If a person is experiencing difficulty breathing comfortably, the chances are that the difficulty stays with them no matter what they’re doing, be it sitting, standing, or walking. So it’s not surprising that conventional scans or breathing tests, carried out with the patient lying on a couch or sitting in a chair, don’t always tell us what the problem is. With an exercise test (cardiopulmonary exercise test to give it its full title, often abbreviated to CPEX or CPET) measurements are made as the patient gradually gets up to full tilt on an exercise bike or treadmill. Questions are being asked of the body. How much can it do? If there’s anything wrong, it should be easier to see when the patient is being physically pushed.
‘How much can the body do?’ could also be re-phrased as ‘how much oxygen is the body getting?’ Your body needs to get oxygen into the lungs, across into the blood, then around the body to the muscles that require it. If you can’t get any more oxygen in, then you have to switch from aerobic to anaerobic metabolism, meaning your muscles are operating without oxygen. Anaerobic exercise is maintainable for a short sprint, but you may know from experience that you won’t be able to sustain this pace for very long.
Oxygen uptake is usually called VO2, with ‘V’ being the volume of oxygen. In the early days of exercise physiology, you had to collect all the air you breathed out in one minute into a large bag and then look at the oxygen content. Air contains 21% oxygen, so if you know the volume of air breathed in a minute and how much oxygen there is left in the expired air, you can work out how much has been taken up by the body. As technology has advanced and with the development of faster analysers, we can work out VO2 on a breath-by-breath basis and display the data in real time.
VO2 at the point you can’t exercise any longer is called VO2max. It is the single best indicator of your fitness, or indeed your health in general. The better your VO2max, the longer you’re likely to live and the more likely you are to survive an operation or general anaesthetic. During such tests heart rate and CO2 output are also recorded during exercise. Manipulating the data also allows us to look at how good your lungs are, how much blood your heart can pump with each beat, when you pass your anaerobic threshold, and much more. Such tests result in acres of numbers and graphs, and hopefully a diagnosis.
Featured image credit: Running Man by Kyle Cassidy. CC BY-SA 3.0 via Wikimedia Commons.