The Role of DLNO in the Functional Assessment of Patients with Idiopathic Pulmonary Fibrosis

Background and Objectives: Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive interstitial lung disease characterized by alveolar-capillary membrane remodeling and impaired gas diffusion. The diffusing capacity of the lung for nitric oxide (DLNO) has been proposed as a physiological parameter reflecting membrane diffusing capacity and pulmonary vascular involvement, potentially providing complementary information to diffusing capacity of the lung for carbon monoxide (DLCO). This study aimed to evaluate the role of DLNO in the functional assessment of patients with IPF and its correlation with clinical and echocardiographic outcomes. Materials and Methods: This observational, retrospective study included 35 consecutive IPF patients receiving antifibrotic therapy between February and December 2023. All participants underwent plethysmography, combined single-breath DLNO and DLCO testing, six-minute walk test (6MWT), mMRC dyspnea scale assessment, and echocardiography for the estimation of a higher probability of pulmonary hypertension (PH). Results: DLNO was significantly lower in males compared to females (49.3 ± 16.7% vs. 74.6 ± 16.1%, p < 0.001), with a reduced DLNO/DLCO ratio in men. DLNO correlated with oxygen therapy requirement (p = 0.010) and lower oxygen saturation during the 6MWT (p = 0.021). Patients with higher echocardiographic probability of PH showed markedly reduced DLNO values (17.6 ± 7.6%, p = 0.016) and higher FVC/DLNO ratios (2.31 ± 0.85 vs. 1.65 ± 0.64, p = 0.023), together with lower DLCO levels (p = 0.037). Conclusions: DLNO may complement DLCO in the evaluation of gas exchange and alveolar-capillary dysfunction in IPF. Although preliminary, these findings support the potential clinical utility of DLNO as an adjunct parameter in the functional characterization of IPF. Further multicenter studies are warranted to confirm these results.