The Bristol Model

Added: 2026-05-13

[00:00:00]Let's talk about the Bristol model for total intravenous anesthesia. Now, to be adequately anesthetized for surgery, you need a concentration of propofol in your brain of between two and six micrograms per milliliter.

[00:00:16]A super chilled out person who's also got a remifentanil infusion and has been pre-medicated with temazepam is going to need much less, and a super anxious person who has just taken cocaine and is being only sedated with propofol is going to need much more. That's why you have quite a wide range.

[00:00:31]Now, normally what you would do to achieve this is you would use a TCI or target controlled infusion pump, and you just tell it the weight, age, height, and whatever other parameters it asks for of your patient, and then tell it what you want it to target, and it uses its own algorithm to work out how much to infuse to aim for the target you have asked for.

[00:00:53]Now, the concentration in the brain is the effect site concentration cuz that's where the drug is having its effect. You can also target a plasma concentration.

[00:01:02]So, some of them have a CET or a target effect site, and some of them are CPT target plasma concentration. It doesn't matter too much once you're at equilibrium, because they will be pretty much equal. It's mainly when you're doing an induction dose whether it matters. But, for the purposes of this video, we're just going to say we're aiming for either a plasma or an effect site concentration of between two and six micrograms per mil. So, what if we don't have a pump and we want to try and achieve this manually? That's where the Bristol model comes in. So, to figure this out, they tested a whole bunch of healthy volunteers by giving them different rates of infusions of propofol and then testing their plasma concentrations to work out what the right rate is at what time to achieve a certain concentration. And these healthy volunteers were given temazepam as a pre-medication and three mics per kilo of fentanyl. So, this is dramatically going to reduce how much propofol you need for general anesthesia, but it doesn't really matter because for the purposes of this, it's not going to affect the actual concentrations of propofol.

[00:02:02]And the Bristol model will get you a propofol concentration of 3 micrograms per mil in the plasma.

[00:02:10]And the idea is this infusion regime will achieve this within a couple of minutes and hold it there for the duration of the operation. And here's our graph of plasma concentration of propofol over time. And the Bristol model is divided into three blocks of 10 minutes.

[00:02:27]So, you have your first 10 minutes, your second 10 minutes, and then onwards for however long the operation lasts. What you do is you give a bolus of propofol of 1 mg per kilo.

[00:02:39]And what this is going to do is jump your plasma concentration up nice and high.

[00:02:44]You then follow this 1 mg per kilo bolus with a 10 mg per kilo per hour infusion for the first 10 minutes.

[00:02:54]And this is just enough to maintain a plasma concentration of around 3 micrograms per mil.

[00:03:02]Remember, propofol redistributes really, really quickly. So, if you didn't give that infusion, this would just suddenly drop all the way back down to zero very, very quickly. So, that infusion slows that descent and brings it down towards the target of 3 micrograms per mil.

[00:03:17]If we continue to give 10 mg per kilo per hour indefinitely, our plasma concentration would then start to climb as we saturate the fat stores in the body and we don't need quite so much.

[00:03:27]So, what we do for the second 10-minute block is drop it to eight.

[00:03:31]And the idea that this maintains your steady plasma concentration.

[00:03:36]And again, that redistribution is going to continue, but it's going to gradually slow further down until you only need 6 mg per kilo per hour from then on.

[00:03:47]And then you continue 6 mg per kilo per hour for however long you need for the rest of the operation. So, it's 1 mg per kilo bolus, 10 mg per kilo per hour infusion for the first 10 minutes, then eight for the next 10 minutes, and then six from then on. And the idea is this gives you a nice spike in plasma concentration, which will then drop down to the 3 microgram per mil target that we're aiming for.

[00:04:10]Why do we want this spike? Because of this spike, this slight overshoot, will allow our effect site concentration, which is what really matters, to come up a bit more quickly.

[00:04:21]And meet in the middle at 3 micrograms per mil, because that's really what matters is the effect site concentration. So, this is how you can do TIVA with propofol without a pump.

[00:04:32]However, as you can see, it is quite labor-intensive, and it's much more clunky than just using a TCI pump. So, we wouldn't recommend using it, but academically, it's useful to know how this would work. And this is essentially what your TCI pumps are doing. They're doing a much more complicated version of this, but they are estimating the bolus required, the initial infusion rate, and then what is subsequently needed to maintain that steady plasma concentration. So, if you program your TIVA pump next time you use it, have a look at what it's actually infusing in terms of mils or milligrams per kilo per hour on the pump, and you might see that it's actually quite similar to this original Bristol model.