Exertional ventilation/carbon dioxide output relationship in COPD: from physiological mechanisms to clinical...

There is well established evidence that the minute ventilation (V'_E)/carbon dioxide output (V'_CO ) relationship is relevant to a number of patient-related outcomes in COPD. In most circumstances, an increased V'_E/V'_CO reflects an enlarged physiological dead space ("wasted" ventilation), although alveolar hyperventilation (largely due to increased chemosensitivity) may play an adjunct role, particularly in patients with coexistent cardiovascular disease. The V'_E/V'_CO nadir, in particular, has been found to be an important predictor of dyspnoea and poor exercise tolerance, even in patients with largely preserved forced expiratory volume in 1 s. As the disease progresses, a high nadir might help to unravel the cause of disproportionate breathlessness. When analysed in association with measurements of dynamic inspiratory constraints, a high V'_E/V'_CO is valuable to ascertain a role for the "lungs" in limiting dyspnoeic patients. Regardless of disease severity, cardiocirculatory (heart failure and pulmonary hypertension) and respiratory (lung fibrosis) comorbidities can further increase V'_E/V'_CO A high V'_E/V'_CO is a predictor of poor outcome in lung resection surgery, adding value to resting lung hyperinflation in predicting all-cause and respiratory mortality across the spectrum of disease severity. Considering its potential usefulness, the V'_E/V'_CO should be valued in the clinical management of patients with COPD.