[Dynamic hyperinflation -- the main mechanism of decreased exercise tolerance in patients with COPD].

Decreased exercise tolerance in patients with COPD is the result of involvement in variable proportion of three mechanisms: ventilatory limitation, muscle dysfunction and cardio-vascular involvement (inadequate intake of oxygen at tissue level). Ventilatory limitation is caused by the combination of increased demand and decreased ventilatory capacity Increased ventilatory demand is the result of exercise worsening of ventilation-perfusion imbalance, and decreased ventilatory capacity is the result of decreased elastic recoil and dynamic obstruction. The consequence is the expiratory flowlimitation, leading to inefficientexpiratory muscle activity and dynamic hyperinflation. Dynamic hyperinflation is a result of structural abnormalities in COPD producing mechanical disorders that limit ventilation. Dynamic hyperinflation has some beneficial effects by facilitating maximal exhalation. Negative effects ofhyperinflation are: (1) decreased tidal volume ability to grow properly at exercise, which causes mechanical ventilatorlimitation; (2) decreased functional capacity of inspiratory musles (by increasing elastic load with respiratory muscle fatigue and increase work ofbreathing); (3) exercise hypoxemia and carbon dioxide retention; (4) impairmentof cardiac function during exercise by decreasing venous return and cardiac output. Evaluation of pulmonary hyperinflation is a useful tool for better characterizing the effects of disease and for monitoring the response of therapeutic interventions on exercise tolerance of patients with COPD.