Evaluation of the effect of short term activation/deactivation of biventricular pacing (BivP) of the CRT (during routine CRT interrogation) on vascular function as assessed via retinal vessel analysis (RVA), in patients treated with CRT.

Heart failure is a condition in which the heart becomes unable to maintain the body's need of blood supply. Congestive heart failure can be considered a syndrome but the final common path of many cardiovascular diseases. Recently, the investigators of this study confirmed an increased impairment in retinal microvascular function in patients with ischemic heart failure compared to patients with stable coronary disease.

If medication fails to improve ejection fraction, cardiac resynchronization therapy (CRT) is the guideline-recommended treatment for patients with advanced heart failure and bundle branch block.

The question remains if CRT changes purely hemodynamics by synchronizing the heart or has potential impact on the microvasculature to cause reverse remodeling of the failing heart. Measuring retinal vascular function might increase knowledge on the effects of cardiac resynchronization therapy.

Inclusion Criteria:

• Informed Consent as documented by signature
• Patients ≥ 18 years of age, male or female, diagnosed with advanced heart failure
• Implanted as well as activated CRT device for at least 3 months prior to Visit 1

Exclusion Criteria:

• Current acute decompensated HF
• Documented pacing dependency
• Documented AV-Block II (Mobitz Typ 2) or III in patient's history
• History of hypersensitivity or allergy to Tropicamide
• Acute coronary syndrome, stroke, transient ischemic attack, cardiac, carotid or other major cardiovascular (CV) surgery, percutaneous coronary intervention (PCI) or carotid angioplasty within 3 months prior to Visit 1.
• History of heart transplant, on a transplant list or with ventricular assistance device (VAD).
• Presence of any other disease with a life expectancy of < 6 months
• Presence of significant endocrine diseases, including primary hyperparathyroidism, Cushing's disease, adrenal insufficiency, pituitary tumors, primary hyperaldosteronism, manifest hyperthyroidism or genetic endocrine disorders
• Presence of active acute infectious diseases.
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc
• Women who are pregnant or breast feeding
• Known narrow-angle glaucoma
• Known epilepsy (flicker-light could trigger a seizure)

It is not yet decided in what way there will be data sharing.