Sudden cardiac arrest is one of the leading causes of death in Europe. The recommended treatment for cardiac arrest is cardiopulmonary resuscitation (CPR) and electrical defibrillation with an automated external defibrillator (AED). The chain of survival, which connects all links of basic and additional resuscitation procedures and post-resuscitation care, is important for successful survival. Most patients after out-of-hospital cardiac arrest die in intensive care units due to the withdrawal of active treatment, based on the prediction of a poor neurological outcome. Prognostic indicators are influenced by many factors, including the patient's neurological condition. This should be taken into account when predicting outcome and a multimodal approach based on clinical examination, electrophysiological examinations, imaging studies and biological markers should be used. Based on the professional literature, we asked ourselves the research question of whether it is possible to achieve changes in cerebral oxygen saturation values using simple tests and whether such changes in cerebral oxygen saturation coincide with other methods used to assess neurological outcome and brain damage in patients after primary out-of-hospital cardiac arrest. The patient will have a device installed to detect cerebral oxygen saturation (NIRS) throughout the entire hospitalization period. The level of oxygen in the brain is influenced by various factors: mean arterial pressure, cardiac index, carbon dioxide level in the blood, hemoglobin level in the blood, depth of sedation, body temperature. We will attempt to achieve changes in cerebral oxygen saturation with two simple tests. The first test is based on increasing blood flow through the brain through a controlled gradual infusion of a drug that raises blood pressure. The patient will receive such an infusion immediately upon admission and stabilization in the Department of Intensive Internal Medicine and then in the period of 6, 12, 24, 48 and 72 hours after admission. The second test is based on passive elevation of the lower extremities, which results in a transient increased cardiac index produced by the heart and a consequent increase in blood flow through the brain. The second test will also be performed on the patient immediately upon admission and stabilization in the Department of Intensive Internal Medicine and then in the period of 6, 12, 24, 48 and 72 hours after admission. Based on the data obtained with the help of the aforementioned tests, we will not decide on the prognosis of the patient's neurological outcome or change or adjust the therapy in any way based on the data obtained. The aim of the study is to demonstrate that changes in cerebral oxygen saturation values in patients after primary out-of-hospital cardiac arrest, obtained with simple tests, coincide with other methods used to assess neurological outcome and brain damage.