Inclusion Criteria:  Study A and B  Patients referred for conventional defibrillator implantation or generator change. Adults (age > 18 years) Willing to take part and able to give consent.  Study C  Patients with defibrillators who are undergoing a VT ablation. Adults (age > 18 years) Willing to take part and able to give consent.  Study D  Patients with defibrillators already implanted who are able to exercise and raise their heart rate substantially Adults (age > 18 years) Willing to take part and able to give consent.  Study E  Patients with a defibrillator already implanted admitted with atrial arrhythmias, ventricular tachycardia or RV lead fracture. Adults (age > 18 years) Willing to take part and able to give consent.  Exclusion Criteria:  Exclusion criteria for studies A-E:  Unable to give consent. Children (age < 18 years) Pregnant women.  Exclusion criteria for studies A and B if undergoing VF induction:  Intracardiac thrombus AF without regular anticoagulation Severe aortic stenosis Severe ischaemic heart disease Decompensated heart failure  Exclusion criteria for study D  Recent heart attack or stroke (within the last month) Thromboembolic event undergoing active treatment Severe aortic stenosis Severe ischaemic heart disease Hypertrophic obstructive cardiomyopathy Uncontrolled ventricular arrhythmias Physically unable to exercise, for example due to arthritis.