• ventricular fibrillation
• electrocardiography
• defibrillators, implantable

The prediction of sudden cardiac death remains the ‘holy grail’ of cardiology to enable optimal targeting of preventative therapies in at-risk groups. This challenge is made greater by the fact that the numerical majority of patients at risk of sudden death due to ventricular tachycardia/fibrillation (VT/VF) have ejection fractions greater than the 35% cut-off employed in guidelines for internal cardiac defibrillators (ICDs) despite this being the crudest but most effective differentiator as highlighted by Myerberg and Junttila.1 Although ICDs have proven to be highly efficacious, less than 1 in 10 of the implanted devices are actually needed. Hence, a large number of patients are unnecessarily exposed to their complications such as infection and inappropriate shocks, and needlessly contribute to the escalating cost, estimated as in excess of 2 billion euros per annum in Europe alone. Mechanistically, electrical instability of conduction and repolarisation underlies the development of VT and its degeneration into VF due to wavebreak of the activation wavefronts. Leaving aside the impact of plaque rupture causing acute coronary occlusion and major ischaemic insult to trigger VF, we are left with the concept of a myocardial substrate susceptible to re-entry and subsequent VF.

Conceptually, the electrical behaviour of the heart can be viewed as an oscillator or pendulum which must be constrained within a tight dynamic envelope (figure 1). Excessive changes in conduction and repolarisation disturb this equilibrium, setting up the conditions for ventricular arrhythmias. Two schools of thought currently evaluate the substrate depending on the perspective of the investigators’ technology focusing on fibrosis with imaging using the latest cardiac magnetic resonance imaging (CMR) techniques or dissecting the fundamental electrical behaviour of the heart. The earliest insight into the role of electrical instability comes from Lewis’ observation of T wave alternans degenerating into VF.