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.