PURPOSE: Interpretation of spirometric measurements may pose significant challenges in the presence of concomitantly decreased FEV1 and FVC and normal or near-normal FEV1/FVC ratios. Conditions which may cause interpretative difficulty include incomplete inhalation or exhalation, coexistent restriction and obstruction, or small airway collapse during exhalation. In order to distinguish among these possibilities, we considered alternative measurements of obstruction, such as: area under expiratory flow-volume loop (AEX), the ratio between AEX and approximated predicted AEX (AEX ratio), and MEF/FVC.
METHODS: 21,253 consecutive patients who underwent both spirometric and plethysmographic studies in the Cleveland Clinic Pulmonary Function Laboratory between 01/01/1996 and 12/31/2005 were included. We used the standard definitions for obstructive, restrictive, and mixed ventilatory defects, while small airways disease was defined conservatively as MEF<0.35•MEFpred, with a normal FEV1/FVC ratio. The approximated predicted AEX (AEXapprox) was estimated as follows: AEXapprox = 1/2 [PEFpred•FEVPEF + (1/2 FVCpred –FEVPEF)•(PEFpred + MEFpred) + 1/2•MEFpred•FVCpred]. AEX index was then tested for its capacity to differentiate between different spirometric patterns.
RESULTS: Mean (±standard deviation) values of AEX index were: 1.01±0.29, 0.52±0.26, 0.31±0.24, 0.18±0.11 and 0.16±0.12 in the normal, obstructive, restrictive, mixed and small airway disease patterns, respectively. The model did not distinguish between mixed spirometric defects and small airway disease, while the differences between the other categories were significant. When compared with MEF/FVC, the AEX index distinguished well among the spirometric patterns (Figure). For an AEX index threshold value of 0.30, results were: sensitivity 0.89, specificity 0.68, area under the ROC curve [AUROC] 0.78; at a threshold AEX index of 0.40, results were: sensitivity 0.85, specificity 0.71, and AUROC 0.78. AEX index also distinguished well among different classes of severity of obstruction.
CONCLUSION: AEX index may be easily derived from the available spirometric parameters and can be used to diagnose and quantify the degree of obstruction.
CLINICAL IMPLICATIONS: The main value of this new measure is to diagnose obstruction that is co-existent with restriction on standard spirometry and to better quantify the severity of small airway disease.
DISCLOSURE: Octavian Ioachimescu, None.