30% of heart failure patients that receive a device for cardiac resynchronization therapy fail to show clinical improvement. The reason for lack of response is still unclear but factors such as scar tissue in the heart musculature, inadequate lead placement, device-settings and the degree of dyssynchrony before implant seems to be important. In this study, these factors are further investigated.

Cardiac resynchronization therapy (CRT) is an established therapy for patients with severe heart failure, depressed left ventricular function and a wide QRS-complex. Large clinical trials have demonstrated unequivocal improvements in functional status, morbidity and mortality. However, 30 % of heart failure patients treated with a CRT-device do not benefit clinically. Several factors have been suggested to be important for the response to CRT such as mechanical dyssynchrony, presence of scar tissue in the myocardium, and device-optimization (among others). It is the purpose of this study to investigate the importance of these factors.

100 patients with ischemic cardiomyopathy, eligible to CRT according to current guidelines, will be included. Patients are randomised to two arms. One group will have atrioventricular (AV)-optimization after implantation, the other AV -and interventricular (VV)-optimization. After 4 months patients are crossed-over to the other arm. Preimplantation patients are MR-scanned and low-dose dobutamine stress-echocardiography is performed. Furthermore patients will be examined by echocardiography and evaluation of clinical status

1. Mechanical dyssynchrony can predict response to CRT. b. Measures of mechanical dyssynchrony is related to myocardial viability and conduction.
2. Individual optimization based on conduction times will increase benefit to CRT. b. The effect of adding VV-optimization is related to myocardial viability.
3. > 30 % of non-viable tissue globally in the myocardium is predictive of lack of CRT- response. b. Non-viable tissue located in the area of the left ventricular lead is predictive of non-response.

Inclusion Criteria:

• LVEF</= 35%, QRS-duration>/= 120 ms, NYHA-class II- IV.
• Ischemic heart disease (> 50% stenosis in 1 or more major epicardial coronary artery or prior PCI or CABG.)
• Optimal treatment ( beta-blocker, ACE-1 or ARB and spironolactone)

Exclusion Criteria:

• Pregnancy
• Unstable angina pectoris
• Chronical atrial fibrillation
• Severe valvular disease
• Dementia or mental retardation
• Severe claustrophobia
• Acute myocardial infarction < 3 months
• Severe health condition threatening short-term survival
• Severe kidney insufficiency, GFR < 35 ml/min/1.73 m2
• Metal implants contraindicative of magnetic resonance scan