A possible, non-invasive method of measuring dynamic lung compliance in patients with interstitial lung disease using photoplethysmography.

BACKGROUND Measuring lung compliance is useful for evaluating interstitial lung disease (ILD) progression because reduced lung compliance by fibrosis progression is the main primary cause of decreased vital capacity. However, because of the invasiveness of the method, requiring the insertion of a balloon into the esophagus, lung compliance is rarely measured. A recently developed, possible method estimates intrathoracic pressure using fingertip photoplethysmography. This method non-invasively measures the lung dynamic compliance (Cdyn) by simultaneously measuring tidal volume. We evaluated the efficacy of this method in assessing ILD. METHODS We conducted a single-center, observational cross-sectional study to evaluate the efficacy of this method in subjects with ILD as compared with that in healthy control subjects. The main outcome was the estimated Cdyn (eCdyn) determined using this method. We also evaluated potential confounding factors of eCdyn in the baseline characteristics. RESULTS In the ILD group (n = 14) the median eCdyn was significantly lower than that in the control group (n = 49) (0.122 vs. 0.183; P = 0.011). In univariate regression analysis, eCdyn was significantly correlated with height, weight, forced vital capacity, forced expiratory volume in one second, diffusing capacity for carbon monoxide, and usual interstitial pneumonia (UIP). In multivariate regression analysis, both weight (β = 0.49, P = 0.011) and UIP (β = 0.52, P = 0.007) were significantly associated with eCdyn. CONCLUSIONS We demonstrated a significant reduction in Cdyn in subjects with ILD using photoplethysmography. This non-invasive method might be a novel, promising tool for evaluating fibrosis progression in ILD.