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) <or= 0.40. Mean LVEF was 0.29 +/- 0.13. ICA was detected at either the apex or base in 11 patients. ICA was concordant with MTWA in 87% (59 of 68) of the patients. ICA occurred at a greater magnitude than MTWA (3 +/- 2 mV vs. 2 +/- 2 microV, P <0.05). At a mean follow up of 4 years, 50% (34 of 68) of patients had an ICD implanted. Of patients with either a positive ICA or MTWA test, 49% (8 of 17) had an ICD implanted with appropriate shock in 75% (6 of 8) of the patients. Of patients with both normal ICA and MTWA tests, 51% (26 of 51) had an ICD for primary prevention, and appropriate ICD therapies occurred in 27% (7 of 26). After 1 year follow-up in patients with ICDs, the positive predictive values of ICA and MTWA in predicting ventricular arrhythmias were 14% and 17%, respectively. The negative predictive values for ICA and MTWA were both 82% at 4 years.
Conclusion: ICA is detectable from the right ventricle and can predict the patients at low risk for ventricular arrhythmias. Future applications of ICA may provide an integral part of arrhythmia detection and development of algorithms to divert shock therapy in ICDs.