A consequence of chronic RV pacing is a process of electrical remodeling that alters myocardial repolarization to reflect the altered depolarization induced by the RV lead. When RV pacing is discontinued, the altered repolarization persists for several weeks. This phenomena is traditionally described as "T-wave memory" based upon the 12-lead ECG appearance of inverted T-Waves. The investigators are using the ECGI technique to produce three dimensional electroanatomical images of this phenomena in patients with dual chamber pacemakers. Echocardiography will also be used to image the mechanical effects of RV pacing and T-wave memory. The images will show the spatial distribution of altered repolarization and allow us to correlate any mechanical consequences of this phenomena that may exist.

Inclusion Criteria:

• Implanted dual chamber pacemaker for sick sinus syndrome with no history of complete heart block.

Exclusion Criteria:

• Persistent atrial fibrillation that is not suppressed by medications.

Patients of a pacemaker clinic