Study objectives: To determine the correlates of static lung volumes in patients with airways obstruction, and to determine if static lung volumes differ between asthma and COPD.
Patients and methods: We examined the data from all of the adult patients (mean age of 69) who were referred to a pulmonary function laboratory from January 1990 through July 1994 with an FEV1/FVC ratio of < 0.70 and tested using a body plethysmograph. Correlates were determined using regression analysis.
Measurements and results: Of the 4,774 patients observed with evidence of airways obstruction, 61% were men. Self-reported diagnoses included asthma, 19%; emphysema or COPD, 23%; chronic bronchitis, 1.5%; and alpha1-antiprotease deficiency, 0.6%. Fifty-six percent of the patients did not report a respiratory disease. The degree of hyperinflation, as determined by the residual volume (RV)/total lung capacity (TLC) ratio, or the RV % predicted (but not the TLC % predicted), was strongly associated with the degree of airways obstruction (the FEV1 % predicted). Patients with moderate to severe airways obstruction and high RV and TLC levels were more likely to have COPD than asthma. Of the 1,872 patients with a reduced vital capacity determined by spirometry testing, 87% had hyperinflation as defined by the RV/TLC, and 9.5% had a low TLC (with less severe airways obstruction).
Conclusion: In patients found to have airways obstruction by spirometry, the additional measurement of static lung volumes added little to the clinical interpretation.