Hypertrophic Obstructive Cardiomyopathy (HOCM) is an inherited cardiac condition which causes the heart muscle to become abnormally thick causing obstruction of blood flow in the heart. This causes debilitating symptoms including shortness of breath, blackouts and chest pain. Current treatments are not ideal as the medication is often poorly tolerated or ineffective. People with HOCM can often have an Implantable Cardioverter Defibrillator (ICD) to shock them out of dangerous arrhythmias. ICD's can also be used as pacemakers and are a promising treatment option, since they can alter the sequence of the heart muscle contraction thereby relieving the obstruction to the blood flow, making it easier for the heart to pump. The study will recruit patients who already have an ICD/pacemaker or who are scheduled to have an ICD / pacemaker implanted. For patients who are due to have a device implanted high precision haemodynamic, echocardiographic and electrical measurement techniques will be used to assess whether adjusting the position of the pacing lead (at the time of implant) can bring about changes in LVOT gradient and blood pressure. These patients with a new device and also patients who already have a device in situ will then go on to have atrioventricular delay (AV Delay) optimisation so we can assess what the optimum AV delay should be programmed at in order to bring about the most improvement in LVOT gradient and blood pressure. Patients will then be recruited into a medium term double blinded randomised crossover study. They will have optimum RV pacing settings turned on for 3 months. They will then return and be crossed over and have optimum RV pacing turned off for a further 3 months. The primary outcome will be to see if optimum RV pacing being turned on is effective in improving symptoms and quality of life.