An adult patient (aged ≥40 years) with ASA II-IV who was scheduled for elective open heart surgery under cardiopulmonary bypass (CPB) in our clinic between March 1, 2022 and October 30, 2022 will be prospectively included in this study. Patients included in the study will be divided into two groups. At least two of the preoperative biomarkers (Mg\<0.85 mmol/L, Hgb\<8.5 mmol/L, proBNP\>480 pg/mL, CRP\>5 mg/L) that are thought to be closely related to acute kidney injury after cardiac surgery Group I, patients who meet at least two of the preoperative biomarkers (Mg\>0.85 mmol/L, Hgb\>8.5 mmol/L, proBNP\<480 pg/mL, CRP\<5 mg/L) Group II will be included in the study.Standard monitoring such as electrocardiogram, non-invasive blood pressure and pulsoximetric saturation (SpO2), cerebral oximetry (bSO2, NIRS) will be performed in all open heart surgery patients. To prevent possible kidney damage, colloid fluids and mannitol will not be used as prime solution and replacement fluid in both groups. As in routine open heart surgery, mean arterial pressure (MAP) will be allowed to vary between 55 and 70 mmHg, the volume deficit will be replaced with erythrocyte suspension and/or crystalloid replacement in the form of 500 ml boluses, keeping the hematocrit above 25%. In case of need for a vasoactive agent, dopamine and/or adrenaline infusion is started in the standard open heart surgery procedure. After the operation, the patients are transferred to the cardiovascular surgery intensive care unit (ICU) under the effect of orotracheal intubation and conscious anesthesia. Demographic characteristics of patients (age, gender, height, weight, body mass index, diabetes mellitus, hypertension, peripheral vascular disease, smoking history, hyperlipidemia, atrial fibrillation history, left ventricular ejection fraction) European Cardiac Operative Risk Assessment System (EuroSCORE), baseline bSo2, preoperative magnesium, proBNP, CRP, hemoglobin, serum urea, creatinine value and calculated glomerular filtration rate, cystatin C value, diagnosis, surgical procedure, cardiopulmonary bypass time and cross clamp time will be recorded. Patients with high preoperative Cystatin C levels will be excluded from the study.Serum urea, creatinine concentration and GFR values will be evaluated and recorded at admission to the hospital, one day before surgery, admission to the intensive care unit, and in the mornings of the following 7 days. Cystatin C concentration will be evaluated and recorded the day before surgery, on the first postoperative day (24 hours after surgery).Heart apex, mean arterial pressure, Spo2, bSO2, hemoglobin and hematocrit concentration, lactate, hourly urine volume, diuretic requirement, replaced blood and fluid volume, vasoactive agents before induction of anesthesia {1}, directly before skin incision {2}, after sternum detachment {3}, 20 min after aortic cross clamp {4}, 40 min after aortic cross clamp {5}, 20 min after aortic cross clamp removal {6}, 20 min after CPB release {7}, and {8}eight operations will be recorded in time 60 minutes after leaving the CPB. In the postoperative period, daily total balance, diuretic requirement and vasoactive requirement will be recorded 7 days after the operation. Postoperative complications of the patients, length of stay in the intensive care unit, duration of mechanical ventilation, 28-day mortality will be recorded. The incidence of acute kidney injury will be compared between the patients in Group I and Group II according to the criteria of the Global Result of Improvement of Kidney Disease (KDIGO) criteria and postoperative cystatin C levels during the intraoperative and postoperative period.