[Pulmonary function testing of dyspnea complaint by the pulmonologist]

Dyspnea results from an imbalance between ventilatory demand (linked to CO₂ production, PaCO₂ set-point and wasted ventilation-physiological dead space) and ventilatory capacity (linked to passive-compliance, resistance-and active-respiratory muscles-components of the respiratory system). Spirometry and static lung volumes investigate ventilatory capacity only. Ventilatory demand (increased for instance in all pulmonary vascular diseases due to increased physiological dead space) is not evaluated by these routine measurements. DLCO measurement, which evaluates both demand and capacity, depicts the best statistical correlation to dyspnea, for instance in obstructive and interstitial pulmonary diseases. Dyspnea has multiple domains and is inherently complex and weakly explained by resting investigations: explained variance is below 50%. The diagnostic strategy investigating dyspnea has to distinguish complaints related or not to exercise because dyspnea can occur independently from any effort. Cardiopulmonary exercise testing (V'O2, V'CO2, V'E and operating lung volumes measurements) allows the assessment of underlying pathophysiological mechanisms leading to functional impairment and can contribute to unmask potential underlying mechanisms of unexplained dyspnea although its "etiological diagnostic value" for dyspnea remains a challenging issue.