Heart failure is a major health problem in Canada. Recent advances in medical and device therapy have helped to reduce the morbidity and mortality of patients with this problem. Among these treatments, cardiac resynchronization therapy (CRT) has very recently been shown to be effective to improve functional class, quality of life and exercise tolerance of the patients with the most severe symptoms of heart failure and a prolonged duration of the QRS on the 12-lead Electrocardiography (ECG).

Resynchronization of the failing ventricle is currently achieved by pacing the left and right ventricles simultaneously with specialized electrodes and a cardiac stimulator. However, controversy persists concerning the optimal configuration for cardiac pacing in these patients. Right ventricular pacing alone has been shown to be deleterious in some patient populations. The benefits of biventricular pacing in heart failure patients may be due primarily to left ventricular stimulation and may, in some patients, be decreased by the presence of simultaneous RV stimulation. Preliminary data from our own animal work suggest that in the majority of cases, LV stimulation alone is better than RV stimulation, and that BiV stimulation represents an intermediary situation between LV and RV stimulation.

Inclusion Criteria (for Greater and Lesser EARTH):

Patient must answer "yes"

1. Does the patient require an ICD or an ICD replacement?
2. Does the patient have a documented LVEF less than or equal to 35% measured in the previous 6 months (without major concomitant clinical event)?
3. Does the patient have a QRS duration < 120 ms?
4. Is the patient in sinus rhythm?
5. Was the six-minute walk test distance less than 400 meters and limited by heart failure symptoms?

Exclusion Criteria:

Patient must have answered "NO" to all of the exclusion criteria

1. Does the patient have:

   • Indication for permanent ventricular pacing?
   • Chronotropic insufficiency?
   • Second or third degree AV block, either persistent or intermittent?
   • A pacemaker or an ICD which is paced in ventricular chamber more than 5% of the time?

2. Does the patient have a reversible cause of LV dysfunction such as post-partum cardiomyopathy, tachycardia induced cardiomyopathy, acute myocarditis or acute toxic cardiomyopathy (including acute alcoholic)?

3. Did the patient have myocardial infarction or cardiac surgery in the 6 weeks preceding the pre-implant visit?

4. Does the patient have a moderate or severe cardiac valve stenosis?

5. Is the patient's capacity to walk is limited by reasons other than heart failure symptoms (e.g., angina, intermittent claudication, severe lung condition or arthritis)?

6. Does the patient have severe coexisting illnesses making survival > 6 months unlikely?

7. Is the patient pregnant and/or nursing?

8. Is the patient unable or unwilling to consent or to comply with follow-up requirements?

9. Is the patient participating in another clinical study potentially interfering with the present trial?

10. Does the patient have a resynchronization system in place?