Hallmarks of fibrotic interstitial lung disease (f-ILD) include severe hypoxemia, dyspnea and exercise limitation. Although ambulatory oxygen (O2) therapy is widely prescribed, standard low-flow O2 systems (nasal prongs) fail to meet patients' inspiratory demand on exertion resulting in incomplete correction of hypoxemia and limited symptomatic relief. Nasal high-flow O2 therapy (NHFO2) delivers heated, humidified, and O2-enriched air at high flow rates. It has recently emerged as a promising alternative to overcome the pre-specified limitations: NHFO2 is more effective in correcting hypoxemia and reducing dyspnea vs standard O2 therapy and consistently improved exercise capacity in f-ILD. In fact, NHF per se may exert independent physiological benefits such as washout of the anatomical dead space and reduced work of breathing. However, the respective effect of respiratory support and improved oxygenation on dyspnea and exercise tolerance remain unexplored in f-ILD. To address this gap in knowledge, this prospective, randomized-controlled trial aimed to disentangle the i) independent and ii) combined effects of respiratory support and supplemental O2 on dyspnea and exercise tolerance in patients with f-ILD.

Inclusion Criteria:

• diagnosis of fibrotic interstitial lung disease (regardless of etiology)
• age ≥18 years
• modified Medical Research Council scale ≥1
• stable condition (no acute exacerbation within the preceding 3 months)

Exclusion Criteria:

• inability to perform a cardiopulmonary exercise test (CPET) on a bicycle ergometer
• participation in a pulmonary rehabilitation program within the last 6 months

Individual participant data will be available from the contact person on reasonable request.