Cardiopulmonary exercise testing (CPXT) was once primarily utilized as a research tool. It is now more widely available and has a substantial evidence base supporting its application in everyday care in the management of a wide array of clinical conditions and situations. Despite its widespread applicability, the actual utilization of this form of testing in clinical practice is woefully inadequate likely largely due to a lack of awareness. The goal of this chapter is to provide a basic overview of the general principles of CPXT, clinical indications, and key elements of interpretation.
CPXT is most commonly used to objectively and reproducibly establish the degree of exercise limitation, identify the underlying mechanism of this limitation, and specifically identify the relative contributions of cardiac and pulmonary impairment to a patient’s exercise intolerance. There is an abundance of literature supporting the more widespread use of this form of testing in several clinical settings including the evaluation of unexplained dyspnea, perioperative risk assessment, functional significance of valvular disease, disability assessment, and, most notably, the prognostic assessment of patients with chronic systolic heart failure. In the latter case, CPXT is now considered a critical element in determining a patient’s eligibility for advanced congestive heart failure (CHF) therapies such as cardiac transplantation and left ventricular assist device (LVAD) support. The key clinical applications and contraindications for CPXT are listed in Table 14.1.
CPXT Protocol
Pre-exercise assessment includes resting electrocardiogram (ECG), oxygen saturation, blood pressure (BP), and pulse, as well as resting spirometry including vital capacity, FEV1 (forced expiratory volume in 1 second), and MVV (maximum voluntary ventilation). During exercise, the patient remains connected to an ECG monitor as well as facemask or mouthpiece which is in turn connected to a gas analyzer. A pulse oximeter and sphygmomanometer are attached.
Incremental exercise is then performed to maximal effort either on a bicycle or treadmill utilizing any of several standard exercise protocols, such as a modified Bruce or Naughton protocol. It is important to note that high increments of workload, such as in the Bruce exercise protocol, are avoided to reliably measure steady state conditions within each stage of exercise. Table 14.2 demonstrates and defines the most common elements of respiratory gas analysis during exercise. The primary physiologic measures assessed include peak oxygen consumption (PvO2), anaerobic threshold (AT), and respiratory exchange ratio (RER) or respiratory quotient (RQ).
The Normal Cardiopulmonary Response to Exercise
Under normal conditions, incremental exercise work is accomplished by physiologic changes including a reduction in systemic vascular resistance, augmentation of stroke volume, and a progressive rise in heart rate (HR) all leading to progressive rise in cardiac output. Maximal exercise is normally limited by heart rate reserve, with maximal predicted HR not exceeding 220−age. Beyond this limit, cardiac output can no longer be augmented further to sustain incremental physical workload. Normal individuals exhaust their cardiovascular reserve at peak exercise. In contrast, lung mechanics are not limiting in healthy individuals. A ventilatory reserve, or breathing reserve (BR), of 30% to 50% would be considered normal for a healthy control exercising maximally. As a result, oxygen desaturation during exercise does not occur in a maximally exercising healthy individual and, additionally, does not occur during maximal exercise in those with significant circulatory dysfunction when there is no intrinsic pulmonary abnormality.
During exercise with incremental workload, oxygen consumption and carbon dioxide production rise in a parallel linear fashion. As an individual’s exercise effort begins to exceed their ability to deliver blood flow and oxygen to working muscles, anaerobic metabolism begins. This is defined as the anaerobic threshold (AT). Energy-starved muscle begins to produce lactic acid leading to a disproportionate rise in expired CO2 relative to oxygen consumption (Figure 14.1). Identification of AT is an easily derived measurement during CPXT and should occur at approximately 40% to 65% of the VO2 max in a healthy individual.
Table 14.1 CLINICAL INDICATIONS AND CONTRAINDICATIONS FOR CARDIOPULMONARY EXERCISE TESTING
Indications
Evaluation of breathlessness or fatigue of unknown cause
Cardiac ischemia
Risk stratification and assessment of prognosis in heart failure
Direct measurement of functional capacity
Disability determination
Assess functional significance of valvular heart disease
Perioperative risk assessment
Risk of lung resection surgery
Congenital heart disease: Assessment of functional capacity and prognosis
Absolute Contraindications
Acute myocardial infarction
Acute myocarditis
Critical symptomatic aortic stenosis
Severe uncontrolled heart failure
Uncontrolled arrhythmia
Severe resting hypoxia
Aortic dissection
Table 14.2 ABBREVIATIONS AND DEFINITIONS OF KEY COMPONENTS IN CPXT
VO2 (oxygen uptake)
Amount of oxygen extracted from inspired gas per unit time—may be expressed as an absolute value (mL/min) or corrected for weight (mL/kg/min)
VCO2
Amount of carbon dioxide exhaled from the body per unit time (usually, per minute)
VO2 max
Maximum oxygen uptake achievable (confirmed by repeated tests), despite further work rate increases
Peak VO2
Highest VO2 achieved during presumed maximal effort (as indicated by RER >1.15), for that test
R (or respiratory exchange ratio, RER)
Ratio of carbon dioxide output to oxygen uptake (VCO2/VO2)
VE
Volume of air inhaled or exhaled by the body in 1 min
MVV (maximum voluntary ventilation)
The maximum potential ventilation achievable (estimated as forced expiratory volume in 1 s [FEV1] × 40)
Anaerobic threshold (AT)
Exercise limit above which the subject’s anaerobic high-energy phosphate production supplements aerobic metabolism
Breathing reserve
The difference between maximum voluntary ventilation and the achieved maximum exercise minute ventilation
CPXT, cardiopulmonary exercise testing.
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