Heart failure (HF) is one of the clinical syndromes with the highest morbidity and mortality rates among cardiovascular diseases. Despite pharmacological treatments, symptoms persist in a significant proportion of patients, and mortality cannot be prevented. Therefore, device-based therapies, particularly cardiac resynchronisation therapy (CRT), have gained an important place in the treatment algorithm.CRT aims to correct mechanical dyssynchrony by enabling simultaneous contraction of the left and right ventricles. It also aims to reduce sudden cardiac death rates with its defibrillator feature. Current guidelines emphasise that CRT provides significant benefits in terms of symptomatic improvement and long-term survival when appropriate patient selection is made. However, not all patients who meet the criteria have benefited equally from CRT. It is difficult to predict in advance which heart failure patients will benefit from CRT.The aim of the study is to analyse blood gas parameters obtained from the coronary sinus during the procedure in cases of symptomatic heart failure with low ejection fraction despite optimal medical treatment and in patients with indications for CRT according to current guidelines. This analysis aims to contribute to the prior identification of patients who may benefit.

Inclusion Criteria:

1. Patients diagnosed with heart failure, with an EF <35, and in heart failure stages 2-3-4 despite optimal medical therapy.
2. Those who agree to participate in the study.
3. Patients aged 18 years and older.
4. Patients with a QRS duration of 135 ms or more on ECG.

Exclusion Criteria:

1. Patients with EF 35 or higher.
2. QRS duration on ECG less than 130 milliseconds.
3. Not receiving the maximum tolerable dose of medication for KY.
4. Absence of left bundle branch block morphology on ECG.
5. History of acute myocardial infarction within the last 3 months.
6. Estimated life expectancy of less than 1 year.
7. History of cardiac surgery within the last 3 months.
8. Elective cardiac surgery planned.
9. Patients scheduled for CRT prior to nodal ablation.
10. Patients with intracardiac shunts.
11. Those who refuse to participate in the study.
12. Chronic lung diseases (COPD, emphysema, bronchitis, etc.). -

Heart failure patients with an ejection fraction below 35, NYHA physical capacity of 2-3, and QRS duration longer than 135 ms on ECG, whose symptoms persist despite optimal medical treatment.

Individual patient data will be added to the data system with the written consent of the patients and will be destroyed after statistical analysis. Other researchers are not involved in this process; therefore, these data will not be shared with other researchers.