The purpose of the study is to characterize atrial arrhythmias in patients indicated for Cardiac Resynchronization Therapy (CRT) and to monitor changes in atrial arrhythmias while CRT is provided.

The combination of congestive heart failure and atrial fibrillation is a common co morbidity, although the exact prevalence of AF in the heart failure population is still unclear. Recent studies show a prevalence of AF ranging from about 10% to 50%, although the type of AF observed and investigated in these studies is not always clearly described.

A number of mechanisms attributed to congestive heart failure may contribute to the development of AF Experimental congestive heart failure promotes sustained AF by ionic remodeling and increased interstitial fibrosis. In contrast to tachycardia-mediated AF, in congestive heart failure no shortening of atrial refractoriness occurs. Atrial tissue stress caused by congestive heart failure may also contribute to promotion of AF by inducing triggered activity, affecting atrial refractoriness properties or resulting in increased tissue mass supporting re-entry [31]. Existence of these mechanisms suggests that treatment of congestive heart failure may also influence the development and progression of AF in these patients. Conversion of chronic AF has been observed in patients with congestive heart failure treated with biventricular pacing Ventricular ionic remodeling likely underlies the increased risk for proarrhythmia in heart failure patients exposed to antiarrhythmic drugs, prolonging the action potential duration , which therefore should be avoided in patients with congestive heart failure.

The independent prognostic significance of AF in heart failure patients is still not completely clear. Results from some recent studies suggest no independent prognostic significance of AF in heart failure patients Result from other large studies on congestive heart failure suggest an independent prognostic effect of AF in patients with AF and congestive heart failure, associated with an increased risk for pump failure death and all-cause mortality a significantly reduced 1-year survival and a higher mortality among heart failure patients who developed AF A recent review with regard to the mortality in studies on congestive heart failure suggests that concomitant AF does not have an additional effect on the mortality in patients with severe heart failure, but does increase the mortality in the setting of mild-to-moderate heart failure This observation may be attributed to the fact that the atrial contribution to left ventricular filling is limited in patients with severe diastolic dysfunction, whereas the atrial contribution may still be of hemodynamic importance in mild-to-moderate heart failure

Inclusion Criteria:

• Patient is willing and able to comply with the protocol
• Patient is willing to sign written informed consent
• Patient is expected to remain available for Follow-up visits
• Patient age is 18 years and older
• patient is on a stable medication regimen (including beta blockers) for at least 4 weeks prior to enrollment
• Baseline criteria: patients should meet all of the following criteria, to be determined at the baseline assessment procedure within 4 weeks prior to device implantation: - New York Heart Association functional classification III or IV
• QRS duration > 130 ms
• Left ventricular ejection fraction < 35% measured by echocardiography left ventricular end diastolic dimension > 55 mm measured by echocardiography

Exclusion Criteria:

• Patients with unstable angina or who have experienced an acute myocardial infarction or received coronary artery revascularization (CABG) or coronary angioplasty (PTCA) within 3 months prior to enrollment or who are candidates for CABG or PTCA
• Patients who have experienced CVA or TIA with permanent disability within 3 months prior to enrollment
• Patient on, or anticipated to require, intravenous inotropic drug therapy
• Patients with severe primary pulmonary disease (such as cor pulmonale)
• Post heart transplant patients and patients on an urgency list for cardiac transplantation
• Supine systolic blood pressure greater than 170 mm
• Patient who are not expected to survive for 8 months of study participation due to other medical conditions
• Women who are pregnant or with child bearing potential and who are not on a reliable form of birth control
• Serum creatinine greater than 250 mol/l
• Untreated hyperthyroidism
• Patients enrolled in any concurrent (drug and/or device) study
• Patients with an existing implantable cardioverter defibrillator (ICD) or indications for an ICD including those patients with sustained VT within the previous month
• Patients with permanent atrial arrhythmias. Permanent atrial arrhythmia is defined as an arrhythmia for which any possible type of cardioversion is not considered or that is recurrent within 24 hours from an attempted cardioversion
• Patients with contraindications for implantation of a cardiac pacing device
• Patients who are already implanted with a cardiac pacing device for purposes other than Cardiac Resynchronization Therapy