100 pt multicentre study 50 pts randomised to triventricular pacemaker, 50 to biventricular pacemaker Feasibility study with 6 month outcome data

100 patients with QRS duration (120-150ms LBBB) for device insertion as per ESC guidelines 2013 will be recruited to the study from 10 centres. Patients will undergo the pre-operative work up that is typical of their institution (for example at the GSTT site this will include cardiac MRI, cardiopulmonary exercise testing (CPET), 6 minute walk test , bloods including Nt pro BNP, Minnesota living with heart failure questionnaire (MLWHFQ), 2D/3D echocardiogram and the collection of demographic data.)

Patients will then be randomised in a 1:1 fashion to receive either the Triventricular device or the conventional biventricular device.

Participants will then be followed up as per the implanting institution's typical follow up. (for example at GSTT this will include 6 week atrio-ventricular and ventricular-ventricular optimisation (if patients have a device that does not autooptimise) and review by a clinician as well as a 6 month follow up at which point the CPET, 6 minute walk test , MLWHFQ, bloods including Nt pro BNP, 2D/3D echocardiogram and the collection of demographic data will be repeated.)

Regardless of the institution's own follow up, a 6 week (4-8 week) and 6 month (5-9month) follow up appointment will form part of the study protocol. The 6 week check will be take the form of a standard clinical pacing check with physician input if it is deemed necessary as per the host institution's usual standard of care/ (usually one would imagine if a problem of therapy delivery is noted)

The 6 month follow up will allow for patient and device follow up. This will require 2D echocardiography, 6 min walk test, MLWHFQ and NtproBNP measurement. Where possible institutions will be encouraged to perform CPET and 3D echocardiography pre implant and at 6 months.

For determining the " response rate", a positive response will be defined as an absolute reduction of LVESV by 15%. In order to determine the magnitude of response then the change in LVESV will be assessed as a continuous variable.

Inclusion Criteria:

• Class 1b Indication for CRT (LBBB QRS120-150ms) as per ESC guidelines 2013
• Participant is willing and able to give informed consent for participation in the study.
• Male or Female, aged 18 years or above.
• 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 participant who is pregnant, lactating or planning pregnancy during the course of the study.
• 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.
• Insufficient capacity to consent to the study
• QRS >150ms and Non-LBBB morphology 120-150ms