With the increase in the elderly population in the world, the incidence of vertebral deformities is increasing and vertebral surgery has become one of the most commonly performed surgeries. Blood loss in vertebral and spinal canal deformity surgeries varies according to the surgical procedure, duration of operation and type of anesthesia. With the increase in blood loss, blood and blood product transfusion also increases. Although many technical and pharmacological methods have been applied to reduce blood transfusion in surgical patients, there is still no consensus on when to transfuse, and there are still institutional and individual differences in transfusion practices. In the literature, there are studies showing that some centers never use blood for the same surgery in patients, while in other centers blood is almost always used \[4\]. This has led to the comment that "Blood transfusion is a practice that takes place according to the physician, not the patient." The World Health Organization (WHO) defines anemia as hemoglobin (Hb) concentration \<13 g/dl for men and \<12 g/dl for women. Although these Hb concentrations stated in the literature try to define the lowest acceptable hemoglobin, transfusion practices are based on multiple physiologic parameters (tachycardia, hypotension, ST segment changes) rather than an isolated laboratory measurement (Hb and hematocrit values), The decision to transfuse blood is known to be difficult to standardize as it has to be made taking into account the need for increased oxygen extraction rate, new-onset wall motion abnormalities on transesophageal echocardiography, cardiac output and oxygen content of arterial blood, central venous oxygen saturation, base deficit, lactate level, etc.). The decision to transfuse blood is based on the hope of increasing tissue oxygen delivery (DO2), followed by cellular oxygenation. Improving DO2 has been shown to reduce both morbidity and mortality in the perioperative period. Tailoring DO2 to O2 consumption using specific targets seems promising. The common criterion used to start blood transfusion therapy is to determine the Hb level and to show that the DO2 has fallen to the critical Hb level, which is assumed to be compromised. With this view triggered by the Hb level detected at the critical level, transfusion is performed and it is seen that the Hb level is increased to a safe level after transfusion. Although it can be easily demonstrated that transfusion therapy increases Hb level and consequently DO2, it requires further research to determine how this treatment reflects on the general oxygen supply/consumption balance. Indeed, for this purpose, monitoring of mixed central venous oxygen saturation (SvO2) is a prerequisite. This method, which requires a pulmonary artery catheter, is currently used in very limited situations and indications and central venous saturation (ScvO2) value is generally used instead of SvO2.

Anesthesia management of the patients included in the study and the interventional procedures to be performed will be decided by the responsible anesthesiologist.

The initial ORI value of the patient will be determined as PO2 value between 100-120 mmHg in arterial blood gas (ABG) and will be recorded on the case report form every 15 minutes. The preoperative Hb value will also be recorded on the case report form, and in patients with intraoperative expected blood loss \>500 ml and for whom blood transfusion is foreseen, the hemoglobin value and ORI value will be recorded by the responsible anesthesiologist at the time of the indication of erythrocyte transfusion. Accordingly, the relationship between the initial ORI value and the ORI value at the time of the decision for erythrocyte transfusion will be examined.