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* COPD patients suffer from reduced expiratory flow. As a result, the lungs may not fully empty before the next breath begins. This is particularly likely if the patient has to breathe faster as a result of physical activity.1 Because the lungs do not fully empty, they become progressively over inflated with each breath. This process is known as air trapping, which leads to hyperinflation.1 Air trapping reduces the ability of the patient to breathe in, which causes the sensation of breathlessness that typifies the disease. O'Donnell DE, Webb K. The etiology of dyspnea during exercise in COPD. Pulmonary and Critical Care Update 14, Lesson 15.http://www.chestnet.org/downloads/education/online/Vol14_13_18.pdf. Accessed 24 February 2004. * New * 随着认识的深入，对于COPD的理解不再局限于气道和肺。关于COPD营养不良的研究越来越