Srivas Chennu, University of Cambridge
Patients often say that one of their worst nightmares when undergoing surgery is that they might wake up in the middle of their operation. And, scarily enough, it does happen. But now scientists at Cambridge University have discovered a unique brainwave pattern that can signal when a person has been effectively anaesthetised, which should, they say, make surgery safer. Chris Smith spoke to Srivas Chennu who is leading the project…
Srivas - There are 60,000 surgeries done in the United States alone every day. Right, now the challenge doctors face is how much drug to give somebody to to put them under and how do you know that they are actually unconscious before you actually cut them open. While doctors are pretty good at solving that problem, one or two of every thousand people report having woken up during surgery. Now that doesn’t seem like a lot, but if you add up the numbers, it’s 20-30,000 people every year.
Chris - It’s also a pretty big deal for the person who wakes up.
Srivas - Absolutely. It’s a very traumatic, very terrifying experience; this is the number one thing that people report after surgery as having caused them traumatic distress. Not the actual operation itself, not the fact that they were cut open, but the fact that they were aware of what was going on. They really don’t like that and we’d like to prevent that, as scientists and as doctors.
Chris - Why is that such a hard nut to crack? Why doesn’t an anaesthetist have a simple test they can do that tells them this person is asleep?
Srivas - That’s a really good question. It’s basically asking, why don’t we have a test for consciousness. Now, this exactly what we’d all like to have; a test of the level of consciousness just like you have a test for the level of blood pressure. In the current situation, doctors don’t have that, so they do very simple things like measuring blood pressure or measuring,sort of, the movement that a person has before deciding - this person's unconscious,we can proceed with surgery. So they’re using indirect measures, but what we’d like to do is go ask the brain how conscious it is and then decide whether somebody’s ready for surgery or not.
Chris - How did you do that?
Srivas - So what we did here is to use a measure called EEG, which stands for Electroencephalography. So what we do is basically put these little electrodes on the scalp of a person who is being anaesthetised. It’s completely painless, completely non-invasive; they don’t really feel it and it measure tiny electrical currents that the brain’s producing all the time, and it gives us a couple of key signatures that tells us how conscious the brain is.
Chris - Right. So, I guess what you're saying is you take somebody, you measure those signatures when they’re awake, you put them to sleep. You know they were asleep because you can ask them later, but you can see how those signatures changed when they were saying - well not saying, they were awake or not and that means you’ve then got a sort of EEG signature of unconsciousness.
Srivas - Yes, and we went further. What we did was while we measured the EEG, we got an object of measure of how aware they were and got them to press a button when they heard a ‘bing’ and another button when they heard a ‘bong’ - just so that we know they were responsive. We also measured the amount of sedatives and anesthetic in their blood so that we can say, okay what in the EEG, what in the brain links to the amount of drug in the blood, and what in the EEG links to the fact that this person says they’re aware. So we can link the two to find out what other useful signatures in the brain then link to the fact that people say they’re aware and, when does that stops, when do they become aware.
Chris - Anaesthetists often say, there’s a really big difference between individuals. They might give you just a whiff of a drug, you’d be out like a light. I might take the amount it would take to knock out an elephant; we’re both similarly statured people. Did you see that sort of relationship that there was a tiny amount of drug needed in some people people to induce the same state of unconsciousness compared with others?
Srivas - That was a fascinating bit of our data. What we did was we gave people just enough drug to put them on the cusp of consciousness that allowed us to basically find people, some of whom were given a particular level of drug were completely under and others, with the same level of drug, were fully alert, fully responsive. We don’t know why this is the case, but our data gives us clue because what is suggests is that the way the brain activity looks like before they get the drug seems to affect who is going to go under and who isn’t, and that’s a fascinating insight for now applying this idea to being able to figure out how much of the drug to give somebody to make sure they’re under.
Chris - Right. So not only is this potentially a monitoring tool for making sure someone is asleep and they’re staying asleep, but also it could be used predictively; how much drug am I likely to need to give this person to get them off in the first place?
Srivas - Exactly. Currently what is done in modern clinical settings is they have a thing like a ‘marsh model’, as it’s called where they take your body weight and you body mass and say they have some internal model to calculate how much drug you need. Clearly, the model doesn’t do the job every time. What we’d like to do is enhance these sort of models to bring in the brain and say well based on this person’s brain, this is how much drug they would need to make sure they were under for surgery.
Chris - You only looked here at a small number of people; this is just a preliminary test. Surely you’re looking at healthy, fit individuals. What about those people who wake up on the theatre table. Would it not know be appropriate now to go and find who they are and see if you can get the similar sorts of tests and then see what your brain EEG measures would predict about them?
Srivas - Absolutely. I think this is exactly the next step. This study is very much a science study you know, we don’t actually go the clinic yet, but the next step would be to try and make some sort of device that we can trial in that context because there, you are talking about people who are no longer healthy, who already have chronic conditions. In those cases, you really don’t want to risk overdosing such people so, what we try to do, is exactly that; try to take this to the operating theatre and see how well it can perform in the real world.