Cardiac Resynchronisation Therapy Versus Rate-responsive Pacing in Heart Failure With Preserved Ejection Fraction

All subjects will receive a biventricular pacemaker at implantation. Programming will initially be to dual-chamber, dual-function rate-responsive pacing for 12 weeks; following reassessment, device will be reprogrammed to biventricular pacing for a further 12 weeks. Optional study extension: if incremental benefit is shown for biventricular pacing above dual chamber, the mechanism will be sought using echocardiographic evidence and the devices will be optimised according to this mechanism of action to see whether further benefit can be achieved.