DETERMINE-reduced - Dapagliflozin Effect on Exercise Capacity Using a 6-minute Walk Test in Patients With Heart Failure With Reduced Ejection Fraction

Inclusion Criteria: * Provision of signed informed consent prior to any study specific procedures * Male or female, aged ≥18 years * Documented diagnosis of symptomatic HFrEF (NYHA functional class II-IV), which has been present for at least 8 weeks * LVEF≤40% * Elevated NT-proBNP levels * Patients should receive background standard of care as described below: All HFrEF patients should be treated according to locally recognised guidelines on standard of care treatment with both drugs and devices, as appropriate. Guideline-recommended medications should be used at recommended doses unless contraindicated or not tolerated. Therapy should have been individually optimised and stable for ≥4 weeks (this does not apply to diuretics) before visit 1 and include (unless contraindicated or not tolerated): * an ACE inhibitor, or ARB or sacubitril/valsartan and * a beta-blocker and * if considered appropriate by the patient's treating physician; a mineral corticoid receptor antagonist * 6MWD≥100 metres and ≤425 metres at enrolment and randomization. Exclusion Criteria: * Presence of any condition that precludes exercise testing * Participation in a structured exercise training programme in the 1 month prior to screening or planned to start during the trial * Receiving therapy with an SGLT2 inhibitor within 4 weeks prior to enrolment or previous intolerance of an SGLT2 inhibitor * Type 1 diabetes mellitus * eGFR \<25 mL/min/1.73 m2 (CKD-EPI formula) at enrolment, unstable or rapidly progressing renal disease at time of randomisation * Systolic BP \<95 mmHg on 2 consecutive measurements * Systolic BP ≥160 mmHg if not on treatment with ≥3 blood pressure lowering medications or ≥180 mmHg irrespective of treatments, on 2 consecutive measurements * Current acute decompensated HF or hospitalisation due to decompensated HF \<4 weeks prior to enrolment * MI, unstable angina, coronary revascularization ablation of atrial flutter/fibrillation, valve repair/replacement, implantation of a cardiac resynchronization therapy device within 12 weeks prior to enrolment or planned to undergo any of these operations after randomization. * Stroke or transient ischemic attack within 12 weeks prior to enrolment. * Primary pulmonary hypertension, chronic pulmonary embolism, severe pulmonary disease including COPD. * Previous cardiac transplantation or implantation of a ventricular assistance device or similar device, or implantation expected after randomization * HF due to infiltrative cardiomyopathy, active myocarditis, constrictive pericarditis, cardiac tamponade, known genetic hypertrophic cardiomyopathy or obstructive hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy/dysplasia, or uncorrected primary valvular disease