HIPACE High Frequency Low Energy Pacing to Terminate Fast Ventricular Arrythmias (HIPACE)

People that have survived, or are at high risk of a lifethreatening ventricular arrhythmia are routinely offered a defibrillator (ICD). An ICD is an implanted device which can recognise a ventricular arrhythmia and then try to treat it, either by rapidly pacing the heart or by shocking the heart. Currently, very fast ventricular arrhythmias, known as VF, can only be treated with a shock. Patients find shocks painful and this study is intended to find out whether it is possible to pace the heart at very high rates during VF. It is hoped that if this is possible, further research could lead to effective treatment of VF with pacing rather than shocks, and therefore better outcomes for patients.

The primary outcome measure is to determine whether it is possible to obtain ventricular capture of the heart by repetitive pacing at very high rates during ventricular fibrillation.

Inclusion Criteria: Patient is older than 18 years of age; Any patient with a Medtronic cardiac implantable defibrillator undergoing defibrillation testing; No contra indication to defibrillation testing Patient is able to give informed consent. Exclusion Criteria: Patient is younger than 18 years; Patient is not able to give informed consent or is unwilling to participate in the study; Patient has a contraindication to defibrillation testing