The objective of this multicentre prospective therapeutic study is to evaluate the effects of CRT on the right and left ventricular function of patients with CHD in the medium and long term. Patients who reject the randomization or cannot be paced with right ventricular pacing alone will be enrolled in group C with continuous biventricular pacing, which is the main group in this study. Optional: Immediately after implantation the patients are divided into group A and B (randomized, single blind (for the patient), cross-over design).

The treatment and the completing follow-up examination will take approximately 18 months and includes seven visits - one previous to the CRT and six at certain times afterward. At selected time intervals echocardiographic 3D and Tissue Doppler Imaging to evaluate the global and regional ventricular function are performed. Subjective quality of life assessment (questionnaire) will also be performed at the defined follow-up intervals, and if applicable (optional) also objective assessment of the physical performance (VO2 max).

55 patients also including children and adults with CHD are planned to be included in the study. The main target is to provide evidence of the effectiveness of CRT with biventricular stimulation in terms of improved ventricular function (ejection fraction and QRS interval).

Inclusion Criteria:

• Patients with congenital heart defects
• Heart failure of the Morphologically right or left system ventricle with an absolute ejection fraction of < 40% in the failed ventricle
• NYHA classes II-IV
• Optimal drug therapy for heart failure according to the ESC guidelines
• QRS interval ≥ 120 ms (demonstrated in at least two relevant ECG derivations, depending on existing bundle branch block symptoms)
• Patients with chronic stimulation of the subpulmonary ventricle by a 1- or 2-ventricle pacemaker, which also meets the above-mentioned criteria
• Patients with morphologically right ventricle in system position and significant insufficiency of the systemic atrioventricular valve, if they also meet the criteria listed above

Exclusion Criteria:

• PTCA, cardiomyoplasty or myocardial infarction / unstable angina pectoris or cerebral insult within the 6 weeks preceding the planned cardiac resynchronisation therapy
• Patients with pulmonary hypertension / Eisenmenger's syndrome
• Life expectancy < 1 year due to a non-cardiac disease
• Anticipated poor compliance by the patient
• Pregnancy and breastfeeding
• Known or persistent abuse of prescription medicines, recreational drugs or alcohol