Obstructive? Restrictive? Or a ventilatory impairment?

Since 1991, the guidelines for interpretation of lung function tests endorsed by the American Thoracic Society (ATS) have recommended that the presence of airflow obstruction be determined by a reduction in the ratio of FEV1 to FVC (or vital capacity), but that the severity of the obstructive impairment be indicated by the reduction in FEV1 itself, expressed as a percent of its predicted value (FEV1 % predicted). [1] Both of these choices have a good rationale and were again recommended in the 2005 ATS/European Respiratory Society joint guidelines. [2] The FEV1/FVC ratio has an inherent correction for the variance in FVC, which contributes to that of FEV1, so the normal range for the ratio is tighter than for FEV1, increasing its sensitivity for the recognition of abnormal airflow. The recommended National Health and Nutrition Examination Survey III reference data [3] show that the normal FEV1/FVC ratio decreases from about 0.85 at age 20 years to 0.75 at age 70 years, and the lower limit of normal (LLN), representing the fifth percentile of a healthy nonsmoking reference population of the same age, is almost exactly 0.10 below the predicted value throughout this age range. For the FEV1, the fifth percentile LLN ranges from 17% to 25% below the predicted value over this age range. The FEV1/FVC ratio decreases steadily with the early progression of disease, but varies more with advanced disease. If FVC is maintained, the ratio continues to fall, but as air trapping reduces the FVC, along with further reduction in FEV1, the decrement in the ratio of the two is blunted. For example, a patient with FEV1 of 1 L and FVC of 2 L is clearly more advanced than one of the same age and size with an FEV1 of 1.8 L and FVC of 3.6 L, but both have an FEV1/FVC of 0.50. Thus, the FEV1 % predicted more reliably tracks the advancing severity of disease and correlates, albeit not tightly, with increasing symptoms, disability, and morbid outcomes. [4] , [5]