Why do a cardiopulmonary exercise test?

This chapter describes how a maximal cardiopulmonary exercise test (CPET) assesses the exercise capacity of an individual. It identifies whether exercise is limited by the heart or lungs, or by another factor. The test can be used to elucidate the cause of symptoms such as breathlessness. By quantifying exercise capacity, a CPET can be used as a tool to plan physical training. Information from a CPET helps quantify the risk of a surgical intervention and is used in planning post-operative care. Performance in a CPET is a predictor of subsequent mortality. A CPET should be preceded by a full clinical history and a thorough clinical examination. The test is best interpreted alongside the results of simple preliminary investigations.

Respiratory compensation point

This chapter describes how acidaemia stimulates ventilation in the later stages of a cardiopulmonary exercise test (CPET). This happens after the anaerobic threshold, once the capacity of the blood to buffer lactic acid has been used up. The respiratory compensation point (RCP) can be identified from an increase in the slope when minute ventilation (VE) is plotted against carbon dioxide output (VCO2), or from a rise in the ventilatory equivalents for carbon dioxide (VeqCO2). The presence of a clear RCP indicates that the subject has made a fairly maximal effort during the CPET. An RCP also argues against significant lung disease, since it implies the ability to increase ventilation in response to acidaemia.

Oxygen pulse

This chapter outlines how dividing the volume of oxygen uptake (VO2) by the pulse rate gives an estimate of the stroke volume of the heart. The amount of oxygen taken up with each heartbeat is called the oxygen pulse (O2 pulse). It should increase steadily on exercise to a value above 10 ml/beat and may continue to rise during the recovery phase. A low O2 pulse can be an indicator of low cardiac output. If the maximum VO2 (VO2max) is normal, caution should be used in the interpretation of a low O2 pulse. Sometimes the O2 pulse is abnormal because of a fall in peripheral arterial oxygen saturation (SpO2) or mixed venous oxygen levels.

Oxygen saturation

This chapter describes how desaturation during a cardiopulmonary exercise test (CPET) is uncommon. A fall of more than 4% from resting values is considered abnormal. Oxygen desaturation is usually caused by lung or pulmonary vascular disease, reflecting ventilation–perfusion inequality or impaired diffusion. Occasionally, a right-to-left shunt will open up in the heart during exercise. Some elite athletes show exercise-induced arterial hypoxaemia, when the ability of their muscles to utilize oxygen cannot be met by the subject’s ventilatory capacity.

Ventilation

This chapter describes how the amount of air going in and out of the lungs increases on exercise. The predicted value for minute ventilation (VE) is calculated for each subject from their own forced expiratory volume in one second (FEV1), rather than taken from published tables. Normally, ventilation does not limit exercise and VE does not reach 80% of the predicted value. If VE at peak exercise is more than 80% of predicted, it is likely there is something wrong with the lungs. VE increases by a combination of a larger tidal volume and an increase in breathing frequency. The pattern of increase is normally gradual. An erratic pattern suggests dysfunctional breathing.

Ventilatory equivalents

This chapter shows how dividing the minute ventilation (VE) by the volume of carbon dioxide exhaled, or the volume of oxygen taken up, gives the ventilatory equivalents (VeqCO2 or VeqO2, respectively). VeqCO2 show how much ventilation is needed to get a given volume of carbon dioxide out of the body. In a normal subject, the VeqCO2 fall gradually during exercise, as ventilation–perfusion matching improves, to a value of <30. In lung disease, the lowest value remains >30. Beyond the anaerobic threshold (AT), VE increases in order to get rid of CO2 produced from buffering of lactic acid. Since there is no corresponding increase in VO2, the VeqO2 start to rise, giving one of several ways of looking at the AT.

Preoperative cardiopulmonary exercise testing

This chapter discusses how the results of a cardiopulmonary exercise test (CPET) can be used for preoperative surgical planning. A low preoperative maximum oxygen uptake (VO2max) is associated with a poor outcome. The lower the VO2max, the worse the prognosis. Use of the anaerobic threshold is less reliable. The CPET may identify clinical problems which can be optimized prior to surgery. Pre-habilitation can improve the chances of a good outcome from surgery.

Interpretative strategy

This chapter outlines the approach to producing a cardiopulmonary exercise test (CPET) report. A CPET is rarely diagnostic and should be looked at in the context of the clinical background and what key question is being asked. Cardiovascular, ventilatory, and gas exchange responses are looked at in turn, then reviewed in a systematic and iterative way. If the maximum oxygen uptake (VO2max) is within the normal range, abnormalities seen in other parameters should be interpreted cautiously.

Pre-test assessment

Pre-test assessment describes the next step after deciding to do a cardiopulmonary exercise test (CPET). The indication for the test must be defined, with clear end points. A CPET is a very safe test. There are well-defined conditions which increase the risk. The most serious side effects are related to heart problems, most commonly seen in subjects with unstable heart disease. Careful scrutiny of the resting electrocardiogram (ECG) is imperative prior to the test. If the subject is unwell, e.g. with a viral illness or an exacerbation of asthma, the test should be postponed. On the day of the test, the subject should take their usual medication. Caffeine and alcohol should be avoided on the day of a CPET. A light meal should be taken at least 30 minutes beforehand.

Exercise prescription

This chapter outlines the health benefits of exercise. A cardiopulmonary exercise test (CPET) can reassure the subject that exercise is safe for them. Exercise can be prescribed using heart rate zones derived from the CPET. Exercise should be taken for at least 30 minutes each day, or more intense exercise for 20 minutes three times per week. A common daily target for steps is 10,000.