Recent studies have raised concerns about ERP-related SCA. However, little data on risk stratification of ERP are available. In 1961, Wasserburger et al. defined early repolarization as an ST-segment elevation at the J junction of the QRS complex accompanied by downward concavity of the ST segment and suggested that it is a normal variant . Subsequent long-term studies failed to find any adverse consequences [13, 14]. In 2000, Gussak et al. suggested that an ERP may cause malignant arrhythmias by showing that the ERP reflecting dispersion of repolarization in arterially perfused canine left ventricle wedge preparations facilitates ventricular arrhythmia by phase-2 re-entry . Case control studies have shown that the prevalence of ERP was statically higher in the idiopathic ventricular fibrillation group than in the controls. In 2008, Haissaguerre et al. and Nam showed a significant association between ERP and ventricular fibrillation in their case control studies [1, 2]. Furthermore, subsequent studies based on general population cohorts suggested that an ERP is associated with sudden cardiac death in Western and Asian populations [4–6].
However, it is inappropriate to compare previous studies with recent studies suggesting ERP is associated with SCA because the definition of ERP in recent studies differs from that of Wasserburger . The classical definition of ERP focused on the downward concavity of the ST segment in the lateral leads because of its critical association with myocardial infarction and pericarditis . On the other hand, a new definition suggested by Haissaguerre et al. only requires a slurred or notched J-point elevation of >0.1 mV in two contiguous inferior or lateral leads; ST-segment elevation is not necessary. In a recent study, Tikkanen et al. subgrouped the ERP into an ascending ST segment and horizontal ST segment by ST-segment morphology . The definitions of ascending ST segment and horizontal ST segment in the present study originated from their work. These definitions represent ERP subgroupings as one compatible with the old description and another that was not originally considered to be early repolarization . They showed that an ERP with a horizontal/descending ST segment, but not an ERP with an ascending ST segment, was associated with an increased risk of sudden cardiac death in the general population . Uberoi et al. also found no significant association between an ERP with an ascending ST segment and cardiac mortality in their outpatient-based retrospective population study . Another recent study by Rosso et al. reported that J-point elevation with a horizontal ST segment has a three-fold higher odds ratio for idiopathic ventricular fibrillation compared with J-point elevation only (OR, 13.8 and 95 % CI, 5.1–37.2 vs. OR, 4 and 95% CI, 2.0–7.9, respectively) and concluded that the combination of J-point elevation with a horizontal ST segment improves the ability to distinguish malignant forms of ERP . Interestingly, the ERP itself, which was suggested to be a predictor of sudden cardiac death, was not associated with SCA in this study; however, an ERP with a horizontal ST segment was associated with SCA. This may have resulted not only from the small number of subjects in this study, but also the low relative risk of early repolarization per se . The results of this study support the conclusions of the studies by Tikkanene et al. and Rosso et al. Other suggested malignant features (J-point elevation of >0.2 mV and ERP location in the inferior wall) were associated with SCA, and this result is also consistent with those of previous studies [3, 5]. To the best of our knowledge, this study is the first case control study to examine the prevalence of malignant ERP features in patients with SCA. The prevalence of the ERP in the control group was 6.7%. This result is comparable with that in a study by Tikkanen et al., who showed that the prevalence of ERP was 5.8% among 10,864 middle-aged North European subjects . According to the study by Uberoi et al., the prevalence of the ERP with an ascending ST segment among 29,181 Americans was 2.3%, and it was 3.3% in the control group of this study .
Explanation for the favorable outcomes of an ERP with an elevated ST segment is not sufficient. In the study by Tikkanen et al., the subjects in the ERP group with an ascending/upsloping ST segment were younger and had a lower blood pressure, lower heart rate, shorter QTc duration, and higher prevalence of left ventricular hypertrophy on ECG. These results suggest that an ERP with an ascending ST segment might occur secondary to ECG changes in athletes . In addition, Bastiaenen et al. showed that early repolarization with a horizontal ST segment may be associated with abnormal depolarization based on the finding that subjects with early repolarization with a horizontal ST segment had a longer QRS duration . However, the evidence for abnormal depolarization in early repolarization syndrome is still controversial , and the association between early repolarization with a horizontal ST segment and late depolarization has not been well studied.
First, the number of subjects was small. Further large and prospective population-based studies are needed to confirm the risk of early repolarization with a horizontal ST segment. Second, electrophysiology tests were not performed in this study. Recent studies demonstrated that the prevalence of abnormal late potentials, which are usually associated with depolarization abnormality, is not low, suggesting that the ERP is associated with depolarization abnormality [18, 19]. Thus, further studies involving large numbers of patients with SCA and associated ERP are needed to understand the underlying pathophysiology of malignant ERP. Third, the provocation to induce coronary spasm could not be performed in the aborted SCA group because of the inversed risk-benefit balance. Obtaining a detailed history during the post-cardiac arrest follow-up would limit the possibility of coronary spasm.