Cardiopulmonary exercise testing

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Most lung function tests assess lung function using maximum expiratory/inspiratory manoeuvres at rest, but, although providing very useful information, the procedures are unnatural. More often than not, patients complain of dyspnoea during exercise. A number of tests to assess the patient’s performance while exercising have been developed. There are three main reasons for performing an exercise test:
• to identify a cardiac or respiratory cause for exercise limitation
• to quantify functional disability
• to assess the response to treatment
Exercise tests can fall into two categories:
• Simple tests of exercise capacity such as the six-minute walk test or shuttle test
• Sophisticated, progressive multistage exercise tests, usually to maximal effort, during which a wide variety of cardiac and pulmonary variables are measured continuously to assess the pattern of cardio respiratory limitation on exercise
When ordering an exercise test, it is essential to have a clearly formulated clinical question you wish the test to answer, and order the correct test for that clinical indication or question. The results of progressive exercise tests are often not clear-cut and so must always be interpreted within the patient’s clinical situation.
Two methods of varying exercise workload are commonly used; the treadmill and the cycle ergometer. Each device has advantages and disadvantages. In our lab we have chosen to use the cycle ergometer. One of the chief advantages of the cycle ergometer is that the workload is independent of the weight of the patient and VO2 (target oxygen uptake) can be reasonably estimated if the pedaling speed and resistance are carefully measured and better stability of the patient for blood pressure and oxygen saturation monitoring. Patients of different weight, walking on a treadmill at the same speed and slope perform different work and different walking patterns, or stride length, may also affect the actual amount of work being done. Patients who grip the handrails of the treadmill may use their arms to reduce the amount of work being performed. For these reasons estimating VO2 from a patient’s weight and the speed and slope of the treadmill may produce erroneous results. VO2max (maximal oxygen uptake) has been shown to be measured slightly higher (approximately 7% - 10%) on a treadmill compared to a cycle ergometer.
In the exercise test we perform the patient is asked to carry out progressively increasing workloads of exercise while their respiration is monitored breath by breath, their cardiovascular responses are also observed by recording their heart rate and ECG continuously during the exercise period and into recovery.
The exercise is a test of the three components involved in heavy physical exercise:
1. respiration
2. the cardiovascular system (both the heart and peripheral circulation)
3. the condition of the peripheral musculature.
4. It is important to note that the results of exercise testing may indicate the organ system limiting exercise capacity, but will not provide a diagnosis. In many patients there may be a combination of factors (such as skeletal muscle deconditioning in patients with advanced cardiac disease).
We calculate the following variables for each breath and provide a running average over a set number of breaths for tidal volume, oxygen consumption and carbon dioxide production as well as continuously monitoring heart rate and oxygen saturation. The responses of these variables are usually compared to workload and compared with a predicted range of normal values.
In health, exercise tolerance is limited by the ability of the cardiovascular system to supply oxygen to the musculature. When the ability to supply oxygen to the muscles is exceeded, any additional work is generated by anaerobic metabolism, producing lactic acid. In order to compensate for this metabolic acidosis, ventilation increases and carbon dioxide is eliminated at a greater rate than would be expected with respect to oxygen uptake referred to as anaerobic threshold. Anaerobic exercise is difficult to maintain causing the person to stop exercising due to fatigue. In health this usually occurs at 60-70% of the VO2max.
In order to interpret a study appropriately it is important to look at the responses throughout exercise as well as the maximal values achieved. They should be interpreted in the context of the clinical presentation. It is also important to bear in mind the reason for test cessation – was this really a maximum exercise test?

Vanessa