CPET Explained...for Regular People - by Todd Davenport
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Cardiopulmonary Exercise Testing Explained...for Regular People<br>Part 1: CPET Procedure
Todd Davenport<br>May 22, 2026
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In a previous post, I described how cardiopulmonary exercise testing (CPET) has changed the game in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) research and clinical practice. CPET has provided a wealth of objective physiological evidence supporting what patients have always told us. When done two days in a row, CPET can be used to assess the failure to recover after exertion, which appears central to ME/CFS pathophysiology.1<br>CPET is often discussed in the patient, scientific, and clinical communities. However, I find it that even very informed people may not know very much about the test itself, how the data is collected, and what the data means. Even my own phone autocorrects my speech-to-text dictations of “CPET” to “Sea Pet” and “CPAP.” It must think I own a massive marine aquarium where…I do sleep studies?<br>The next couple of posts will describe CPET. This first one will discuss the procedure. The next one will describe the data, how we analyze it, and how we interpret it.<br>Hopefully you will find both posts to be understandable even if you are not involved in research and clinical practice. And if you are, I hope these posts are an accessible reminder and primer to share with your patients.<br>What’s a “CPET?”
CPET test is a supervised physical stress test.2 We use it to evaluate how the body responds to physical exertion as demands increase.2 In healthy people and most clinical populations (even very ill ones), CPET results are highly reproducible from one day to the next. ME/CFS stands apart because reproducibility often breaks down after exertion.3-9 This is finding is often held up as an objective marker of PEM.5 More on that in a future post.<br>How a CPET Looks
The type of CPET set-up we use involves pedaling a stationary bicycle.1 We use a stationary bike rather than a treadmill to improve safety, and to precisely control and measure workload. This allows us to avoid the risk for falling, stop the test quickly if needed, and to pick important timepoints during the test during later analysis. The stationary bicycle has an automatic brake. This allows us to apply a known amount of resistance to the pedals throughout the test, making it progressively harder. The person just needs to maintain a constant rate of pedaling throughout the test.<br>We start by placing a lightweight mask over the person’s nose and mouth. The mask is connected to a plastic tube going into a machine that analyzes the gas composition of each breath. The computer uses a sensor to measure how much oxygen is consumed, breath by breath, from exhaled air. We know the content of oxygen in room air and we measure the gases expired each breath for oxygen and carbon dioxide. The balance of oxygen and carbon dioxide that we measure during CPET reflects what we call “gas exchange.”<br>We also keep track of heart rate, blood pressure, and how hard the person is pushing against the pedals continuously throughout the test. This setup allows us to see how efficiently the body is producing energy, how those systems change as the test becomes more challenging, and how much work all these changes are resulting in.<br>Typically, each CPET session would begin with unloaded cycling.2 This means the person pedals at zero resistance for three minutes while we gather some baseline data. Resistance then increases gradually in a continuous fashion until the individual reaches their limit, typically within eight to twelve minutes. Continuous application of resistance is called a ramping protocol.<br>However, we do not start our CPETs with three minutes of unloaded cycling.1 We found even unloaded cycling was too taxing on the patient. People were going through the test and hitting their maximal exertion too quickly to gather enough data to be able to comment on the physiology of what happened. Instead, we have modified the protocol to have a person do a 3-minute seated rest period on the bike before the ramping protocol starts.<br>We call the test a maximal CPET because, if the person is able, we are trying to achieve a certain set of accepted objective criteria that ensure accurate interpretation.1 Unlike a lot of so-called submaximal tests that involve a lower amount of physical exertion for a longer period of time, people work only briefly at a maximal level.<br>If the person needs to stop the CPET at any time and for any reason, we stop immediately. The patient remains in control at all times.
Image from: From Function to Physiology and Back in Adults with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Davenport, T. E. 19 Jan 2024. University of Portsmouth.<br>Recovering After CPET
For many patients, the most difficult part of the two-day CPET begins after leaving the laboratory. Symptoms often worsen within hours of the first test...