The lungs and heart are irrevocably linked in their oxygen (O2) and carbon dioxide (CO2) transport functions. Functional impairment of the lungs often affects heart function and vice versa The steepness with which ventilation (V'E) rises with respect to CO2 production (V'CO ) (i.e. the V'E/V'CO slope) is a measure of ventilatory efficiency and can be used to identify an abnormal ventilatory response to exercise. The V'E/V'CO slope is a prognostic marker in several chronic cardiopulmonary diseases independent of other exercise-related variables such as peak O2 uptake (V'O ). The V'E/V'CO slope is determined by two factors: 1) the arterial CO2 partial pressure (PaCO ) during exercise and 2) the fraction of the tidal volume (VT) that goes to dead space (VD) (i.e. the physiological dead space ratio (VD/VT)). An altered PaCO set-point and chemosensitivity are present in many cardiopulmonary diseases, which influence V'E/V'CO by affecting PaCO Increased ventilation-perfusion heterogeneity, causing inefficient gas exchange, also contributes to the abnormal V'E/V'CO observed in cardiopulmonary diseases by increasing VD/VT During cardiopulmonary exercise testing, the PaCO during exercise is often not measured and VD/VT is only estimated by taking into account the end-tidal CO2 partial pressure (PETCO ); however, PaCO is not accurately estimated from PETCO in patients with cardiopulmonary disease. Measuring arterial gases (PaO and PaCO ) before and during exercise provides information on the real (and not "estimated") VD/VT coupled with a true measure of gas exchange efficiency such as the difference between alveolar and arterial O2 partial pressure and the difference between arterial and end-tidal CO2 partial pressure during exercise.