Postoperative bleeding is still an important concern after major abdominal surgery. Postoperative bleeding is the most important determinant of morbidity and death. It results in longer hospital stays, the necessity for extra radiological tests, interventional radiology treatments, and the possibility of reoperation. Early detection of postoperative bleeding reduces morbidity and mortality with appropriate intervention.Simultaneous arterial and venous blood gas measurements can be used to quickly identify the degree of bleeding and the need for replacement in serious trauma patients brought to the emergency room.Lactate levels, venous oxygen saturation, and the differential in partial pressure of carbon dioxide between arterial and venous blood can all be used to indicate the requirement for replacement, as demonstrated in several studies. The purpose of this study is to evaluate lactate levels, venous oxygen saturation, and arterial-venous partial pressure of carbon dioxide difference in blood gas analysis in order to assess the severity of bleeding in patients undergoing major abdominal surgery as early as possible and perform replacement as soon as possible.

Inclusion Criteria:

• Patients with major abdominal surgery
• Postoperative intensive care unit following
• Accepting to participate in research

Exclusion Criteria:

• Age under 18 years old
• Pregnant patients
• Do not accept to participate of study

Patients will have surgery at Ankara Bilkent City Hospital's General Surgery Clinic and then be transferred to the critical care unit.

IPD will not share other researchers.