| CHAPTER | ||
| 23 | Depolarization Abnormalities in Arrhythmogenic Right Ventricular Cardiomyopathy | |
| Fabrizio R. Assis, MD; Harikrishna Tandri, MD |
INTRODUCTION
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a familial cardiomyopathy characterized by predominantly right ventricular (RV) dysfunction and a high prevalence of malignant ventricular arrhythmias.1–3 Recent data suggests varying degrees of left ventricular (LV) involvement in the majority of cases of ARVC and left-dominant variants with primary LV disease.4–7 The 12-lead surface electrocardiogram (ECG) plays an important role in the diagnosis of this rare genetic cardiomyopathy. The characteristic appearances on the 12-lead ECG precedes structural changes and are characterized broadly into depolarization and repolarization abnormalities. This chapter discusses the diagnostic utility of various depolarization abnormalities seen in arrhythmogenic cardiomyopathy and their value in risk stratification.
Abnormalities in cardiac depolarization on the surface electrogram was one of the first diagnostic findings reported in arrhythmogenic cardiomyopathy as early as the 1980s. This finding, termed “parietal block,” was considered a major diagnostic criterion for ARVC.8,9 Parietal block represented selective prolongation of depolarization in the RV with preserved depolarization pattern in the LV. This was measured by comparing the QRS duration in the right precordial leads with the left precordial QRS duration. The premise of parietal block was that the right precordial leads represented RV activation, and the left precordial leads predominantly measured LV depolarization. This may not be entirely true; however, slowing of conduction within the ventricle was recognized ever since the disease was described.2 Different measurements of such regional activation delay were proposed and collectively reflect the same pathophysiological phenomenon. The slowing of conduction was attributed to patchy fibrosis mostly distributed within the RV myocardium. This data precedes genetic testing and our current understanding of the pathogenesis of ARVC.
PATHOPHYSIOLOGY OF DEPOLARIZATION CHANGES IN ARVC
Conduction slowing in ARVC occurs by more than one mechanism, as we know today. To better understand the pathophysiology behind conduction slowing, one must understand the normal pattern of electrical activation within the ventricles.
During normal sinus rhythm, the endocardium of the LV toward the apex is activated first with near-simultaneous activation of the RV at the insertion of the right bundle branch in the mid-anterior wall of the RV. Conduction in the LV through the Purkinje fiber network is very well described; in contrast, a robust Purkinje network has never been demonstrated in the RV. As such, propagation of the electrical activity in the RV relies on cell-to-cell propagation reaching the RV outflow tract and the basal subtricuspid region toward the end of the QRS. Therefore, defective cell-to-cell electrical communication is more likely to slow conduction in the RV, compared to the LV. It is not coincidental that the latest-activated regions in sinus rhythm, such as the RV outflow tract and the basal subtricuspid region, are the first to be affected in ARVC.
A defect in the cardiac desmosomes results in disruption of the cellular gap junctions and affects connection trafficking to the gap junctions.10–12 This results in both mechanical and electrical dysfunction, which may play a critical role in the structural and arrhythmic manifestations of ARVC. All the depolarization criteria in ARVC are partly related to this lack of electrical coupling between cells. In advanced ARVC, interstitial fibrosis and replacement fibrosis lead to further electrical delay.
DEPOLARIZATION ABNORMALITIES IN ARVC
RV activation delay is the electrical hallmark of the disease and sets the stage for all depolarization abnormalities. Several depolarization changes have been reported as valuable ECG parameters to diagnostic assessment in ARVC (Table 23.1). However, although those features represent different measurements of the same underlying phenomenon (slow conduction within RV substrate), they vary in diagnostic value across different studies (Table 23.2), and only a few of them are objectively included in the revised Task Force Criteria (Table 23.3).14
Table 23.1 Definitions of ECG Depolarization Parameters in ARVD/C
| Parameters | Definition |
|---|---|
| Right precordial QRS prolongation | QRS duration in V1–V3 ≥ 110 ms |
| Localized precordial QRS prolongation | QRS duration (V1 + V2 + V3)/ QRS duration (V4 + V5 + V6) ≥ 1.2 |
| Precordial QRS amplitude ratio | QRS amplitudes (V1 + V2 + V3)/ QRS amplitude (V1 + V2 + V3 + V4 + V5 + V6)* |
| Prolonged S-wave upstroke | Nadir of S wave to isoeletric line ≥ 55 ms |
| Epsilon waves | Distinct waves of small amplitude that occupy the ST segment in the right precordial leads |
| Parietal block | QRS duration in V1–V3 that exceeds the QRS duration in lead V6 by > 25 ms** |
| Complete right bundle branch block | QRS duration ≥ 0.12 s, secondary R wave in right precordial leads, and wide S wave in leads I and V6 |
| Incomplete right bundle branch block | QRS duration ≥ 100 ms and < 120 ms, secondary R wave in right precordial leads, and wide S wave in leads I and V6 |
| QRS dispersion | Difference between the maximum and minimum QRS complex duration occurring in any of the 12 ECG leads ≥ 40 ms |
| QRS fragmentation | Deflections at the beginning of the QRS complex, on top of the R wave, or in the nadir of the S wave |
| Terminal activation duration | Longest duration in V1–V3, from the nadir of the S wave to the end of all depolarization deflections (including not only the S-wave upstroke but also both late fractionated signals and epsilon waves) ≥ 55 ms |
*Recently proposed ECG criterion for arrhythmic risk stratification in ARVC (potential surrogate for RV myocardial mass and arrhythmogenic substrate).29
**Parietal block may represent the presence of an epsilon wave that merges with the QRS complex and appears as a soft R wave.
Table 23.2 Sensitivity and Specificity of ECG Depolarization Abnormalities in ARVD/C
| Parameters* | Sensitivity | Specificity |
|---|---|---|
| QRS in V1–V3 ≥ 110 ms9,16,18,20,34–37 | 26–80% | 82–100% |
| Localized precordial QRS prolongation9,16,18,20,35,36 | 36–100% | 92–100% |
| S-Wave upstroke in V1–V3 ≥ 55 ms14,16,18,20,36,37 | 34–95% | 94–100% |
| TAD ≥ 55 ms16,18 | 71–85% | 96% |
| Epsilon waves14,16,20,34,35,37 | 17–61% | 92–100% |
| QRS fragmentation18 | 85% | ** |
| QRS dispersion ≥ 40 ms20,24 | 44–90% | 77–99% |
| Signal-averaged ECG13,38–40 | 47–100% | 90–100% |
| Complete right bundle branch block14,34 | 6–20% | – |
| Incomplete right bundle branch block14,34 | 14–17% | – |
*See Table 23.1 for parameter definitions.
**A valid specificity value could not be drawn due to few numbers of controls (n = 2).
Table 23.3 Depolarization/Conduction Abnormalities Criteria in Original (1994) and Revised (2010) Task Force Criteria Recommendations
Epsilon waves are considered to be pathognomonic of ARVC. Shown in Figure 23.1
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
