CRT In Narrow QRS Heart Failure: Mechanistic Insights From Cardiac MRI And Electroanatomical Mapping

Inclusion Criteria: * • Participant is willing and able to give informed consent for participation in the study. * Male or Female, aged 18 years or above. * NYHA grade III-IV heart failure * LVEF\<35% * QRS duration \<120ms * On optimum medical therapy for heart failure * Female participants of child bearing potential must be willing to ensure that they or their partner use effective contraception during the study and for 3 months thereafter * Able (in the Investigators opinion) and willing to comply with all study requirements. * Willing to allow his or her General Practitioner and consultant, if appropriate, to be notified of participation in the study. Exclusion Criteria: * • Female participants who is pregnant, lactating or planning pregnancy during the course of the study. * Scheduled elective surgery or other procedures requiring general anaesthesia during the study. * Participant who is terminally ill or is inappropriate for placebo medication * Any other significant disease or disorder which, in the opinion of the Investigator, may either put the participants at risk because of participation in the study, or may influence the result of the study, or the participant's ability to participate in the study. * Contraindication to an MRI scan * Rate uncontrolled atrial fibrillation precluding a cMR * Significant peripheral vascular disease precluding an EP study * A contraindication to anticoagulation * A prosthetic aortic or tricuspid valve * Significant Aortic valve disease * Known LV thrombus * Insufficient capacity to consent to the study