New simpler methods to optimize cardiac resynchronization therapy (CRT) are being evaluated, such as the use of the electrocardiogram.

In this prospective, double-blind, study, the investigators will evaluate:

Primary endpoint: To compare the clinical response to CRT in patients with the programming optimized by QRS versus the nominal suggested by the device.

Secondary endpoints:

1. To compare cardiac remodeling in patients with the programming optimized by QRS versus the nominal suggested by the device.
2. To evaluate the degree of asynchrony in both groups (nominal versus optimization by QRS) with respect to the intrinsic rhythm.
3. To evaluate the echocardiographic improvement in left ventricular (LV) filling in both groups.
4. To evaluate the differences in the optimal atrioventricular (AV) delay with atrial sensing or atrial pacing.

For that, 180 patients with an indication for CRT will be randomized to nominal programming of the device or optimization by the electrocardiogram for a narrower QRS. A clinical and echocardiographic evaluation will be done at baseline, 6 and 12 months.

Inclusion Criteria: patients with standard indication of cardiac resynchronization

• Ejection fraction ≤35%.
• QRS duration ≥120 ms.
• New York Heart Association Functional Class 2-4..
• Left ventricular diameter ≥55 mm.
• Optimal medical treatment.

Exclusion Criteria:

• Patient refusal.
• Cardiopathy with a reversible cause.
• Prevision of heart transplantation.
• Atrial fibrillation.
• Complete AV block.
• AV delay >250 ms.
• Right bundle branch block.
• Severe peripheral vascular disease.
• Other diseases with < 1 year life expectancy