A Proposed Curvilinearity Index for Quantifying Airflow Obstruction

BACKGROUND: Though forced expiratory volume in the first second (FEV₁) is the primary indicator of airway obstruction, curvilinearity in the expiratory flow-volume curve is used to support the quantitative assessment of obstruction via FEV₁. Currently there is no available index to quantify a pathological contour of curvilinearity.

STUDY PURPOSE: We propose a “curvature” index (k_max) and compare FEV₁ values to the index with a sequential sample of spirometry data.

METHODS: The hyperbolic function b₀Q̇ + b₁Q̇V + b₂V = 1 (in which Q̇ = flow rate, V = volume, and b₀, b₁, and b₂ are estimated from the patient's flow-volume data) is fit to a fixed segment of the descending phase of the expiratory flow-volume curve. A previously developed biomechanical interpretation of this relationship associates the coefficient b₁ with the rate of airway-resistanceincrease as exhaled volume increases. A global curvature index k_max = b₁/2(b0b2+b1) is defined to quantify the curvilinearity phenomenon. We used statistics software to determine the k_max of spirometry data from 67 sequential patients, and to determine the relationship of k_max to FEV₁.

RESULTS: Individual k_max estimates appeared to correspond well with the degree of curvilinearity observed and were related in an exponential manner to FEV₁.

CONCLUSIONS: We defined a curvature index to quantify the curvilinearity phenomenon observed in the expiratory limb of flow-volume loops from patients with obstructive lung disease. This index uses data from a major segment of the flow-volume curve, and our preliminary data indicate an exponential relationship with FEV₁. This new index allows the putative association between curvilinearity and obstructive lung disease to be examined quantitatively in clinical practice and future studies.