Using Intraoperative EEG to Predict Post-operative Delirium at Duke

“Both intraoperative EEG monitoring and preoperative cognitive testing should become more standard.”

January, 2021
By Sandra Gordon for ASA

In this Q&A, Miles Berger, MD, PhD, Associate Professor, Anesthesiology Dept (Neuroanesthesiology Division) and Senior Fellow at the Center for the Study of Aging & Human Development at Duke University Medical Center in Durham, North Carolina, discusses his team’s work with intraoperative EEG monitoring to predict post-operative delirium and cognitive impairment.

Q: Dr. Berger, can you describe your work with intraoperative EEG and post-operative delirium?
Many--though clearly not all--anesthesiologists use some type of EEG monitoring in the operating room to either assess the extent to which a patient is adequately anesthetized for surgery. Our work and the work of others has suggested that the waveforms from those EEG recordings can also be used to tell us something about a patient's preoperative neurocognitive status and about which patients may be at highest risk of developing delirium after surgery.

We've identified a decrease in Alpha band (8-12 Hz) EEG power as a predictor of impaired preoperative cognition. Others have confirmed that finding and suggest it may even be a predictor of delirium as well.

Another EEG feature that has been a topic of much study is burst suppression, in which the EEG essentially intermittently flat lines. That's a pattern not seen at all in awake people or even during sleep. A number of papers have demonstrated that burst suppression is associated with increased risk for delirium after surgery and in the ICU setting.

Q: Is EEG intraoperative monitoring the wave of the future for predicting post-operative delirium?
I think so. A group of us, from the Fifth International Neurotoxicity Working Group, published a paper in Anesthesia and Analgesia in 2018 which we suggested that both intraoperative EEG monitoring and preoperative cognitive testing should be implemented more widely, particularly in the care of older adults.

Yet, the majority of older surgical patients are not undergoing preoperatively cognitive testing. (A February 25, 2020 study in Perioperative Medicine found that of 1737 ASA members surveyed, 80.6% of respondents indicated that screening for signs of dementia or preexisting cognitive decline is performed in fewer than 10% of cases.)

Frequently, because of the time pressure we’re under, as anesthesiologists, we're not the best people to suddenly stop and say, ‘Okay, let's do a mini mental status exam before we take you back to the OR.’ And often that's not really the best time to be screening patients because they may be nervous about surgery.

Our group has worked on identifying intraoperative EEG features, brain wave patterns that are indicative of preoperative cognitive impairment, which we think will be also predictive of post-operative delirium.

For the field, this is a bit of a paradigm shift. Many anesthesiologists monitor EEG even though it's not an ASA standard monitor or requirement. Much of the research on EEG and anesthesia has focused on the effects of specific drugs on the EEG waveforms, which operates under the assumption that all patients would respond similarly to a given drug. We now know that's not quite true; different people respond differently to the same drug and will show different EEG waveform patterns.

EEG monitoring has a storied history within anesthesia because it has been promoted as a way to prevent intraoperative awareness. In particular, using a BIS EEG monitor is controversial. Some people will say we shouldn't be using an EEG monitor because it doesn't prevent awareness. The existing data suggests that’s true, at least in patients receiving inhaled anesthetics. But there are multiple reasons to use an EEG monitor in the OR. Since the brain and central nervous system is the target of most of our anesthetic drugs, it’s reasonable to want to have some idea of what the drugs we are administering are doing to a patient’s brain.

Q: Do you screen older surgical patients for perioperative delirium at Duke?
We've started doing preoperative cognitive testing on all patients above a certain age at our preoperative anesthesia screening unit. We also use intraoperative EEG monitoring on roughly one-third of all adult cases at our institution (see Ni K et al, British Journal of Anaesthesia, 2019).

You can recognize on an EEG spectrogram which patients’ waveforms don’t have much Alpha power, and you can make predictions: that a patient who has very little Alpha power is probably having some preoperative/baseline cognitive impairment. Patients with very little Alpha power and frequent burst suppression are at higher risk than normal of post-operative delirium. With burst suppression, it’s one of the easier EEG waveforms to see because it's just a flat line, which should normally not be there.

Q: What would it take if someone wanted to start using EEG monitoring as a way to predict post-operative cognitive impairment?
It depends on two things: one is having the monitors, whether it's a BIS monitor or Sedline monitor or any of the other commercially available EEG monitors and 2) the knowledge and how to interpret the waveforms, and how to use that information to guide clinical care.

In terms of the knowledge, resources are available. ICE-TAP, which is a consortium of those interested in promoting EEG usage in the OR, has a number of learning modules. Patrick Purdon and Emery Brown and colleagues at Massachusetts General Hospital has also created learning modules for providers who want to learn more about the effects of specific anesthetics on the EEG and how to recognize some of these wave forms.

Q: How might EEG monitoring affect clinical practice?
A big question in the field that’s still somewhat unresolved in my mind: What's the best thing to do for older surgical patients with these sort of patterns, who show a lot of burst suppression or very low Alpha power? Is there anything we can do pharmacologically in terms of the drugs we administer their doses to try to improve their outcomes postoperatively in terms of delirium or cognitive function?

There's been a lot of large studies in this area, but I think that still remains an open question. The ENGAGES trial by Michael Avidan MD, Troy Wildes MD and Brad Fritz MD and colleagues examined whether an intervention to reduce intraoperative burst suppression would reduce postoperative delirium; Jacqueline Leung, MD had a study similar to that looking at what happens to postoperative delirium rates if you try to prevent burst suppression. There's some other work in progress right now on what if you try to maximize Alpha power during surgery, including the alpha Max trial currently being led by Jamie Sleigh, Paul Garcia and colleagues. I think we'll learn a lot more in the next few years about what we can do in these situations when we see abnormal brain responses to anesthesia, i.e. what we can do to improve outcomes. The brain is a lot more complicated than virtually any other organ in the body. We're still just figuring out what these EEG waveforms mean in a lot of cases under anesthesia. We’re where EKG was more than 50 years ago, in terms of identifying EKG patterns that are indicative of a heart under strain or having a heart attack.

In the meantime, there's work by Michael Avidan and colleagues showing that a lot of patients don't understand post-operative instructions or remember what we tell them early after surgery. If we had a better idea of who was cognitively impaired at baseline, we could better identify who we may need to spend a little extra time with to make sure they understand their discharge instructions. That's possible now with existing EEG technology because we can look at patients’ EEG responses to anesthetic drugs to determine which patients are likely to have preoperative cognitive impairment.  

With EEG monitoring, I think there will be in an emerging role for an anesthesiologist as part of a multidisciplinary care team in aiding post-operative care in ways in which we haven't traditionally been involved. It doesn't need to take a lot of time. Sometimes I'll say this across the drapes to my surgical colleagues: “Based on how this patient is responding the anesthesia, I think they have some cognitive impairment. You guys may want to spend a little extra time talking to them after surgery to really make sure they understand their postoperative instructions.”

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