REVIEWUnderstanding the Basics of Cardiopulmonary Exercise Testing
Section snippets
WHY MEASURE GAS EXCHANGE?
An important prognostic component of exercise stress testing is the evaluation and quantification of work performed, also known as functional capacity.4 Because energy consumption, generally quantified as caloric expenditure, is difficult to measure during exercise, a more readily measurable metabolic equivalent (MET) was sought to quantify the work performed at various stages of exercise. In humans, total oxygen consumption approximates caloric expenditure, with both parameters increasing
HOW IS GAS EXCHANGE MEASURED?
Like the lungs, the primary function of the cardiovascular system is gas exchange, supplying oxygen and other fuels to working muscles and removing carbon dioxide and other metabolites. The heart, lungs, and pulmonary and systemic circulations form a single circuit for exchange of respiratory gases between the environment and the cells of the body.6, 7 Under steady-state conditions, oxygen consumption per unit time (o2) and carbon dioxide output (co2) measured at the mouth are equivalent to
CARDIOPULMONARY RESPONSES TO INCREMENTAL EXERCISE
In healthy people, predictable physiologic changes occur during exercise. These changes include a reduction in systemic vascular resistance, increases in oxygen extraction, and augmentation of stroke volume (SV) and HR, resulting in an increase in cardiac output (CO). The coupling between oxygen transport and circulatory function is revealed in the Fick equation, which states o2 = CO × (CaO2 - CvO2), in which CaO2 is the arterial oxygen concentration and CvO2 is the mixed venous oxygen content.
WHAT WILL THE TEST RESULTS TELL ME?
Besides precise assessment of peak aerobic capacity (ie, measured METs as opposed to estimating METs), cardiopulmonary exercise testing is invaluable in determining the cause of dyspnea on exertion and determining normality of cardiac and pulmonary responses to exercise. The most common indications for cardiopulmonary exercise testing are outlined in Table 2.
Metabolic derangements can occur at multiple sites within the circuitry of gas exchange, including the consumers at the muscle
CARDIOPULMONARY EXERCISE TESTING IN HF
From a clinical standpoint, cardiopulmonary exercise testing has gained widespread use in the evaluation of patients with advanced systolic HF.27 Although left ventricular ejection fraction does not predict Pko2, numerous studies have confirmed the prognostic utility of measuring gas exchange in patients with systolic HF. In the Veterans Administration Heart Failure Trial, mortality of patients with a o2max of 14.5 mL/kg per minute or less was double that of patients whose Vo max exceeded this
OTHER USES OF CARDIOPULMONARY EXERCISE TESTING
Cardiopulmonary exercise testing is frequently an integral component in the work-up of patients being evaluated for employment disability, in which direct measurement of RER and Pko2 can often discriminate between true disability and malingering. Assessment of Pko2 can be important for patients who wish to return to physically vigorous occupations. This technology is also useful in determining precise functional capacity in young and middle-aged, presumably asymptomatic, patients with moderate
TECHNICAL PERFORMANCE ISSUES
Although most data obtained from cardiopulmonary exercise testing are reliable and reproducible, as with most clinical modalities, there may be pitfalls in collecting and interpreting these metabolic data during exercise. The technician who performs the cardiopulmonary exercise test must be well trained in the calibration and maintenance of the metabolic cart and provide thorough instruction to patients before testing, including the use of preassigned hand signals to relay patient symptoms, as
CONCLUSION
Cardiopulmonary exercise testing remains a relevant modality in the clinician's diagnostic armamentarium for evaluation and treatment of many commonly encountered clinical problems. Measurement of exercise gas exchange provides objective and reproducible indices of functional capacity, generates invaluable information in determining the origin of dyspnea on exertion, and provides unique prognostic capabilities in the assessment of patients with systolic HF. Physicians should be comfortable with
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