Cardiac pacing which involved stimulating the heart electrically with electrical wires that go into the heart is routine practice in the diagnosis and treatment of heart rhythm problems. Clinically this involved the fields of cardiac pacing and electrophysiology. Patients who are at risk of sudden death because of serious heart rhythms that are a result of malfunction of the electrical system of the pumping chambers of the heart (ventricles) are generally implanted with specialised pacemakers that can defibrillate (shock) the heart if a nasty life threatening rhythm should result. Shocks are painful and in order to try and treat these rhythms without shocks, anti tachycardia pacing is performed (this is routine part of the device), which aims to interrupt the rhythm by stimulating the heart electrically. This does not always work and can destabilise the rhythm leading to a shock. REVRAMP is a novel modification of anti tachycardia pacing which involved stimulating the heart through the defibrillator wires in a different way. It appears to work better and seems less likely to destabilise the heart rhythm, hence can reduce painful shocks.

Cardiac pacing which involved stimulating the heart electrically with electrical wires that go into the heart is routine practice in the diagnosis and treatment of heart rhythm problems. Clinically this involved the fields of cardiac pacing and electrophysiology. Patients who are at risk of sudden death because of serious heart rhythms that are a result of malfunction of the electrical system of the pumping chambers of the heart (ventricles) are generally implanted with specialised pacemakers that can defibrillate (shock) the heart if a nasty life threatening rhythm should result. Shocks are painful and in order to try and treat these rhythms without shocks, anti tachycardia pacing is performed (this is routine part of the device), which aims to interrupt the rhythm by stimulating the heart electrically. This does not always work and can destabilise the rhythm leading to a shock. REVRAMP is a novel modification of anti tachycardia pacing which involved stimulating the heart through the defibrillator wires in a different way. It appears to work better and seems less likely to destabilise the heart rhythm, hence can reduce painful shocks.

Inclusion Criteria:

• Participants who are due to have a new defibrillator (including cardiac resynchronisation defibrillator) implant or box change

Exclusion Criteria:

• Contraindications to defibrillator testing e.g. severe untreatable coronary disease Intracardiac thrombus Interruption of anticoagulation Participants undergoing box change, device upgrade or revision Inability or unwillingness to provide informed consent.