Muscle paralysis alone in an awake subject can cause significant drop in the Bispectral Index Score (BIS) to below that normally indicating unconsciousness and deep anaesthesia.pearl
- P J Schuller, S Newell, P A Strickland, and J J Barry.
- Department of Anaesthesia & Intensive Care, Cairns Hospital, PO Box 902, Cairns QLD 4870, Australia email@example.com.
- Br J Anaesth. 2015 Jul 1;115 Suppl 1:i95-i103.
BackgroundThe bispectral index (BIS) monitor is a quantitative electroencephalographic (EEG) device that is widely used to assess the hypnotic component of anaesthesia, especially when neuromuscular blocking drugs are used. It has been shown that the BIS is sensitive to changes in electromyogram (EMG) activity in anaesthetized patients. A single study using an earlier version of the BIS showed that decreased EMG activity caused the BIS to decrease even in awake subjects, to levels that suggested deep sedation and anaesthesia.MethodsWe administered suxamethonium and rocuronium to 10 volunteers who were fully awake, to determine whether the BIS decreased in response to neuromuscular block alone. An isolated forearm technique was used for communication during the experiment. Two versions of the BIS monitor were used, both of which are in current use. Sugammadex was used to antagonise the neuromuscular block attributable to rocuronium.ResultsThe BIS decreased after the onset of neuromuscular block in both monitors, to values as low as 44 and 47, and did not return to pre-test levels until after the return of movement. The BIS showed a two-stage decrease, with an immediate reduction to values around 80, and then several minutes later, a sharp decrease to lower values. In some subjects, there were periods where the BIS was <60 for several minutes. The response was similar for both suxamethonium and rocuronium. Neither monitor was consistently superior in reporting the true state of awareness.ConclusionsThese results suggest that the BIS monitor requires muscle activity, in addition to an awake EEG, in order to generate values indicating that the subject is awake. Consequently, BIS may be an unreliable indicator of awareness in patients who have received neuromuscular blocking drugs.Clinical Trial Registry NumberACTRN12613000587707.© The Author 2015. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: firstname.lastname@example.org.
Researchers induced awake paralysis in 10 volunteers using separately both suxamethonium and rocuronium. Both the BIS A2000 (2003) and BIS Vista monitor (2013) were tested.
BIS decreased immediately after paralysis and did not fully recover until muscle recovery. BIS values decreased to as low as 44, despite the subject being awake.
In more than half of the 20 trials the BIS value decreased to below 60 at some point. In one case this lasted for almost 4 minutes, representing 76% of the total paralysis time for that subject.
This paper is full of many important pearls, and should be read in full.
Regarding common practices in the conduct of BIS-guided anaesthesia:
It has been suggested that a BIS range of 60–75 is suitable for ‘the end of surgery’, but our results show that if neuromuscular block is used, this range is consistent with full awareness.
...and on the use of the Signal Quality Index:
Given that the major cause of patient-related artifact is movement, it is not surprising that the SQI will increase towards 100 when NMBDs are administered, as we found. Unfortunately, the high SQI will indicate that the BIS is at its most reliable exactly when it is performing most poorly in the aware but paralysed patient.