The CRT pacemaker being studied in this clinical trial is approved by the US Food and Drug Administration (FDA) for patients with moderate to severe heart failure, whose hearts pump blood inefficiently. In the MIRACLE EF study, patients who have heart failure with slightly less inefficient hearts will be observed to see if the electrical pacing treatment is better than not getting the treatment. This study is being conducted to support FDA approval of this type of pacemaker for people whose heart failure is less inefficient.