Small Airways Evaluation and Treatment

The COPD patients in the investigators hospital are selected to participate in the investigators study, and strict inclusion and exclusion criteria are to be complied with. All the subjects will be told the details of the study and sign the informed consent before inclusion.HRCT will be used to evaluate the structure of small airway by measuring the thickness of the third stage branches of apical segmental bronchus in the right upper lobe (WT). R5-R20 measured by IOS will be used to assess small airway resistance. The study is divided into two parts, that is the part of AECOPD patients and the part of stable patients . Both of them have three steps . Part 1. AECOPD patients: * Subjects screening : To inquire the history of the disease and to take the IOS and Bronchial dilation test. * Baseline data collection：patients will be arranged to complete a series of questionnaires, as well as HRCT, IOS, pulmonary function test and 6 minute walk test. * Test stage：During their hospital stay, the patients are to be treated with oxygen inhalation and different kinds of drugs, and the patients will followed up for three weeks after discharge. Completing the examinations and questionnaires at each time visit ,and another CT scan at the last visit. Part 2. Stable COPD patients: * Subjects screening : To inquire the history of the disease and to take the IOS and Bronchial dilation test. * Baseline data collection：In the two-week washout period，patients are only allowed take salmeterol/fluticasone 50:250ug 2 times daily, and stop the use of other inhalation drugs and oral glucocorticoid. At the end of this period, complete a series of questionnaires, as well as HRCT, IOS, pulmonary function test and 6 minute walk test. * Test stage： patients are randomized to take either beclomethasone / formoterol (100:6ug 2inhalations twice daily ) or budesonide / formoterol (160:4.5ug 2inhalations twice daily) for three months.Completing the examinations and questionnaires at the end of each month, another CT scan at the last visit.