BACKGROUND Cardiac resynchronisation therapy (CRT) implant rates continue to escalate but device optimisation is rarely performed despite being routinepractice in the CRT trials. This thesis seeks to address the uncertainties contributing to failure of uptake of atrioventricular (AV) optimisation, and examines best management of the patient following CRT. METHODS AND RESULTS Echocardiographic iterative AV delay optimisation was used in both rest and exercise studies. A significant drop in NT-proBNP (median fall 474 pg/ml) was seen within 7 days of optimisation compared with patients requiring AV adjustment. The larger the change in AV delay made (>50ms change), the greater the reduction in NT-proBNP levels. During exercise the majority of patients [94%] demonstrated a marked reduction in optimal AV delay. With rate adaptive AV delay activated (adaptive shortening of AV delay with exercise), a 10% improvement in peak V02 and exercise duration was seen. 77 patients were followed over a median 16.3 months, revealing that more than a third of patients required further AV delay readjustment. Multivariate analysis shows the degree of reverse remodelling post CRT dictated the requirement for repeat optimisation. The additional benefits of the CRT optimisation clinic resulting in a multifaceted patient assessment via simultaneous clinical, echocardiographic and device interrogation are demonstrated. Finally, using remote monitoring device diagnostic data, patients with atrial high rate event (AHRE) burden >3.8 hours/day were 9 times more likely to develop thromboembolic complications and 4 times more likely to experience the composite endpoint of death from cardiovascular cause, thromboembolic events and admissions for heart failure or atrial fibrillation. CONCLUSION AV delay device optimisation at both rest and exercise provides additional benefits for the patient over that with CRT implant alone. The setting of the optimisation clinic facilitates a 3600 appraisal of the patient with additional benefits over that of AV delay optimisation alone. Ongoing remote monitoring helps to detect, risk assess and manage patients most at risk from HF decompensation, death and thromboembolic events. The concept of CRT as an isolated procedure is evolving into that of a remotely monitorable, adjustable component of a complex long term condition.