Effect of effort versus volume on forced expiratory flow measurement

Recently, the validity of the American Thoracic Society (ATS) standards for selection of spirometric test results has been questioned based on the finding of inverse dependence of FEV1 on effort. I carried out a study to determine how the test results of FEV1 and FEF25-75 are affected by use of effort-based criteria (highest PEFR) as compared with the volume-based criteria (largest FVC, largest sum of FVC and FEV1-ATS criteria). Randomly sampled routine pulmonary function test results obtained from 10 normal subjects and 12 patients with COPD were analyzed. Although selection based on the largest FVC yielded test results identical to that obtained by the ATS criteria, selection based on the highest PEFR resulted in a decline of mean FEV1. The latter, however, was accompanied by an even more significant decrease of FVC, suggesting bias of this criterion toward selecting the maneuvers with lower FVC. Furthermore, the individual values of FEV1 were often found to decrease with effort only when the accompanying FVC were also lower. In contrast, FEF25-75 were more prominently affected by the negative effect of effort regardless of the values of accompanying FVC. It appears that fully inflated lungs (the largest FVC) favorably shift the balance of two opposing influences of effort, one of which positively acts on the effort-dependent portion of the flow-volume curve. FEV1 thus is little affected by effort once the lungs are fully inflated. Accordingly, the volume-based selection criteria remain valid. This argument does not apply to FEF25-75, which is prominently affected by the negative effect of effort.