We will address these questions with a prospective, observational study comparing patients implanted with the Synergy device with matched heart failure controls. The two study centres will be Royal Brompton & Harefield NHS Foundation Trust (Harefield Hospital) and University Hospitals Leuven, Belgium. We will recruit a total of 64 patients. The primary outcome will be change in peak VO2 at 6 months. Secondary outcomes will include functional parameters, profiling of circulating microRNA, multimodal echocardiographic assessment and comprehensive assessment of platelet activation and coagulopathy.