Cardiac resynchronization therapy shows great promise as a method to improve ventricular function in heart failure due to dilated cardiomyopathy. It is applied to patients with class III and IV symptomatic heart failure on optimal therapy. Typical entry criteria in the trials testing resynchronization have used the electrocardiogram and a prolonged electrocardiogram (ECG) QRS interval as criteria for dyssynchrony. This is recognized to be quite limited as a tool for identifying patients. Several new echocardiographic methods for identifying dyssynchrony have been proposed. The purpose of this study is to systematically examine these multiple echocardiographic measurement techniques in three conditions:

1. in the patient's native rhythm simulated by changing the pacing system to atrial pacing only;
2. in conventional dual chamber pacing (DDD mode); and
3. during biventricular pacing. All patients entered into this study will already have had a biventricular pacemaker placed.

Inclusion Criteria:

• All patients who have previously had a resynchronization pacemaker system placed at the University of Wisconsin Hospital or at the Middleton Veterans Hospital are potentially eligible for this study.
• The study is only open to patients as enumerated above, and also patients who have good imaging windows.

Patients who have previously had a resynchronization pacemaker system placed at the University of Wisconsin Hospital or at the Middleton Veterans Hospital are potentially eligible for this study.