Area under the expiratory flow-volume curve (AEX): actual versus approximated values

Previous work has shown that area under the expiratory flow-volume curve (AEX) performs well in diagnosing and stratifying respiratory physiologic impairment, thereby lessening the need to measure lung volumes. Extending this prior work, the current study assesses the accuracy and utility of several geometric approximations of AEX based on standard instantaneous flows. These approximations can be used in spirometry interpretation when actual AEX measurements are not available. We analysed 15 308 spirometry tests performed on subjects who underwent same-day lung volume assessments in the Pulmonary Function Laboratory. Diagnostic performance of four AEX approximations (AEX_1-4) was compared with that of actual AEX. All four computations included forced vital capacity (FVC) and various instantaneous flows: AEX₁ was derived from peak expiratoryflow (PEF); AEX₂ from PEF and forced expiratoryflow at 50% FVC (FEF₅₀); AEX₃ from FVC, PEF, FEF at 25% FVC (FEF₂₅) and at 75% FVC (FEF₇₅), while AEX₄ was computed from all four flows, PEF, FEF₂₅, FEF₅₀ and FEF₇₅ Mean AEX, AEX₁, AEX₂, AEX₃ and AEX₄ were 6.6, 8.3, 6.7, 6.3 and 6.1 L²/s, respectively. All four approximations had strong correlations with AEX, that is, 0.95-0.99. Differences were the smallest for AEX-AEX₄, with a mean of 0.52 (95% CI 0.51 to 0.54) and a SD of 0.75 (95% CI 0.74 to 0.76) L²/s. In the absence of AEX and in addition to the usual spirometric variables used for assessing functional impairments, parameters such as AEX₄ can provide reasonable approximations of AEX and become useful new tools in future interpretative strategies.