FRC%TLC and RV%TLC to assess effects of lung disease: Comparison to FEV1%FVC

Background: Functional Residual Capacity (FRC) iis the point we breathe at and is the balance point between the chest wall and lung compliance. Hyperinflation is the ratio of FRC to total lung capacity (TLC), whilst gas trapping is the residual volume (RV) to TLC ratio. Methods: A retrospective analysis of patients attending for static and dynamic volumes was undertaken. Static lung volumes were measured by body plethysmography. Airflow obstruction was defined as and FEV1%FVC <-1.64SR and a raised FRC%TLC and RV%TLC defined as >+1.64SR. Results: 3335 data sets were reviewed (1768 M; median age 54yrs (4-96). 1867(56%) had an FEV1%FVC <-1.64SR. Of these 132 (7.1%) had an FRC/TLC ratio >+1.64SR, of which 30(1.6%) had an SR>3.50. Similar results, but not always in the same patient was observed for the RV%TLC. There were significant correlations (p<0.0001) between FEV1%FVC SR and both FRC%TLC SR (r2=0.12) and RV%TLC SR (r2=0.31). ROC curves showed that for the FRC%TLC ratio, the AOC=0.81, sensitivity=81.7 (95%CI - 80.4 to 83.0) and specificity=55.9 (95%CI 54.3 to 57.6). For RV%TLC ratio, the AOC=0.77, sensitivity=77.3 (95%CI - 75.8 to 78.71) and specificity=53.6 (95%CI 51.9 to 55.3). Conclusion: The FRC%TLC is raised in 7% of patients without airflow obstruction, as is the RV%TLC. The worse the airflow obstruction, the greater the ratio observed. This suggests that the FEV1%FVC may not be account for some of the early changes in airway function.