Cardiac resynchronisation therapy (CRT) is a specialised type of pacemaker used in patients with severe heart failure to improve symptoms and survival. Approximately one third of patients treated with CRT do not notice significant improvement in their symptoms and this may be due to inadequate co-ordination between the upper and lower chambers of the heart (atrioventricular dyssynchrony).

The investigators propose a new method to achieve atrioventricular synchrony in CRT based on measurements of electrical conduction from within the heart. Patients referred for CRT implantation at Castle Hill Hospital are eligible to participate. During CRT implantation, additional measurements, will be recorded from within the heart. After implantation, device settings will be adjusted to either standard or electrophysiologically-optimised settings with cross-over at 4 months.

The investigators hypothesis is that patients with optimised settings will derive additional benefit compared to patients with standard pacemaker-determined settings.

Inclusion Criteria:

• Clinical diagnosis of heart failure
• Referred for cardiac resynchronisation therapy (CRT) with or without defibrillator
• In sinus rhythm
• Patients must have a telephone
• Ability to give informed consent
• Ability to complete the study

Exclusion Criteria:

• Severe valve disease or previous valve replacement
• Previous atrial flutter ablation
• Chronic kidney disease Stage 3 or above
• Recent myocardial infarction
• Ongoing cardiac ischaemia
• Infiltrative cardiomyopathy
• Wolff-Parkinson White syndrome
• Presence of second or third degree heart block
• <90% biventricular pacing at 3 months
• Hypertrophic cardiomyopathy
• Pregnancy or breastfeeding
• On treatment for hypothyroidism or hyperthyroidism