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Figure 1 Schematic representation of flow-volume curves in a normal subject (left) and in a patient with severe chronic obstructive pulmonary disease (COPD) (right). The outermost curves correspond to a forced maneuver, the innermost curves to tidal breathing. In the normal subject, the maximal expiratory envelope is way above the tidal one. In the COPD patient, maximal expiratory flows are reduced at all lung volumes and dramatically so at low lung volumes. Tidal expiratory flow reaches the maximal expiratory flow, defining flow limitation. As compared with normal subjects, residual volume (RV) and functional residual capacity (FRC) are shifted toward total lung capacity (TLC), defining hyperinflation: RV in the patient approximately corresponds to FRC in the normal subject. COPD = chronic obstructive pulmonary disease; FRC = functional residual capacity; TLC = total lung capacity; RV = residual volume.