What is CPET?
Cardiopulmonary exercise testing (CPET) is, as it sounds, the analysis of both cardiac and pulmonary data during exercise. The combination of this data with respiratory gas analysis makes CPET useful in a wide array of clinical applications. The purpose of this article is to review briefly the physiology behind CPET, and to summarize some of the applications toward which CPET can be applied, with special attention to endurance athletes.
CPET as a clinical technology has been available for a long time, but its widespread use has been limited in the past because of the complexity of test interpretation and the generally poor reimbursement for the testing itself. Despite these barriers, an immense amount of useful clinical information can be obtained with CPET.
Exercise requires the precise coordination of multiple body systems. The initial seconds of exercise are powered by adenosine triphosphate (ATP) obtained from the creatine phosphokinase reserves in skeletal muscle. For exercise to continue, however, oxygen delivery to working muscle must occur. Initially, ventilation (breathing) remains constant while cardiac stroke volume (blood per beat) increases, followed by a progressive increase in heart rate as the demand for increased cardiac output continues. As long as the cardiovascular and pulmonary systems can respond to increasing levels of exercise by corresponding increases in oxygen delivery, exercise can continue at a relatively comfortable level (with a brief recovery period afterward), limited only by the amount of fuel (glycogen) in the muscle.
Most people cannot exercise to the upper limits of the capability of their lungs, and are ultimately limited by the capacity of their cardiovascular system. At the point that exercise levels increase beyond the capability of the body to deliver oxygen to drive this exercise, either exercise must cease, or the body must begin utilizing forms of energy that don’t use oxygen. This “anaerobic” use of energy reserves is distinguished by a more rapid consumption of glycogen stores (600% faster) and the accumulation of muscle lactate, which is then transferred into the circulation, where it is converted into CO2 and exhaled into the air.
CPET and anaerobic threshold
CPET, which measures both O2 consumption and CO2 production, can identify the point at which CO2 levels start to increase at the airway. This correlates to the lactate, or anaerobic threshold (AT) where energy systems begin to change over. Because this threshold demarcates the point at which glycogen utilization is either efficient or inefficient, advice for athletes in training is usually to compete just below, and train just above this point. This is because the anaerobic threshold is an intensity threshold that is particularly sensitive to improvements as aerobic conditioning increases. As the anaerobic threshold improves, so to does the ability of a endurance athlete to compete at a higher pace before non-oxidative fuel consumption sets in (which burns through fuel faster and prolongs recovery times). Thus, the anaerobic threshold can be used in cyclists, distance runners, or other elite endurance athletes as a marker for the establishment of optimized training zones.
During testing, capillary blood lactate levels can be drawn as well in order to confirm findings on gas analysis. This is sometimes useful in a well-conditioned endurance athlete, because as fitness level increases, the anaerobic threshold moves toward VO2 MAX and can sometimes be difficult to distinguish with gas analysis alone. Since heart rate closely tracks oxygen consumption in healthy people, once an athlete knows their proper training zone (around the AT), they can center their workouts around a heart rate zone that is specific to them. Use of newer heart rate and pace monitors increases the consistency of each training run, and makes consistent gains more likely.
In marathon runners, the best zone for training endurance is at blood lactate levels between 2.5 and 3.5 mmol/liter. Training at higher lactate levels is often counterproductive. The pace at 2.5 mmol/liter is usually predictive of an athlete’s marathon performance time. Race pace should be no faster than this, and is usually just under it. As training progresses and cardiovascular endurance improves, the AT will shift, and the athlete’s heart rate training zones (and peak expectations for marathon performance) will shift as well.
VO2 MAX as a metric
VO2 MAX, which is the maximal rate of O2 utilization the body can achieve, is what is commonly referred to as “fitness” level. VO2 MAX has been shown to be an independent predictor of cardiovascular mortality, and thus is a great supplement to traditional 12 lead exercise stress testing. VO2 MAX can also be used to help establish intensity thresholds for exercise prescription, but is less useful as a training marker than the AT.
Using CPET to diagnose asthma-limited and high-fitness athletes
CPET also incorporates lung function data during exercise, which allows the examiner to determine whether a patient has exercise limitations of pulmonary origin (such as asthmatic patients with poor exercise capacity, or high level elite athletes whose cardiovascular fitness is so high that their limiting factor is their lungs rather than their heart). Whether full lung data is obtained depends on the history of the individual being tested.
To review, some of the uses of CPET in athletes are:
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