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It is hypothesized that elevated BNP level correlate with an elevated thoracic impedance/fluid index as measured separately by CRT-D devices and external impedance cardiography. Ultimately, it is also hypothesized that both BNP and thoracic impedance/fluid index measurements are predictive of atrial and ventricular arrhythmias.

The primary aim of this study:

1. To assess the correlation between elevated brain natriuretic peptide levels and elevated thoracic impedance/body fluid index as measured by selective biventricular resynchronization devices and an external impedance cardiography device.
2. Correlate impedance measurements and brain natriuretic levels with occurrence of atrial and ventricular arrhythmias as assessed by interrogation of biventricular devices.

Inclusion Criteria:

• Male or female between 18 to 85 years of age.
• All patients with biventricular implantable cardio-defibrillators(manufacturer: Medtronic/ model: InSync 7299/7297 or newer).
• LVEF <35%
• NYHA III/IV
• QRS >120 msec
• Pt willing and able to sign informed consent.
• Conventional heart failure therapy
• Clinically stable for six months.

Exclusion Criteria:

• age less than 18 years of age
• age greater than 85 years of age
• Creatinine > 2.5 mg/dl.
• End stage liver disease complicated by ascites as determined by electronic medical record review.
• women who are pregnant, lactating, or plan to become pregnant during the course of the study.
• Patients who are heart transplant candidates with expected transplantation within the next six months.
• Life expectancy due to non-cardiac cause less than one year.
• Anticipated problem with compliance.
• Critical valvular stenoses/insufficiencies.
• Morbidly obese patients(>300 lbs.)
• In patients whom impedance cardiography was not able to be performed because of inability to place sensors.
• Planned or known need for revascularization procedures within three months.

Potential candidates with criteria for implantable cardiac resynchronization therapy who have or will undergo implantation of a Medtronic CRT-D device with thoracic impedance measurement capabilities, will be eligible to enter the study if they meet all of the inclusion criteria and none of the exclusion criteria. Patients will be enrolled in the study either at the time of device implantation or if they have already undergone device implantation will be enrolled at next routine outpatient clinic visit.