I like knowing how deeply anesthetized my patients are, not just that they are unaware. How much a patient is unaware as a continuum is a lot more important that the binary notion of awake vs asleep. I do this with a BIS monitor (and could do the same with Hospira’s PSA monitor) and make lots of decisions based on it and I think provide better care for the effort. My reasons for saying this follow.
Hemodynamic and analgesia decisions. The BIS monitor is a hypnosis monitor. There are three other important elements of an anesthetic: analgesia, neuromuscular blockade, and sympathetic reflex control. Let’s say the patient is hypertensive. There are three possible causes for it: insufficient hypnosis, insufficient analgesia, and sympathetic reflexes. Given a BIS value less than 60 then I can give an analgesic trial and if that does not work treat the blood pressure primarily. I might choose to use the anesthetic as my antihypertensive or I might choose to use a specific antihypertensive drug. Whatever I choose, I am doing it rationally. My non BIS-using colleagues just dump in the anesthetic for everything, BP, analgesia, and sympathetic control; thus explaining the observation that BIS values, blinded to the anesthesiologist, tend to be much lower than when they are not blinded. To avoid the dreaded awareness we all overdose. As an aside Bennett etal presented some interesting data at this year’s ASA on a possible analgesia monitor, so one day we will be able to quantitatively monitor all four elements of an anesthetic.
Overdosing with anesthesia. While we are talking about overdosing maybe it is useful to recall that Terri Monk and colleagues report a higher death rate at one year in association with such an overdose method….it seems that these anesthetics leave a biological lesion long after the case is done. Now I agree that this is just an association study but then there is the annoyingly supportive material published by Eckenhoff, Wei, and by Xie in separate publications indicating dose and time related effects of volatile anesthetics (especially isoflurane) exposure to produce amyloid depositioin (Alzheimers protein) and apoptotic programmed cell death in brain tissue. One wonders if similar bad things happen in nonneural tissue to account for Monk’s observations. In addition, Bohnen etal have their underpowered study that almost showed an effect of cumulative anesthetic exposure to predispose to the genesis of Alzheimers disease. I am quite sure that more data will be forthcoming that systematic anesthetic overdosing is not a good thing and that the most valuable use of BIS monitoring won’t be to avoid too light anesthesia but quite the opposite….to prevent too heavy anesthesia.
Burst suppression. The device has a handy modality on it called the suppression index. It tells you what percent of the EEG is flat. Very handy for when the brain is swelling out of the head and you have to run maximal doses of thiopental or propofol. Once the eeg is fully suppressed there is no advantage and lots of disadvantages to increasing the dose further. An abstract at a neurosurgery meeting a couple years ago compared this processed EEG calculation of burst suppression with raw eeg with bedside evaluation of the waveform. They were reported to be comparable methods with non comparable cost (raw EEG + neurologist = $$$))
Ischemia detection. Occasionally for severe ischemic events this unit will give a faithful reading of something bad happening. I would not rely on it to say all is well but it sure has detected and confirmed bad things at times.
Another reason is MONEY!... I did a retrospective review of the financial impact of introducing BIS to our ambulatory care center when I was at WVU using CompuRecord software to track anesthetic usage. Similar to what I stated earlier about too-deep anesthesia when not using BIS, we found significant financial savings accrued from BIS use just in terms of less anesthetic drug use. In addition we found shorter times to leave the OR after the end of the case, better Aldrete scores in the PACU and shorter times to being discharge ready from PACU. It was difficult to translate these latter advantages into dollars because we could not indentify an FTE to eliminate. Interestingly, however, in this context of high volume high turnover anesthetics, the money saved in anesthetic usage per case did not exceed the cost of the many BIS patches that were needed. However, the financial advantages for long cases is undisputed. I never published this work but someone else did later report the same thing basically.
Oh yes. Prevention of awareness. I do use the BIS for that but the way I and others use it to run patients less deep I am a bit surprised we don’t see a higher incidence of awareness through its use.
So, here is a classic example of how I use it: Patient presents for extensive back surgery in the prone position. I know that if I let the patient have prolonged surgery, hypotension or anemia that the risk of blindness, although still low, rises. I also know that intraocular pressure tends to rise with increasing time. I don’t ever want to have to deal with that blindness problem in any of my patients and I do quite a few of these cases every year. Without the BIS monitor I would have to overdose then run phenylephrine. However, with the BIS monitor my instructions to my residents are this: BIS 55, MAP>80, Hb>10. Once the anesthetic dose is just right to produce the desired BIS then we manipulate the blood pressure with fluids, analgesics, and vasoactive drugs….all centered around the BIS monitor.
So, it seems that the BIS monitor really has multiple uses: save money, titrate analgesia, sensible use of antihypertensive meds, prevention of anesthetic toxicity, and prevention of awareness. I cannot think of a reason for a thoughtful anesthesia practitioner to not use it.
the following added on dec 3, 2007:
A colleague advises me that there is a recent cochrane review that also supports use of bis monitoring for things other than preventing awareness: