The purpose of the Screening for Advanced heart failure IN stable ouTpatientS (SAINTS) study is to determine the prevalence of advanced heart failure (HF) in symptomatic patients with HF and reduced left ventricular ejection fraction (HFrEF), corresponding to the New York Heart Association functional class II-III. Recognition of advanced HF is a challenge for physicians and under referral for advanced management is a considerable problem. There are excellent treatment options for patients with advanced HF, i.e. heart transplantation or left ventricular assist device (LVAD) implantation, and outcomes with these therapies are considerably better if patients are treated before irreversible end-organ damage occurs. International consensus highlights the importance of timely recognition and referral of these patients to advanced HF centers. The investigators aim to screen patients with symptomatic HFrEF who are followed in Danish HF clinics in the Copenhagen region with echocardiography, cardiopulmonary exercise test, 6 minute walk test, and NT-proBNP. The investigators hypothesize that 20% of patients with symptomatic heart failure followed in HF clinics in the Copenhagen Region will fulfill the modified criteria for advanced HF from the HFA-ESC (primary end-point in the study)(Reference 1). Patients who are identified with advanced HF will be offered right heart catheterization, guided by ultrasound and inserted through the internal jugular vein, determining pulmonary capillary wedge pressure, cardiac index, central venous pressure, mean pulmonary artery pressure, and central venous oxygen saturation. Patients not fulfilling criteria for advanced HF will be offered right heart catheterization consecutively until 50 patients have been examined (this group will be a comparator group to the patients with advanced HF). Patients identified with advanced HF will be offered listing for HTx or LVAD if an indication without a contraindication is present. Patients who fulfill the primary endpoint of modified HFA-ESC criteria for advanced HF, and are ineligible for, or unwilling to undergo HTx or LVAD implantation will be invited to participate in the SAINTS B study.