Background. Acute Respiratory Distress Syndrome (ARDS) results in significant hypoxia, and ARDS is the central pathology of COVID-19. Inhaled prostacyclin has been proposed as a therapy for ARDS, but data regarding its role in this syndrome are unavailable. Therefore, we investigated whether inhaled prostacyclin would affect the oxygenation and survival of patients suffering from ARDS. Methods. We performed a prospective randomized controlled single-blind multicenter trial across Germany. The

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... ial was conducted from March 2019 with final follow-up on 12 th of August 2021. Patients with moderate to severe ARDS were included and randomized to receive either inhaled prostacyclin (3 times/day for 5 days) or sodium chloride. The primary outcome was the oxygenation index in the intervention and control groups on Day 5 of therapy. Secondary outcomes were mortality, secondary organ failure, disease severity and adverse events. Findings. Of 707 patients approached 150 patients were randomized to receive inhaled prostacyclin (n=73) or sodium chloride (n=77). Data from 144 patients were analyzed. The baseline oxygenation index did not differ between groups. The primary analysis of the study was negative, and prostacyclin improved oxygenation by 20 mmHg more than NaCl (p=0.17). Oxygenation was significantly improved in patients with ARDS who were COVID-19-positive (34 mmHg, p=0.04). Mortality did not differ between groups. Secondary organ failure and adverse events were similar in the intervention and control groups. Interpretation. Although the primary result of our study was negative, our data suggest that inhaled prostacyclin might be a more beneficial treatment than standard care for patients with ARDS.