple “awake number” from 0 to 100. Zero would be dead, and 100 would be full-on amphetamine binge, I guess. I’ve never seen 100. The company proclaims >80 is awake, 60-80 sedated and 40-60 deep anaesthesia suitable for surgery.

One problem with BIS is that it’s a proprietary algorithm held secret by the company making them. We don’t really know what exactly it measures or how it works out the BIS number it displays. We do know that it’s supposed to reflect brain activity and EEG in some way. In clinical practice, it often has a pretty good association with perceived depth of anaesthesia: It falls at induction, and you often get an abrupt rise in BIS just before (or as) the patient is waking up. So it must be good? Or could it be influenced by other things, like muscle depolerising activity?

Neuromuscular blockers and BIS
The best indication for BIS would be to use it in patients that are muscle relaxed and cannot communicate that they are awake. BIS would tell us. It has been hypothesised that NMB affects BIS. On the other hand, some have hypothesised that NMB naturally affect BIS because NMB also affect the awake state. So a group of anaesthesiologists set out to investigate.

Study
11 anaesthesiologist made a kind of “Flatliners” club, and tested BIS on themselves while awake, but muscle relaxed with neuromuscular blockers (NMB), and BVM bagged by their colleagues. Using the isolated forearm technique (where you tourniquet one arm to stop neuromuscular blocker to reach and affect that one forarm, and leave it for hand signaling to the outside world while the rest of your body is flaccid). And they used two different, but current, BIS monitors.

They tested both suxamethonium and rocuronium to test both depolerasing and non-depolarising NMB (and probably also to avoid yet another sux vs roc war on twitter). Sux until effect wore off, and roc was reversed with sugammadex after 15 minutes.

Results
Both BIS monitors failed the same way with both sux and roc: they both reported BIS values as low as 44 and 47, while the patient was clearly awake, and answering questions and solving math problems, signalled thorugh their working forearm. Most subjects reached a BIS below 70, and over half reached a BIS below 60 on NMB alone, the low values lasting for up to several minutes.

Conclusion
BIS seems to incorporate muscle activity into its algorithm, making it unreliable in patients that have received a neuromuscular blocking agent – which, unfortunately, is the best indication for BIS. If it worked as advertised.

Use with care. Possibly use as an extra alarm if rising, but don’t trust it. And don’t titrate depth of anaesthesia to lighter levels due to low BIS.

Or don’t use at all.

Response of bispectral index to neuromuscular block in awake volunteers, BJA, 2015.

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3 Responses to BIS: RELAX DON’T DO IT?

  1. nfkb says:

    Hello,

    you write “And don’t titrate depth of anaesthesia to lighter levels due to low BIS.
    Or don’t use at all.”

    But what about using it without NMBA. In ENT surgery I almost all the time avoid NMBA and I find usefull to titrate hypnotic agents with the BIS. Would you write the same message for anesthesia without NMBA ?

    Thanks

    • Thomas D says:

      Thanks for your comment, Remi! We’re not sure how good BIS is without NMB. Probably decent. But with some lag. I just find that without NMB, I’m thinking the patient can give me other signs they are waking – but BIS *could* be an early marker, although I have no great proof and not very much faith.

      About the titration to lighter levels of anaesthesia, you can probably use it like that without NMB, as again, the patient should be able to respond if too light. I know I use it like a depth of anaesthesia monitor sometimes. If I do, I tend to be even more vigilant of other signs of light anaesthesia, though. I also feel that titrating too light, the patient can almost reach the surface, and then you get a spike in the BIS a few seconds later, so I avoid that.

      So, yes, I do use it, but don’t trust it very much. Just my personal opinion, though. How do you feel about it? Useful?

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