Clinical significance of a flattening index derived from the flow-volume loop

An excessive decrease in the flow at the late expiratory portion of the flow volume loop (FVL) or "flattening" reflects small airway diseases (SAD). The flattening is subjectively assessed by visual inspection. In this study, we hypothesized that an objective flattening index (FI) would correlate with age and that an abnormal FI is associated with SAD.

PFT reports with FEV1/FVC ≥0.7, FEV1 ≥80%pred, and FVC ≥80%pred were downloaded from electronic medical records. We digitized and converted FVLs to data points. We calculated the slopes of segment AB (between the peak expiratory flow [point A] and the point corresponding to 75% of the expiratory vital capacity [point B]) and segment BC (between point B and the point where the expiratory portion of the FVL intersects with the x-axis [point C]). Moreover, 75% is chosen because it roughly corresponds to the start of small airways. FI is the angle formed by segment AB and segment BC. We also extracted demographic data from PFT reports.

A total of 490 PFTs (161 men and 329 female) were included. FI was 156° ± 12° in men and 160° ± 14° in women (p = 0.0079). FI correlated negatively with age and BMI in both genders. The threshold value (mean-1.64 × SD) was 136°for men and 137°for women. A total of 21 PFTs (4.3%) in 15 patients had FI

FI decreased with increasing age and BMI. It can be a useful parameter on PFTs to assist clinicians in identifying SAD.