Development and Evaluation of a Small Airway Disease Index Derived From Modeling the Late-Expiratory Flattening of the Flow-Volume Loop

Abstract

Excessive decrease in the flow of the late expiratory portion of a flow volume loop (FVL) or “flattening”, reflects small airway dysfunction. The assessment of the flattening is currently determined by visual inspection by the pulmonary function test (PFT) interpreters and is highly variable. In this study, we developed an objective measure to quantify the flattening. We downloaded 172 PFT reports in PDF format from the electronic medical records and digitized and extracted the expiratory portion of the FVL. We located point A (the point of the peak expiratory flow), point B (the point corresponding to 75% of the expiratory vital capacity), and point C (the end of the expiratory portion of the FVL intersecting with the x-axis). We did a linear fitting to the A-B segment and the B-C segment. We calculated: 1) the AB-BC angle (∠ABC), 2) BC-x-axis angle (∠BCX), and 3) the log ratio of the BC slope over the vertical distance between point A and x-axis [log (BC/A-x)]. We asked an expert pulmonologist to assess the FVLs and separated the 172 PFTs into the flattening and the non-flattening groups. We defined the cutoff value as the mean minus one standard deviation using data from the non-flattening group. ∠ABC had the best concordance rate of 80.2% with a cutoff value of 149.7°. We then asked eight pulmonologists to evaluate the flattening with and without ∠ABC in another 168 PFTs. The Fleiss’ kappa was 0.320 (lower and upper confidence intervals [CIs]: 0.293 and 0.348 respectively) without ∠ABC and increased to 0.522 (lower and upper CIs: 0.494 and 0.550) with ∠ABC. There were 147 CT scans performed within 6 months of the 172 PFTs. Twenty-six of 55 PFTs (47.3%) with ∠ABC <149.7° had CT scans showing small airway disease patterns while 44 of 92 PFTs (47.8%) with ∠ABC ≥149.7° had no CT evidence of small airway disease. We concluded that ∠ABC improved the inter-rater agreement on the presence of the late expiratory flattening in FVL. It could be a useful addition to the assessment of small airway disease in the PFT interpretation algorithm and reporting.

Development and evaluation of a computerized algorithm for the interpretation of pulmonary function tests

“…In the 4,711 PFTs with FEV1/FVC≥0.7, we calculated SADI [ 13 ]. The distribution of SADI was Gaussian ( Fig 5 ).…”

“…One unique feature of our computerized algorithm was the SADI that measured the flattening of the late expiratory limb of the FVL, an assessment of small airway dysfunction [ 13 ]. The assessment traditionally relied on visual inspection of the flattening that is subject to significant intra- and inter-rater variability.…”

“…The clinical significance for SADI needs further investigation in the future. Our previous study that used a greater SADI showed that approximately 50% of the patients with small SADI had small airway disease on CT scans [ 13 ].…”

“…The algorithm also included a small airway disease index (SADI) reported previously by our group [ 13 ]. The SADI is a measurement of the late expiratory flattening of the flow-volume loop (FVL).…”

“…This is currently evaluated by visual inspection of FVL by the PFT interpreters and is subject to significant inter-rater variabilities [9]. We have developed a small airway disease index (SADI) based on the angle formed by two linear segments: one is from peak expiratory flow to the point on the curve corresponding to 75% of the expiratory vital capacity, which roughly corresponds to the start of small airways [10][11][12], and the other is from this point to the end of the curve where the expiratory portion of the FVL intersects the x-axis [13].…”