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Significance of FEV BACKGROUND: Small airways are known to be affected early in the course of chronic obstructive pulmonary disease (COPD); however, traditional spirometric indices may not accurately identify small airways disease. RESEARCH QUESTION: Can FEV3/FEV6 identify early airflow abnormalities and predict future clinically important respiratory-related outcomes, including development of COPD? STUDY DESIGN AND METHODS: We included eight hundred thirty-two current and former smokers with post-bronchodilator FEV1/FVC ≥0.7 from the SPIROMICS cohort. Participants were classified as having a reduced pre-bronchodilator FEV3/FEV6 based on lower limit of normal (LLN) values. Repeatability analysis was performed for FEV3 and FEV6. Regression modeling was used to evaluate the relationship between baseline FEV3/FEV6 and outcome measures including functional small airways disease on thoracic imaging and respiratory exacerbations. Interval censored analysis was used to assess progression to COPD. RESULTS: FEV3/FEV6 <LLN at baseline, defined as reduced compared to FEV3/FEV6 ≥LLN, was associated with lower FEV1, poorer health status (St. George's Respiratory Questionnaire score), more emphysema, and more functional small airways disease on quantitative imaging. FEV3 and FEV6 showed excellent agreement between repeat measurements. A reduced FEV3/FEV6 was associated with increased odds of a severe respiratory exacerbation within the first year of follow-up and decreased time to first exacerbation. A low FEV3/FEV6 was also associated with development of COPD by spirometry (post-bronchodilator FEV1/FVC <0.7) during study follow-up. INTERPRETATION: FEV3/FEV6 is a routinely available and repeatable spirometric index which can be useful in the evaluation of early airflow obstruction in current and former smokers without COPD. A reduced FEV3/FEV6 can identify those at risk for future development of COPD and respiratory exacerbations.