Mechanisms and Innovations in Cardiac Resynchronisation Therapy

This is a single centre acute electrical/haemodynamic and observational study of a subgroup of patients with heart failure. A total of 60 will be recruited. The acute study involves within patient comparison of biventricular and conduction system pacing using detailed electrical mapping and high precision haemodynamic measurement protocol. Participants will have permanent conduction system pacing if; * Baseline left bundle branch block and QRS \> 140ms or QRS \>150ms and any QRS morphology AND * Conduction system pacing is successful at reducing QRS duration by 20ms or more with satisfactory pacing parameters. Participants who receive permanent conduction system pacing will have follow up at 6 weeks, 3 months, 6 months and 12 months. Non-invasive markers including mechanical activation pattern (echocardiography and cardiac MRI) and electrical activation pattern (ultra-high frequency electrocardiography) will be used to identify the characteristics of patients who benefit from conduction system pacing.