Cardiac resynchronization therapy (CRT) involves pacing the left and right side of the heart in order to improve the coordination of the contraction in patients with heart failure. Current selection criteria incorporate the severity of the symptoms, the mechanical function of the heart and the time it takes the electrical stimulation to spread over the left ventricle (as assessed on the standard 12 lead electrocardiogram-ECG). Unfortunately these criteria only seem to select approximately 70% of the patients who might respond to this invasive therapy. Body surface ECG mapping is a new technique that assesses the electrical activation of the heart in more detail than the standard ECG. This study aims to determine whether this new technology may aid current selection criteria in predicting response to CRT.

Inclusion Criteria:

• Age >18 years old
• Fulfils established clinical criteria for CRT implantation (with or without a defibrillator)
• NYHA Class III-IV Heart Failure (or NYHA II with NYHA III/IV symptoms in the preceding 12 months)
• LVEF <35% (Calculated using echocardiography or Cardiac MR) at the time of implantation
• QRS duration > 130ms
• Optimal Tolerated Medical Therapy for Heart Failure

Exclusion Criteria:

• Severe, life threatening non cardiac disease
• Active malignant disease and recent (<5 years) malignant disease
• Prior Heart Transplant
• Recent history of unstable angina, acute coronary syndrome or myocardial infarction within three months of enrolment into the study
• Pregnancy
• Failure to participate in consent process
• Atrial Fibrillation
• Conventional pacemaker in situ
• Heart Failure requiring constant intravenous therapy including diuretics and/or inotropes
• Recent revascularisation procedure i.e. coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI) within the last three months