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.