Area Under the Inspiratory Flow-Volume Curve (AIX): Proposed Normative Values

Abstract

Abstract Area under the inspiratory flow-volume loop (AIX) is a spirometric tool that is potentially useful in characterizing upper and/or lower airflow obstruction. We analyzed AIX in 4,980 single-test, pre- or post-bronchodilator normal spirometry tests recorded in non-smoking, healthy individuals in the Pulmonary Function Testing Laboratory. The mean (95% confidence interval, CI), standard deviation and median (25th -75th interquartile range) AIX were 16.05 (15.79–16.31), 9.08 and 14.72 (9.12–21.42) L2⋅sec− 1, respectively. The mean (95% CI) and standard deviation of the best-trial measurements for square root of AIX (SqrtAIX, which follow a distribution closer to a gaussian distribution, were 3.84 (3.81–3.87) and 1.14; 4.15 (4.12–4.18) and 1.03 in men, and 2.68 (2.63–2.72) and 0.72 L⋅sec− 1/2 in women. The mean (standard deviation) of pre- and post-bronchodilator SqrtAIX were 3.71 (1.17) and 3.81 (1.19) L⋅sec− 1/2, respectively. The mean (95% CI), standard deviation and lowest 5th percentile (lower limit of normal, LLN) of SqrtAIX/SqrtAEX (%) were 101.3 (100.82-101.87), 18.7%, and 71.8%; stratified by gender, it was 102.2 (101.6-102.8), 18.6, and 72.8% in men, and 98 (96.9–99.2), 18.8, and 68.6% in women, respectively. The availability of area under the inspiratory flow-volume curve (AIX) and the derived indices offers a promising opportunity to assess upper airway disease (e.g., involvement of larynx, trachea or major bronchi), especially because some of these measurements appear to be independent of age, race, height, and weight.