Cardiopulmonary Exercise Testing

Take home messages

• CPET is a reliable, repeatable, objective measure of cardiopulmonary function
• An anaerobic threshold below 11ml O2/kg/min is associated with poor postoperative outcomes

Podcast episode

How does this make you feel?

Read the following bullet points and then consider how you'd feel about anaesthetising that person.

• "I swim three times a week and walk my dog twice a day."
• "I'm not as fit as I used to be, but I walk up the hill to the shops every morning."
• "I can manage a flight of stairs but need to pause to catch my breath at the top."
• "I live in a bungalow and feel short of breath walking from the sofa to the fridge."

Hopefully the first one made you feel positively about your job, and as you progressed down the list you starting wondering who you should call before going anywhere near a syringe.

If so, then congratulations, you're already doing cardiopulmonary exercise testing in your head.

Give that hunch a number

We know that patients with heart or lung disease carry a higher risk profile going into major surgery, but it would be nice to know just how much higher it really is.

This is the point of cardiopulmonary exercise testing.

The idea is that CPET is a safe and repeatable, non-invasive way of objectively assessing a whole bunch of useful information about a patient's cardiovascular and respiratory systems.

We love echocardiograms, and lung function tests, however these are invariably performed at rest, and give little-to-no idea of how well the patient's body can cope with any sort of exertion.

They don't necessarily tell us how the patient's body is going to cope when thrown into the arena of surgical stress, cardiodepressant anaesthetic agents and all the other homeostatic insults that occur during an operation.

CPET, on the other hand, assesses the patient's physiological ability to respond to the stress and metabolic demand of major surgery.

Oxygen extraction refers to how efficiently the tissues can pluck oxygen from the blood, and usually you sit around 25% at rest. The more oxygen you can extract from the blood, the less hard the cardiovascular system has to work to deliver more blood to the tissues.

• During exercise, this extraction can reach the giddy heights of 75%
• But after surgery, it's only 30%

This means the remaining demand has to be met by increasing cardiac output - meaning even more of a cardiovascular strain after an operation.

Different types of testing

You can objectively assess cardiopulmonary function in a number of ways, but they're essentially all doing the same thing - make a person exercise, and then see how well they fare.

Six minute walk test

Does what it says on the tin.

If your aortic stenosis means you can't walk more than 300 metres during a six minute walk test, then you have a worse prognosis after aortic valve replacement.

Incremental shuttle walk test

The subject walks back and forth between two cones that are nine metres apart. They have to gradually increase their speed to reach the next cone on time, and the test ends when they fail to reach the cone in the allocated time frame.

This relies a little more on motivation than CPET or the six minute walk.

How CPET is done

You have a stationary bicycle ergometer, and a trolley with all sorts of measury stuff on it.

What the numbers mean

In short - the more exercise you're able to do without dying, having an MI or passing out, the better your chances of surviving major surgery.

VO2 max

This is a measure of oxidative phosphorylation, and therefore aerobic respiration. The more oxygen you're able to consume per kilo of body weight per minute, the fitter you are.

Anaerobic Threshold

The anaerobic threshold is the VO2 at which any further increase in work will result in anaerobic respiration occuring.

The higher the rate of oxygen consumption before you start producing lactate through anaerobic respiration, the fitter you are.

Oxygen Debt

Much as one may need to borrow money at an extortionate interest rate in order to buy a house, a car, or enter a post-graduate medical examination, so does the body's own lactate loan shark offer borrowed energy to the muscle cells of the body in the form of oxygen debt.

• The body can't meet the demands put upon it through oxidative respiration
• It uses anaerobic respiration to foot the immediate bill, and deals with the consequences later
• This ends up consuming more oxygen overall than if the body were able to complete the initial task using aerobic respiration

The is because the cells have to regenerate ATP and phosphocreatine, replace the lost glycogen stores from the muscles and liver, and put all those pesky electrolytes back on the correct side of their concentration gradients.

And that's all before we start thinking about managing the repair of damaged muscle tissue, fixing the myoglobin oxygen stores and getting rid of all the excess heat that was generated during the fight or flight scenario that you've hopefully just survived.

The 9 panel plot

Once we've finished torturing our patient with a stationary bicycle and harvested all sorts of lovely data, we can plot it on a variety of graphs, and the 9 panel plot is the generally accepted method of doing so.

You do not need to be able to comprehensively examine and interpret a 9 panel CPET plot for the anaesthetic exams.

It is helpful, however, to have a vague idea about what you're looking for, and what it might look like.

A good approach is to ask the following questions:

For example:

• An elderly patient with ischaemic heart disease and an aortic aneurysm may finish the exercise early due to fatigue and chest pain, with ST changes demonstrated on the ECG
• His maximum VO2 sat at around 15 ml/kg/min
• His anaerobic threshold was 12 ml/kg/min
• He had no ventilatory limitation

This patient is clearly not the fittest chap, and hence may be a better candidate for endovascular repair rather than major open surgery.

The best video on CPET testing

It's enormous but exceptionally good.

Free CRQ from FRCA revision

Useful Tweets and Resources

References and Further Reading

Preoperative cardiopulmonary exercise testing

Patients with cardiorespiratory disease have an increased risk of perioperative complications. Several prediction tools have been developed in an attempt t

OUP Academicneil.agnewwales.nhs.uk

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