Vascular catheters (VC) are one of the most frequently performed interventional procedures in both critically ill patients in intensive care units and patients scheduled for major surgery. Recognition of VC-related complications has increased in recent years, and among them, VC-related thrombosis (VCRT) is frequently seen. The reported incidence of VCRT is variable, up to an overall rate of 14-40% for symptomatic events, and may result in pulmonary embolism. Ultrasonography is used frequently and effectively to diagnose intravascular thrombosis and occlusion and provide early intervention. It has been observed that the diagnosis of VCRT can usually be made just before VC is removed or when it becomes symptomatic. Because venous thrombosis formation is a result of altered blood flow, vascular endothelial damage, or altered blood structure, VCRT can occur very early because certain risk factors are present at the time of catheterization. The evolution of VCRT may differ from classic deep vein thrombosis (DVT) depending on existing or removed catheter- or patient-related risk factors. Although there are studies in intensive care patients, the clinical significance of VCRT in patients scheduled for major surgery has not yet been clarified. The time from the first day of catheter placement to the onset of VCRT is not clearly known, and the evolution of VCRT size over time before and after VC removal has not been evaluated. For this reason, it was aimed to evaluate VCRT with ultrasound on a daily basis in the period from insertion to removal of the VC and to evaluate it with a prospective study design in order to detect possible risk factors. Adult patients who will undergo major surgery under elective conditions and are planned to have a central venous catheter inserted will be included in the study. Patients who are decided to have VC implanted will be checked for any thrombus in the vascular structure using ultrasound (US) before the procedure is performed. VC size and length, insertion site, and use of ultrasound guidance will be recorded. Ultrasound detection of VCRT is planned to be performed daily before VC insertion and until VC is removed or the patient is discharged from the intensive care unit. If there is no thrombus between the insertion of the VC and the removal of the VC or until the patient is discharged, patients will be considered as having not developed VCRT. When VCRT is detected, the decision to leave or remove the catheter or initiate therapeutic anticoagulation will be made independently by the patient's responsible physician. A linear probe will be used for ultrasound diagnostic imaging of catheter-related thrombosis. Patients will be examined in the supine position. Doppler ultrasound will be used to evaluate the internal jugular (IJV), subclavian (SCV), femoral (FV), or axillary vein on the same side as the IJV. Ultrasound criteria for the diagnosis of VCRT will require the presence of at least two of the following: intravenous echogenic filling defect, lack of compression of the vein, or abnormal color Doppler pattern. The thrombosis size will be evaluated as follows: In the axial plane, the section with the largest thrombosis area will be selected to measure the maximum and minimum distances, and the maximum distance will be defined as the diameter of the thrombosis. In the longitudinal plane, the length of the thrombosis will be measured. The ultrasound will be performed by an experienced specialist who is competent in bedside ultrasound and is not responsible for patients.