Introduction on Sudden Cardiac Death Risk Stratification – An Update
Sudden cardiac death (SCD) can be defined as unexpected death that occurs within one hour of the onset of symptoms or during sleep in a person who was previously stable. The mode of death, which may be due to an arrhythmic or non-arrhythmic cause, depends on the underlying cardiovascular abnormality (mechanical or electrical substrate). SCD remains a major public health problem worldwide and is estimated to account for 15–20 % of all deaths.1 Those deemed to be at highest risk of SCD may benefit from potentially life-saving treatment, such as insertion of an implantable-cardioverter defibrillator (ICD). However, such treatments are expensive and have their own associated risks, such as the risk of long-term infection and need for repeated surgeries and regular check-up. Hence, there is great interest and clinical need in improving methods for risk stratification of SCD to identify those at greatest risk.
Several factors make research into SCD risk stratification difficult and challenging. First, current guidelines and most international randomised studies on SCD have focused on specific high-risk groups – in particular, patients with reduced (below 30–35 %) left ventricular ejection fraction (LVEF) – with the aim of identifying those patients who would benefit most from an ICD. The latest European Society of Cardiology guidelines on ventricular arrhythmias and SCD recommend ICD therapy in patients with symptomatic heart failure (New York Heart Association [NYHA] class II–III) and LVEF ≤35 % after ≥3 months of optimal medical therapy who are expected to survive for at least 1 year with good functional status.2 Similarly, the American College of Cardiology and American Heart Association guidelines recommend ICD therapy in patients with LVEF ≤35 % due to prior myocardial infarction (MI), at least 40 days post MI, or non-ischaemic dilated cardiomyopathy and NYHA class II or III.3 Although low LVEF identifies patients at increased risk for cardiac arrest, the majority of sudden deaths occur in patients with LVEF greater than 30 %.4,5 Indeed, most cases of SCD occur in the general adult population, in people who may be relatively asymptomatic but have an underlying predisposition to SCD. Current guidelines on SCD risk stratification do not adequately cover this general population pool, as acknowledged in the latest European guidelines,2 yet from a population health perspective, this is the group that should be targeted if healthcare providers are to make any meaningful impact on lowering the incidence of SCD worldwide. Second, the mechanisms leading to SCD are complex and multifactorial, making the development of SCD risk scores and algorithms challenging. Even in those at highest risk of SCD, patient factors, comorbidities and underlying cardiovascular substrate abnormalities that predispose to SCD may change over time. Accurate methods for SCD risk stratification should take into account the changing and heterogeneous risk in each individual over time so that the risk is continually revisited and refined. In view of the wide and varying mechanisms and cardiovascular abnormalities underlying SCD, current methods for SCD risk stratification encompass a variety of modalities, which include imaging techniques to look for underlying mechanical or structural abnormalities, assessment for electrical substrate abnormalities and autonomic dysfunction and genetic analyses (see Figure 1).
This review will give an update of current methods and strategies for SCD risk stratification in high-risk individuals (namely patients with ischaemic and non-ischaemic dilated cardiomyopathy) and in the general public. Discussion of SCD in patients with inherited cardiomyopathies, primary arrhythmogenic diseases and genetic determinants of risk is beyond the scope of this review.