Study subjects and general methods
The following study was approved by the institutional review board of Seoul National University Bundang Hospital (B-1108/134-004 and B-0508-023-009). Informed written consent for participation in the study was obtained from all participants.
For validation of new graphic parameters, we analyzed the clinical and baseline pre-bronchodilator best spirometric data of 61 COPD patients from the Seoul National University Airway Registry (SNUAR) and the Korean Obstructive Lung Disease (KOLD) cohort [10]. 40 patients from SNUAR and 21 patients from KOLD cohort were included in the analysis. The SNUAR cohort included stable COPD patients who were prospectively recruited from the outpatient pulmonary clinic of Seoul National University Bundang Hospital. The diagnosis of COPD was made according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2003 criteria. The KOLD cohort consisted of patients with stable COPD, who were prospectively recruited from the pulmonary clinics of 11 hospitals in Korea from June 2005 to September 2009.
Spirometry was performed using a Vmax 22 instrument (Sensor-Medics; Yorba Linda, CA, USA). Lung volumes were measured by body plethysmography (V6200; SensorMedics). Diffusing capacity for carbon monoxide (DLco) was measured by the single-breath method using a Vmax229D (Sensor-Medics). All pulmonary function tests were performed as recommended by the American Thoracic Society and European Respiratory Society.
Graphic analysis methods
For graphic analysis of the EFVC, we used Graphical Analysis 3.4 Vernier Software program. Using the program, we first integrated the area under the curve. Figure 1 shows an example of pre-bronchodilator best EFVC and integration of the area under the curve using the software.
Figure 2 shows the lighted area below the imaginary diagonal line and above the EFVC; we defined this area as Au, which reflects concavity of the curve. Our new graphic parameter, Area of obstruction (Ao) and area of triangle (At) were calculated as follows.
$$ \mathrm{A}\mathrm{o}=\mathrm{A}\mathrm{u}/\mathrm{A}\mathrm{t} $$
$$ \mathrm{A}\mathrm{t}=\mathrm{PEFR}\times \left(\mathrm{F}\mathrm{V}\mathrm{C}\hbox{--} \mathrm{X}\mathrm{p}\right)/2 $$
As Ao approaches to 1, the concavity of EFVC becomes more severe. As Ao gets closer to 0, the EFVC becomes less concave. Xp denotes the lung volume at PEFR.
Because the baseline pulmonary function depends on the patients’ heights and other factors, we needed to define Ao in ratio rather than raw values. In order to calibrate and adjust for such individual differences, we divided Au by At. Using a raw value such as Au may result in wrong direction and inappropriate interpretation of result (see Additional file 1: Table S1).
Another parameter, area of rectangle (AR) were defined as follows; we used the reference equation derived from the Korea National Health and Nutrition Examinations Survey IV to calculate predicted FVC in Korean COPD patients [11, 12].
$$ \mathrm{A}\mathrm{R}=\left(\mathrm{actual}\kern0.5em \mathrm{PEFR}/\mathrm{predicted}\kern0.5em \mathrm{PEFR}\right)\times \left(\mathrm{actual}\kern0.5em \mathrm{F}\mathrm{V}\mathrm{C}/\mathrm{predicted}\kern0.5em \mathrm{F}\mathrm{V}\mathrm{C}\right)\times 100\% $$
The ratio of lung volumes at 75 and 25 % of PEFR (0.25/0.75 V) were also calculated.
Statistical analysis
Statistical analysis was performed using the statistics software, PASW Statistics for Windows, Version 18.0, Chicago: SPSS Inc. The association between clinical and spirometric parameters was determined by using Pearson correlation analysis.