Myocardial Work and Metabolism in CRT

Inclusion Criteria Subject is eligible for CRT implantation according to current European Society of Cardiology guidelines of 2013. * The patient should receive guideline-directed optimal medical therapy for heart failure and left ventricular (LV) function must be severely depressed (LV ejection fraction ≤35%). * The patients should be in NYHA functional class II, III or ambulatory IV. * The patient should present a left bundle branch block (LBBB) with QRS duration of \>120ms or a non-LBBB with QRS \>150ms. * Also patients with conventional pacemaker in NYHA functional class III and ambulatory IV if high percentage of ventricular pacing, are eligible for CRT implantation. * Subject is in stable sinus rhythm at the time of CRT implant and during the last 2 weeks prior to inclusion. * Subject is 18 years or older and able and willing to consent. Exclusion Criteria: * Impossible to obtain LV volumes by echocardiography. * Right bundle branch block. * Permanent atrial fibrillation, flutter or tachycardia (\>100 bpm). * Recent myocardial infarction, within 40 days prior to enrolment. * Subject underwent coronary artery bypass graft (CABG) or valve surgery, within 90 days. * Post heart transplantation, or is actively listed on the transplantation list, or has reasonable probability (per investigator's discretion) of undergoing transplantation in the next year. * Implanted with a LV assist device (LVAD), or has reasonable probability (per investigator's discretion) of receiving a LVAD in the next year. * Severe aortic stenosis (with a valve area of \<1.0 cm2 or significant valve disease expected to be operated on within study period). * Complex and uncorrected congenital heart disease. * Breastfeeding women, women of child bearing potential. * Enrolled in one or more concurrent studies that would confound the results of this study.