An often overlooked and underutilized modality, cardiopulmonary exercise testing (CPET), offers a wealth of information about a patient’s functional status. As a dynamic test, CPET can identify cardiac or pulmonary disease in patients with marginally abnormal diagnostics (electrocardiograms (EKGs), echocardiography, spirometer, etc.), or in those who appear more clinically disabled than their diagnostic tests indicate. Collected data during the procedure include EKG, heart rate, oxygen uptake, and carbon dioxide output. From collected data, minute ventilation, and maximal oxygen consumption (VO2 max) can be calculated, allowing the clinician to assess a patient’s overall cardiopulmonary function. This can be useful in pre-operative risk assessment, the diagnosis of various cardiopulmonary diseases, the evaluation of post-operative recovery, or more general assessment of an individual’s exercise capacity.
Many fundamental anatomical concepts are relevant to CPET. Grossly, the cardiopulmonary axis is the organ system of most significant concern. While the heart and lungs can be subdivided into their component organizations (broncho-pulmonary segments and chambers, respectively), it is crucial to appreciate these organs as a closed unit, as each seamlessly regulates the other. One can follow the path of deoxygenated blood returning from the systemic circulation to appreciate the loop. Via the inferior vena cava, this blood is collected in the right atrium and then pumped through the right ventricle into the pulmonary arteries of each lung. These arteries lead the way to smaller capillaries, just as main bronchi lead air to increasingly smaller bronchioles. Gas exchange occurs at the level of the alveoli, during which waste carbon dioxide is exchanged for freshly inspired oxygen. The re-oxygenated blood is then returned to the heart by way of the pulmonary veins. The left atrium and ventricle are then tasked with the collecting and pumping of this blood back through the systemic circulation. The organs themselves receive their blood supplies through the bronchial and coronary vessels, early bifurcations within the systemic circulatory system. By nature of the test, CPET allows for evaluation of the axis’s baseline function as well as compensatory mechanisms during times of physiological stress. The system is a case study on how structure influences function.
Given the amount of information regarding multiple physiological systems, CPET has relevance as a diagnostic tool for several conditions. Generally, the indications for use are classified into five areas:
In patients with heart failure, both with and without preserved ejection fractions, the test can identify those with exercise intolerance, a common problem in this disease process. Further, a landmark application of CPET to cardiology was the classification of patients with heart failure with reduced ejection fraction based on peak oxygen consumption. Peak oxygen consumption during exercise can also be used to stratify one-year cardiovascular risk in patients with more advanced heart failure. Multiple cardiovascular laboratories have reproduced and validated these findings, such that algorithms have now been in clinical use for years to aid in the management of patients with all stages of heart failure. Cardiologists now use the test in the evaluation of patients with suspected ischemic heart disease, congenital heart defects, valvular diseases, hypertrophic cardiomyopathy, or pulmonary hypertension.
As a tool for risk stratification prior to surgery, CPET has been used for both thoracic and extra-thoracic pathologies. For example, CPET analysis is used to score patients with lung cancer in the Thoracic Revised Cardiac Risk Index, an algorithm set forth by the American College of Chest Physicians.
Though CPET is generally tolerated well, it is not without risk. There are several absolute and relative contraindications for testing, which are compiled from expert opinion and the currently accepted standard of care. The absolute contraindications include:
The relative contraindications that may preclude the use of CPET until the patient is more stable to complete the test include:
Numerous exercise monitoring systems are available for commercial purchase. These systems allow for the measurement of exhaled gas with each breath, as well as the real-time analysis of inhaled and exhaled oxygen and carbon dioxide levels. Volume measurement of inspired and expired air is possible through the use of an airflow transducer. Typically the exercise is conducted on either a stationary cycle ergometer or a motorized treadmill, with reproducibility and patient safety of particular concern. Both modalities can provide increased resistance to elucidate a patient’s work over time, quantified in watts. However, maximal oxygen uptake has been reported to be 5% to 20% greater for individuals on a treadmill than equivalent cycle ergometer work due to the involvement of more muscle groups. Other equipment includes a metabolic cart, which contains a gas analyzer, a computer, and a screen with displays of both 12-lead EKG analysis as well as physiological changes as they occur during exercise.
Given the amount of data collected during the test, CPET is considered a specialized test and, as such, is often performed in cardiac or pulmonary testing centers. Technicians who perform cardiac stress tests and pulmonary function tests possess the necessary skills to perform CPET. Centers located in resource-rich communities may offer a package, which includes a history and physical examination with a specialist, a baseline EKG, an echocardiogram, CPET, and consultations focusing on fitness, healthy eating habits, and optimized disease management.
As stated previously, CPET is performed at specialized centers that often conduct other cardiopulmonary diagnostic tests. Patients should be counseled on the equipment used and data that is collected. If a patient is at significant risk for falls, the test should be performed with equipment to minimize this risk, such as a cycle ergometer.
In non-athletic individuals, maximal oxygen uptake can be achieved with the use of a cycle ergometer. The test is performed in a well-ventilated room equipped with full resuscitation facilities. The computer attached to the metabolic cart displays EKG data as well as physiological data as exercise intensity changes. These data include breath by breath oxygen consumption and carbon dioxide production, with flow calibration performed before each breath. The patient begins at rest to become familiar with the equipment, while baseline heart rate, blood pressure, oxygen saturation, EKG, and gas exchange values are recorded. During a test with a cycle ergometer, saddle height is adjusted, and the patient is asked to pedal at a constant speed prior to a steady increase in work and oxygen consumption. There are multiple protocols available to guide the performer as the activity is increased, including those set forth by Balke and Naughton.
Potential complications of CPET include fatigue and shortness of breath and are often related to the patient's exercise tolerance. Less common complications include cardiac arrhythmias, anginal chest pain, and bronchospasm. Given the potential for more side effects in certain patient populations, such as those with heart failure, it is often suggested to perform the test in the hospital setting under the supervision of a physician.
From pre-operative risk stratification to determine a specific disease process, the use of information garnered from CPET is abundant. While symptoms that may prompt the ordering of the exam are subjective (dyspnea, for example), the data collected are objective and can be used to determine the presence of disease. One crucial aspect of conducting and interpreting CPET is determining if a patient reaches his or her ventilatory threshold. This ventilatory threshold (VT) is defined as the point during exercise at which an individual’s perfusion to active musculature no longer meets metabolic demand. Alternatively, this point is known as the lactate threshold or anaerobic threshold and often begins at 50% to 60% of maximal oxygen consumption. While the specific physiology is still incompletely understood, lactic acid production leads to an increase in hydrogen ions, ultimately coinciding with a rise in carbon dioxide production by way of the intracellular bicarbonate buffering system. Identifying if a patient reaches this threshold can elucidate the nature of one’s intolerance to exercise. Generally speaking, one can exhibit a pattern of response to exercise that is more consistent with cardiopulmonary vascular limitation, or with ventilatory dysfunction. The point at which a patient reaches this threshold may serve as a baseline if the test is performed to develop a comprehensive exercise rehabilitation program.
CPET, by nature of its complex set up and interpretation, is a specialty diagnostic examination. Therefore primary care practitioners must have open lines of communication with consultant specialists and subspecialists. Their recommendations are lost if the information is not readily available between members of the patient's care team. Concerning the conduction of CPET, respiratory therapists, nurses, nursing assistants, and technicians are integral to a successful examination. Without interprofessional collaboration and mutual respect, the performance of the test is difficult, if not impossible.
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