Morphological Analysis of the Flow-Volume Curve for Identifying Fixed Airflow Obstruction: A Scoping Review

Background/Objective: The morphology of the flow-volume curve has emerged as a potential source of additional functional information by enabling the analysis of concavity, slope-based metrics, area-derived measures, and mathematical models extracted from the expiratory tracing. Therefore, the aim of this study was to map and describe the available evidence on morphological analysis methods of the expiratory flow-volume curve for the identification and characterization of fixed airflow obstruction. Methods: A scoping review was conducted following the methodological frameworks proposed by Arksey and O'Malley, Levac et al., the Joanna Briggs Institute, and the PRISMA Extension for Scoping Reviews (PRISMA-ScR). Studies published between 1 January 1990, and 31 December 2025, that evaluated morphological flow-volume curve metrics for the diagnosis or characterization of Chronic Obstructive Pulmonary Disease (COPD) were included, without language restrictions. Searches were performed in PubMed/MEDLINE, Embase, Scopus, Web of Science, IEEE Xplore, OpenGrey, and Google Scholar. Study selection was conducted by independent reviewers, with disagreements resolved by consensus, and data extraction was performed using a standardized form. Results were synthesized narratively and organized according to metric families. Results: The search identified 13,577 records; after duplicate removal and screening, 24 studies met the inclusion criteria. The evidence included observational studies, diagnostic validation studies, longitudinal cohorts, and methodological modeling studies. Identified metrics were grouped into four main categories: concavity and geometric indices of the flow-volume curve, expiratory slope and flow-decay metrics, area-based or volumetric-derived measures, and mathematical or computational models applied to the tracing. Concavity indices, the β-angle, slope-ratio, Peak Index, and the D parameter showed consistent associations with airflow obstruction, emphysema, small airway disease, or functional impairment. Expiratory slope metrics and Flow Decay demonstrated high diagnostic performance in several studies, whereas area-based measures such as AEX, AEX-FV, AreaFE%, and AUC3/AT3 integrated the overall loss of expiratory flow during forced expiration. Mathematical and computational models suggested that the complete shape of the curve contains additional diagnostic and prognostic information, although methodological variability and the need for external validation remain important limitations. Conclusions: Morphological analysis of the flow-volume curve represents a promising approach to complement conventional spirometry in the identification and characterization of fixed airflow obstruction. These metrics may provide additional information regarding airflow limitation, non-uniform lung emptying, emphysema, small airway disease, hyperinflation, and clinically relevant outcomes.