Cardiopulmonary exercise (CPX) testing draws on the recognition that the thorax represents a metabolic gas transport unit, the function of whose requisite components – diaphragm, heart, lungs, rib cage, and corresponding skeletal muscles – is to transport O2 to and CO2 from metabolizing tissues. Unit O2 and CO2 transport must adjust to physiological and pathophysiological stresses that augment the body's consumption of oxygen (V̇O2) and carbon dioxide production (V̇CO2). During strenuous levels of muscular work, for example, V̇O2 may rise eightfold, accompanied by increased V̇CO2. Cardiovascular or ventilatory disease can disrupt the unit's functional integrity. With severe disease, an abnormality in respiratory gas transport may be apparent at rest, when the body's O2 requirements are modest. Resting function is preserved with less severe expressions of disease, but abnormal respiratory gas transport becomes apparent when the unit is stressed by an elevation in V̇O2.
CPX testing includes the monitoring of respiratory gas exchange (V̇O2 and V̇CO2), minute ventilation (V̇E), and its components, tidal volume and respiratory rate, together with blood pressure, heart rate, and the electrocardiogram. CPX testing represents a useful approach in the clinical evaluation of a whole host of disorders and circumstances. This chapter addresses physiological principles and the clinical application of CPX testing in the evaluation of major disorders that impair heart or lung function. The chapter is by no means an exhaustive review. For a more detailed discussion of specific entities, the interested reader is referred to several other textbooks.1,2
Principles, Definitions, and Clinical Application of Cardiopulmonary Exercise Testing
The metabolic gas transport unit, also referred to as the “cardiopulmonary unit,” links metabolizing tissues to the atmosphere and its supply of O2. O2 transport to tissues must be precise and based upon prevailing need. CO2 produced by tissues must be eliminated into the atmosphere in an equally efficient manner.
Resting Oxygen Uptake and Transport
Concepts and calculations pertaining to V̇O2 and O2 content, transport, and extraction are reviewed in Table 34-1. The heart and lungs accommodate to the metabolic requirements of tissues; they must do so on a moment-to-moment basis, according to physiological priorities. Tissue requirements for O2 dictate a certain V̇E and cardiac output. In an average-sized person, resting V̇O2 averages 250 mL/min or 3.5 mL/min/kg body weight (one metabolic equivalent) and is associated with a V̇E of 8 to 10 L/min and cardiac output of 4 to 6 L/min. O2 transport, also termed O2 delivery, ranges between 730 and 1040 mL/min and is more than adequate to satisfy resting V̇O2. On average, 25% of arterial O2 content is extracted ...