 Time to next exacerbation is subject to the investigators power calculation. A previous study has shown that 30 of COPD patients are readmitted again within six month because of an exacerbation. The investigators expect that 12 of patients in the intervention group will have an exacerbation. This corresponds to a hazard ratio of 0.36. Assuming a sample size of 70 subjects for each study group, there is a power of 80 to detect a HR of 0.36 based on a one-tailed test. Additional 14 subjects 7 for each study group have been added to account for drop outs. Therefore, 154 subjects will be investigated in this study. 