T-wave Alternans and Intrathoracic Impedance Measurements

T-wave alternans is a test that is currently being used to risk stratify patients with structural heart disease for sudden cardiac death. The mechanism of T-wave alternans is unclear, but may share a common abnormality with conditions of cardiac fluid overload such as heart failure, which is altered intracellular calcium handling. Current Medtronic implantable defibrillators have the capability of monitoring cardiac fluid status via transthoracic impedance measurements. The purpose of this study is to determine if a correlation exists between T-wave alternans status and cardiac volume status, as determined by transthoracic impedance measurements. Secondarily, the study seeks to examine the relationship between arrhythmia frequency and T-wave alternans or cardiac volume status.