After obtaining ethics committee approval, at least 84 patients who will undergo elective open abdominal surgery under general anesthesia in the operating room of the Ministry of Health Ankara City Hospital will be included in the study. The study is a single center, prospective, observational study.

Weight, height, age, gender, BMI (body mass index), ASA information of the patients will be recorded. Induction and anesthesia management will be applied in accordance with the standard working order of the anesthesia clinic. After the patient is taken to the operating table, non-invasive arterial blood pressure monitoring (noninvasive), 5-lead ECG and pulse oximetry, in addition to standard hemodynamic monitoring, Bispectral Index (BIS), temperature and Massimo ® pulse oximeter (Masimo Corp., Irvine, CA, USA). ) will be monitored. General anesthesia induction will be performed with 0.02 mg / kg midazolam, 1mg / kg lidocaine, 1mcg / kg fentanyl and 2mg / kg propofol after preoxygenation in accordance with the routine practice of the anesthesia clinic, and endotracheal intubation will be performed with a suitable tube for the patient to be provided with 0.6mg / kg rocuronium. BIS target value range will be determined as 40-60, and anesthesia maintenance will be provided by modifying the volatile anesthetic concentration and remifentanil infusion dose (0.1-1 mcg / kg / min) in accordance with hemodynamic values. Ventilation mode and ventilation parameters; tidal volume (6-8 ml / kg according to ideal body weight), respiratory rate (12-16) and end-tidal CO2 values in the range of 30-40 mmhg, inspiratory / expiratory ratio 1: 2, positive end expiratory pressure (PEEP) 5 cm H2O and FiO2: will be set as 50%. After the Allen test to be performed preoperatively, radial artery catheterization and invasive blood pressure monitoring of the non-dominant hand, a 7 fr three lumen central catheter will be placed into the right internal jugular vein with ultrasound (USG) and central venous pressure (CVP) monitoring will be performed. These applications are routinely performed in patients scheduled for open abdominal surgery in our clinic. Preferably 16G or 18G , 2 peripheral venous lines will be placed and fluid will not be given before fluid replacement. Measurements will begin after the patient preparation is complete, after a hemodynamically stable period (defined as the period in which the mean arterial blood pressure (MAP) change is \<10% within 5 minutes), in the supine position, and when the patient is confirmed by a second observer that there is no spontaneous breathing effort. Hemodynamic parameters; heart rate (HR) (beats / minute), central venous pressure (CVP) (mmHg), systolic arterial blood pressure (SAP) (mmHg), diastolic arterial blood pressure (DAP) (mmHg), mean arterial blood pressure (MAP) ) (mmHg), peripheral oxygen saturation (SpO2), PVI and BIS baseline values (baseline 1) will be recorded. The LRM will be automatically adjusted in the mechanical ventilator to be at 30 cm H2O for 30 seconds, and the test will begin, all hemodynamic parameters, especially SAB and PVI, where the maximum change at the end of 30 seconds, will be recorded (after LRM), and the differences initial baseline values will be calculated and recorded as percentages. After the recording of the second values, the crystalloid infusion at 10 ml / kg will be made to the patient through the peripheral venous line within 15 minutes, 10 minutes after the end of the infusion, new baseline values will be taken before the test (baseline 2). After fluid replacement, LRM will be repeated in the same way. The values after LRM (Fluid Challenge-FC) will be recorded and differences between basal 2 values will be calculated and recorded as percentages. Primary measurement parameters are determined as PVI and SAB, and secondary measurement parameters are HR, DAB, OAB, SpO2, CVP and BIS. The research is terminated by calculating the percentage of PVI changes with SAB formed by LRM after fluid challenge.