Electrical Alternans: Physiology and Clinical Significance
Definition and Core Physiology
- Electrical alternans represents beat-to-beat alternation in cardiac electrical properties at the cellular level, arising when heart rate exceeds the capacity of cardiac cells to cycle intracellular calcium, creating electrical heterogeneities between neighboring myocytes that directly promote arrhythmogenesis 1, 2
Cellular Mechanisms
- T-wave alternans reflects repolarization alternans at the single-cell level, occurring when cardiac cells cannot adequately manage calcium cycling at elevated heart rates 1, 3
- The phenomenon is rate-dependent and manifests at relatively lower heart rates (105-110 bpm) in patients susceptible to life-threatening ventricular arrhythmias 4, 5
- Alternans amplifies electrical heterogeneities between neighboring cardiac cells, creating the substrate for reentrant arrhythmias 1, 6
Clinical Risk Stratification
- Exercise-induced T-wave alternans testing is superior to pacing-induced testing for arrhythmic risk prediction, with a target heart rate of 105-110 bpm in adults 2, 4, 6
- A positive test is defined as >1.9 μV alternans with K-score >3 sustained for >2 minutes, and requires special high-resolution electrodes and spectral analysis using Fast Fourier Transform 4, 5
- T-wave alternans predicts sudden cardiac death and major arrhythmic events as well as or better than LVEF, electrophysiological testing, and other markers, with a hazard ratio for 2-year mortality of 4.8 for abnormal T-wave alternans versus 1.5 for prolonged QRS duration 1, 2, 3
Associated Pathological Conditions
- Dilated cardiomyopathy and hypertrophic cardiomyopathy are associated with increased sympathetic nerve activity and fiber disarray, which can provoke T-wave alternans 4, 5
- Both ischemic and nonischemic cardiomyopathy show significant event rates with abnormal alternans, with a relative risk of 2.42 in ischemic heart failure and 3.67 in nonischemic heart failure 3, 7, 8
Management Implications
- Abnormal T-wave alternans identifies patients most likely to benefit from ICD therapy, and is a better predictor than QRS duration for ICD benefit in patients with LVEF ≤30% 3
- Negative alternans with ischemia identifies patients most likely to benefit from revascularization for survival, while positive or indeterminate results warrant aggressive risk factor modification and consideration for primary prevention ICD 3, 6