Quantifying tidal expiratory flow limitation using a vector-based analysis technique

Objective. Tidal expiratory flow limitation (EFL_T) is commonly identified by tidal breaths exceeding the forced vital capacity (FVC) loop. This technique, known as the Hyatt method, is limited by the difficulties in defining the FVC and tidal flow-volume (TV) loops. The vector-based analysis (VBA) technique described and piloted in this manuscript identifies and quantifies EFL_Tas tidal breaths that conform to the contour of the FVC loop.Approach. The FVC and TV loops are interpolated to generate uniformly spaced plots. VBA is performed to determine the smallest vector difference between each point on the FVC and TV curves, termed the flow reserve vector (FRV). From the FVC point yielding the lowest FRV, the tangential angles of the FVC and TV segments are recorded. If the TV and FVC loops become parallel, the difference between the tangential angles tends towards zero. We infer EFL_Tas parallel TV and FVC segments where the FRV is < 0.1 and the tangential angle is within ±18 degrees for ≥5% of TV. EFL_Tis quantified by the percent of TV loop fulfilling these criteria. We compared the presence and degree of EFL_Tat rest and during peak exercise using the Hyatt method and our VBA technique in 25 healthy subjects and 20 subjects with moderate-severe airflow obstruction.Main results. Compared to the Hyatt method, our VBA technique reported a significantly lower degree of EFL_Tin healthy subjects during peak exercise, and in obstructed subjects at rest and during peak exercise. In contrast to the Hyatt method, our VBA technique re-classified five subjects (one in the healthy group and four in the obstructed group) as demonstrating EFL_T.Significance.Our VBA technique provides an alternative approach to determine and quantify EFL_Twhich may reduce the overestimation of the degree EFL_Tand more accurately identify subjects experiencing EFL_T.