The ARETHA trial is a national (Danish), multicenter, randomized, placebo-controlled, double-blind, cross-over, no run-in phase, phase 4, investigator-initiated clinical trial investigating the effect of vericiguat on diastolic pulmonary arterial pressure in patients with heart failure with reduced ejection fraction. Participants will receive both vericiguat and placebo with an intermediate wash-out period. Patients will randomly be allocated 1:1 to two 6-week treatment sequences: either vericiguat first then placebo, or vice versa. The study drug dose will be doubled every 2 weeks (e.g., 2.5, 5, and 10 mg).