Inclusion Criteria:  Patient must be 18 to 75 years of age.  Patient with known clinical diagnosis of Stage C heart failure New York Heart Association (NYHA) Class I-III and on stable medical therapy for at least 12 weeks prior to screening with no significant dose change or new medications added during that period. Specifically:  Patients with a diagnosis of HFrEF, defined as ejection fraction ≤ 40%. OR Patients with a diagnosis of HFpEF, defined as ejection fraction ≥ 50%. Patients with suitable veins and arteries for cannulation or repeated puncture. Patients who are able to lie flat for the duration of IA infusions and related procedures during Visit 2 (approximately 3 hours). Body weight of at least 60 kg and body mass index within the range of 18 to 40 kg/m^2. Male and/or female of nonchildbearing potential. Capable of giving signed informed consent.  Exclusion Criteria:  History of any clinically important disease or disorder which, in the opinion of the investigator, may either put the participant at risk because of participation in the study, or influence the results or the participant's ability to participate in the study. Clinically significant valvular heart disease as judged by the investigator. Congenital heart disease (patients with Patent Foramen Ovale may be included in the study). Clinical diagnosis of heart failure NYHA Class IV.  Occurrence in the last 3 months of any of the following:  Acute coronary syndrome: myocardial infarction or unstable angina. Percutaneous coronary intervention. Cerebrovascular accident or transient ischaemic attack. Heart failure hospitalisation. History of prior dissections. History or suspicion of cardiac amyloidosis. Patients with conditions where vasodilator therapy may be contraindicated. History of cancer in the last 5 years, except for non-melanoma skin cancer. Any clinically important abnormalities in clinical chemistry, haematology or urinalysis. Any positive result at Screening for serum hepatitis B surface antigen, hepatitis C antibody and human immunodeficiency virus. History of severe allergy/hypersensitivity or ongoing clinically important allergy/hypersensitivity, as judged by the investigator.  Abnormal vital signs, after at least 10-minute supine rest, defined as any of the following at Screening:  Systolic blood pressure (BP) > 160 mmHg. Diastolic BP > 90 mmHg.  Any clinically important abnormalities in rhythm, conduction or morphology of the resting 12-lead electrocardiogram as considered by the investigator.  Prolonged QTcF > 450 ms. Family history of long QT syndrome. Second or third-degree AV block, or sinus node dysfunction with significant sinus pause. History of hypersensitivity to drugs with a similar chemical structure or class to AZD3427. Participants who have previously received AZD3427.