Inspiratory resonant frequency of forced oscillation technique as a predictor of the composite physiologic index in interstitial lung disease

• The usefulness of the forced oscillation technique in interstitial lung disease is unclear.
• The composite physiologic index (CPI) is derived from FEV1, FVC, and diffusing capacity.
• We assessed the relationship between the CPI and forced oscillatory parameters in interstitial pneumonia.
• Inspiratory resonant frequency of forced oscillation technique was an independent predictor.
• Fibrosis score was also an independent predictor of the CPI.

The composite physiologic index (CPI), which is derived from FEV1, FVC, and diffusing capacity, has been developed to predict the extent of fibrosis on high-resolution computed tomography (HRCT). However, the relevance to the forced oscillation technique (FOT) is not fully understood. We hypothesized that FOT would independently predict the CPI in interstitial lung disease (ILD). In this cross-sectional study we assessed the relationship between pulmonary function tests, forced oscillatory parameters, and the degree of fibrosis in ILD. Spirometry, evaluation of diffusing capacity for carbon monoxide, and the broadband frequency FOT were performed in 93 patients with a clinical/HRCT diagnosis of ILD. The CPI was calculated and fibrosis extent was measured by HRCT and scored. Univariate analyses revealed that, of the forced oscillatory parameters, inspiratory resonant frequency best correlated with FVC, FEV1, diffusing capacity, CPI, and fibrosis score. In multiple regression analyses, CPI was independently predicted by inspiratory resonant frequency and fibrosis score (model R2 = 0.405, p < 0.0001).