A proposed curvilinearity index for quantifying airflow obstruction

Chang Jiang Zheng, Alexander B. Adams, Michael P. McGrail, John J Marini, Ian A. Greaves

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


BACKGROUND: Though forced expiratory volume in the first second (FEV 1) 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 (kmax) 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-resistance- increase as exhaled volume increases. A global curvature index kmax = b1/√2(b0b2 + b1) is defined to quantify the curvilinearity phenomenon. We used statistics software to determine the kmax of spirometry data from 67 sequential patients, and to determine the relationship of kmax to FEV1. RESULTS: Individual kmax 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.

Original languageEnglish (US)
Pages (from-to)40-45
Number of pages6
JournalRespiratory Care
Issue number1
StatePublished - Jan 1 2006


  • Curvature index
  • Flow-volume curve
  • Forced expiratory volume


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