Rationale: Analysis of maximal expiratory flow-volume curves (MEFVCs) allows for determination of airway obstruction, but quantitative methods to describe these curves are not commonly used.
Objectives: We sought to determine the variability in MEFVC concavity assessment by pulmonary physicians, whether objective indices of concavity can be substituted for subjective expert impression, and whether MEFVC concavity correlates with clinical outcomes.
Methods: A survey of 37 MEFVCs (plus 3 duplicates) was sent to pulmonologists for quantitative assessment of MEFVC concavity. Objective indices (β-angle, ratio forced expiratory flow at 50% of total lung volume to peak expiratory flow rate [FEF50/PEFR], ratio of maximum mid-expiratory flow to FVC [MMEF/FVC], kmax, and averaged flow-volume second derivatives) were calculated for each MEFVC and were correlated with the mean expert score. Both the mean expert scores and the best-performing index were then correlated with hospitalizations.
Measurements and main results: Ninety-two respondents provided usable data. There was substantial variability in concavity scores between subjects, but strong intrasubject reliability. Mean expert score did not differ by physician years of experience. Several indices (β-angle, FEF50/PEFR, FEV1/FVC, MMEF/FVC, FEF50, and forced expiratory flow between 25 and 75% of total lung volume) correlated strongly with mean expert scores. A new variable (β-MMEF) was constructed using coefficients from stepwise linear regression and accurately predicted the mean expert score (R(2) = 0.96). Mean expert score and β-MMEF showed similar odds ratios for hospitalization (2.13 and 2.32, respectively) with identical positive (∼71%) and negative (87%) predictive values. The β-MMEF was also associated with hospitalizations in two independent cohorts of children with asthma and cystic fibrosis.
Conclusions: The β-MMEF is an objective measure of maximal expiratory flow-volume curve concavity and highly correlates with expert impression. Both the mean expert score for expiratory curve concavity and the β-MMEF were associated with increased risk of subsequent hospitalization. The β-MMEF may be a useful biomarker for disease severity in asthma and cystic fibrosis.
Keywords: airway obstruction; asthma; cystic fibrosis; pulmonary function; spirometry.