True about chronic obstructive pulmonary disease (COPD)

In COPD,airflow obstruction is demonstrated by spirometry in which the post- bronchodilator FEV1/FVC is less than 70%.The severity of COPD may be defined in relation to the post - bronchodilator FEV1 which is <30% predicted in very severe type.Measurement of lung volumes provides an assessment of hyperinflation.COPD is characterised by increase in residual volume,residual volume/total lung capacity ratio,and progressive hyperinflation(increased total lung capacity) late in the disease. source : Harrison 20 th edition.