Some time ago I was teaching our foundation (most junior) doctors about sepsis with a focus on the interpretation of blood gases. The teaching session ran along the usual lines of normal ranges, basic physiology and a few cases for discussion. All seemed to go well, but I was left with the impression that the understanding around lactate levels was unclear. It’s difficult to articulate but we were left with the impression that numbers on the blood gas were abstract contructs, that they did not mean that much and that a really abnormal blood gas did not elicit the sort of visceral response that takes place when a junior shows me an acidotic gas in the resus room.
We talked about this as a group and decided to experiment on ourselves in order to ‘feel’ and ‘experience’ physiological abnormality. The premise is that if we are able to personally experience something we might better understand it ourselves and perhaps be able to translate that experience into patient care.
The first thing we discovered is that it both unethical and very unwise to induce severe sepsis in our colleagues. There are many rules against this and the bottom line is just that you can’t do it (we checked). However there are physiologically abnormal states that we can access with less risk of referral to the GMC (the UK regulator). Whilst it is in no way a perfect analogy with sepsis, exercise can provide a state where physiological demand exceeds physiological reserve. It can be an unpleasant and challenging state, and we had a really excited bunch of docs who thought that we should give it a whirl and see what happens…..
The basic set up is to use different modes of exercise as models for physiological challenge. We placed IV cannulae and used these to draw venous blood gases analysed using a standard blood gas analyser in the ED.
Experiment 1. Steady state.
We took some of our fitter docs for a run round the hospital site. This was a group who regularly ran long distance (10km-marathon) and the intention was for the to run for 10 mins at a pace that they would be able to sustain for 10km or more. The purpose of this test was to demonstrate what a blood gas on the edge of aerobic threshold looks like.
As expected this showed us that the lactate at aerobic threshold amongst athletically capable young people was roughly 4. This fits with what we know about exercise physiology, and in all honesty we could have just pointed them to a website on this issue. However, the experience of doing this allows us to discuss lactate levels in the following way.
‘OK, so if Dave is running a 10km run and has a lactate of 4, how does that make you feel when you see a patient with a lactate of 5 or more? Are they going to be able to sustain it? Think of them as running beyond their physiological lactate threshold and consider how long they might be able to do that for’
Experiment 2. Max effort
In this test we take someone who is used to high intensity effort and ask them to sprint at maximal effort for 400m. This reproduces one of the toughest events in the olympics and is designed to create maximal (but short burst) aerobic AND anaerobic effort. This test is designed to reproduce the physiological effort that patients may experience during a grand mal seizure. What was interesting here was the degree of physiological abnormality achievable in such a short space of time AND how long it took to clear. Sadly I’ve lost the gas results for this one, but it was pretty dramatic with a peak lactate of over 28!
Experiment 3. Ramp testing
In many cases critical illness is neither sudden nor immediate. Patients becoming increasingly unwell over a period of time, reach their limits of physiological homostasis, then going beyond to this to critical illness and unless intervention takes place to death.
Our model for this was to steadily increase a physiological load up to and beyond physiological reserve. There are many ways (some rather complex) to do this but we like to keep things simple. We use a set up with a road bike, rolling road and a speedo/power meter to steadily increase load. Following a 5minute warm up period at very low load we steadily increase load at minute intervals until the candidate can no longer continue. They exercise to maximal output defined by their inability to maintain a set speed for the increased load.
For this test we were careful in selecting individuals who were habituated to high intensity physiological exercise.
So how low can you go?
Well this was interesting. Our top two subjects were myself and the marvellous Rick Body……and Rick was more extreme than myself. Let’s have a look at his blood gases at the end of the ramp test. He managed 10 minutes and was pretty much exhausted at the end of the session.
From the learners perspective there were several interesting findings.
Download a full set of graphs hereDownload a full set of graphs here
At peak abnormality Rick’s gas looked like this. If you saw this in a patient then you would consider them to be on the point of no return.
Of note are the lactate curves for the 6 people we tested. Lactate levels rose significantly and persisted for some time postanalysis. In clinical practice you will see similar things post grand mal convulsions – so take this into account when analysing blood gases post tonic clonic seizures.
The legal stuff.
We did get explain and get consent from colleagues before doing this. All levels from clinical director to foundation took part and due to limitation of facilities we had to draw lots from amongst the team as there were far more volunteers than we had time to put through the testing. No-one was coerced into doing it and we have had several requests to re-run the experiment.
We talked to our laboratory services before we did this to ensure that they were happy with the additional analyses placed through the analyser and we ensured that there were no critically ill patients in the ED at the time of the activity. Blood gas machines are like cats as they always appear to be cleaning themselves (saw that quote on twitter can’t remember who said it though).
Times change and I think that if we are going to repeat this again then there may be a little more red tape knocking about so if you’re thinking of doing it make sure that you get the right approvals from your organisation and health care system.
Is this a great model for understanding lactate?
Yes, no, maybe and overall then we think yes if the emphasis is on the ‘understanding’ bit.
- Yes – it gives a series of benchmarks and analogies that clinicians can use to ‘feel’ physiological abnormality.
- No – lactate in patients is far more complex and measured levels are dependent on a range of factors such as clearance, ischaemia and other physiological mechanisms. Exercise is too simplistic a model.
- You could do this better – sure we could. Many hospitals have a physiology lab for the pre-op assessment of patients undergoing high risk surgery. We’ve done these experiments in the past as well and it’s great fun, but difficult to set up, requires a lot of time and technicians to operate the equipment. If you can do that then great, but such detail may not be essential to get the core learning outcomes across.
- These individuals are not like your patients – yes true and that’s a great springboard for discussion when we do this.
So take your pick. We think it helps clinicians understand physiological challenge in health and disease using a model that is easily accessible and memorable. We have no data on whether it makes a difference to patient outcome, but I will leave you with my memory of a conversation I overheard in the ED a few months later.
‘Mr X in resus is big sick. These gases look like he’s running a 400m race and he’s not
going to keep that up for much longer….’
Job done 🙂
- The master at work http://emcrit.org/podcasts/lactate/ listen to Scott on all you need to know about sepsis and lactate.
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- Isaac Pascal Carbon with Campagnolo Chorus groupset, Zonda Wheels, ITM bars and stem, Fizik saddle.