R-wave Optimisation in Cardiac Resynchronisation Therapy Study

Inclusion Criteria * The patient must have a biventricular pacemaker in situ, implanted for NYHA Class III / IV heart failure syndrome with impaired systolic function. * Implantation indication must have included a left ventricular ejection fraction measured on transthoracic echo of \<35% and either * a QRS duration of \>150ms or * a QRS duration of 120 - 149ms with further evidence of dyssynchrony confirmed by echocardiography. * Participants must have \>95% true biventricular pacing over the 1 month before enrolment and throughout the observation period. * The biventricular paced QRS complex morphology in lead V1 of subject's standard 12 lead surface ECG must show an R wave height of \< 1mm or \<20% of the total QRS deflection with standard LV offset settings applied. * The subject's standard 12 lead surface ECG must show a right bundle branch block pattern in lead V1 during LV only pacing. * The participant must be willing to comply with the protocol requirements including travelling to the Royal Bournemouth Hospital for the attendances required for the study and intend to have their long term device follow up at the Royal Bournemouth Hospital at the time of enrolment. * Provision of informed consent Exclusion criteria * Hypertrophic or restrictive cardiomyopathy * Suspected acute myocarditis * Correctable Valvulopathy * An Acute Coronary Syndrome within the last 3 months * Recent (within the last 3 months) or scheduled coronary revascularisation * Treatment resistant hypertension * Severe obstructive lung disease * Pregnancy at the time of enrolment or a desire to become pregnant during the study period * An inability to walk * Reduced life expectancy not associated with cardiovascular disease (less than 1 year) * Unable to provide informed consent