Intracardiac alternans compared to surface T-wave alternans as a predictor of ventricular arrhythmias in humans.

BACKGROUND: Microvolt T-wave alternans (MTWA) measured from the surface electrocardiogram (ECG) is a marker of sudden cardiac death (SCD). Recently, it has been suggested that intracardiac alternans (ICA) detected from the endocardium underlies MTWA and is a marker of electrical instability leading to ventricular arrhythmias. As such, ICA may be used in future implantable cardioverter-defibrillators (ICDs) to monitor periods of electrical instability before ICD therapy. We examined whether electrical instability, as measured by MTWA, can be detected by ICA located from the right ventricle and if ICA can predict ventricular arrhythmias in patients with ICDs. METHODS: Both MTWA and ICA were measured simultaneously during atrial pacing in 68 patients undergoing electrophysiology study (EPS). ICA was measured from unipolar electrograms acquired from a catheter at the apical, mid, and basal regions of the right ventricle in 48 patients and at the apical region alone in 20 patients. Indications for EPS included nonsustained ventricular tachycardia, cardiomyopathy, syncope, near syncope, or palpitation. RESULTS: Fifty-six of 68 patients had cardiomyopathy with left ventricular ejection fraction (LVEF)