The Bispectral Index is a measure of hypnosis, derived by mathematical analysis of the surface electroencephalogram (EEG) using a proprietary algorithm (Aspect Medical Systems, Newtown, MA, USA)1. Commercially available monitors have been available for several years. We were interested in the spectrum of BIS values found in our own patients, and present the results of an analysis of BIS values at different clinical endpoints in patients undergoing Total Intravenous Anaesthesia (TIVA).
A database was developed of all patients for whom BIS
monitoring was used during surgical procedures. Three different BIS monitors
were used during this time (A-1000, A-2000 and A-2000 XP; Aspect Medical
Systems, Newtown, MA, USA). For each patient the BIS values at the following
clinical endpoints occurring in the operating theatre were recorded:
(1) Awake, (2) Loss of eyelash reflex, (3) Thirty seconds
after loss of eyelash reflex, (4) Start of tracheal or laryngeal mask
intubation, (5) Start of surgery, (6) End of surgery, (7) First response to
voice, (8) Eye opening, (9) Patient able to correctly state date of birth.
After they had returned to the ward, patients were visited and asked a series of questions concerning their last memory before, and first memory after general anaesthesia, and other more general questions about their satisfaction with their anaesthetic.
The BIS data pertaining to patients who underwent TIVA were extracted from the database, and separately analysed, to identify the median and ranges of BIS values present at the different endpoints.
One hundred and thirty-six patients in whom anaesthesia was induced and maintained were identified. All patients but one were able to state their date of birth in the operating theatre prior to discharge to the recovery room, but the vast majority of patients had no recollection of this and later believed that they had regained consciousness in the recovery room or ward. The BIS values associated with different clinical endpoints are shown in Table 1:
|
Clinical endpoint |
BIS: median (range) |
|
Awake, pre-induction |
97 (86 – 98) |
|
Loss of eyelash reflex |
64 (31 – 93) |
|
30 seconds after loss of eyelash reflex |
45 (17 – 83) |
|
Start of tracheal or laryngeal mask intubation |
44 (11 – 73) |
|
Start of surgery |
47 (29 – 73) |
|
First response to voice |
80 (46 – 98) |
|
Eye opening |
78 (46 – 98) |
|
Patient correctly states date of birth |
88 (60 – 98) |
At all clinical endpoints a wide range of BIS values was
found - several reasons for this are possible. The trend in an individual patient
is more significant than a single value.