During major surgical procedures performed under general anesthesia, changes in blood flow and oxygen delivery to tissues may occur due to blood loss, hemodynamic fluctuations, and anesthesia-related physiological effects. These changes can lead to impaired tissue perfusion, which is commonly reflected by increased arterial lactate levels. However, lactate measurements are intermittent and may not detect early perfusion abnormalities.

This prospective observational study aims to investigate the relationship between submental tissue oxygenation measured by near-infrared spectroscopy (NIRS), microvascular reactivity assessed by the vascular occlusion test, and arterial lactate levels during major surgery. Adult patients undergoing major abdominal, neurosurgical, or orthopedic procedures will be monitored intraoperatively using non-invasive NIRS techniques, while arterial blood gas analyses will be performed as part of routine clinical care.

The study does not involve any changes to standard anesthesia or surgical management. All treatments and clinical decisions will be made by the responsible care team according to routine practice. By evaluating non-invasive indicators of tissue oxygenation and microvascular function, this study aims to improve the understanding of early intraoperative tissue perfusion changes and their association with metabolic markers.

This prospective, single-center, observational cohort study is designed to evaluate the relationship between intraoperative submental tissue oxygenation, microvascular reactivity, and arterial lactate levels during major surgical procedures performed under general anesthesia.

Major surgery is defined as procedures associated with a substantial risk of blood loss and/or tissue perfusion impairment, requiring invasive arterial blood pressure monitoring and hourly arterial blood gas analysis. Eligible surgical categories include major abdominal, neurosurgical, and major orthopedic procedures. Adult patients aged 18 years and older who meet the inclusion criteria will be enrolled.

All anesthetic and surgical management will be conducted according to standard clinical practice. The study protocol will not influence clinical decision-making. Hemodynamic management, fluid therapy, blood product transfusion, and ventilatory settings will be determined solely by the responsible anesthesia and surgical teams.

Intraoperative tissue oxygenation will be continuously monitored using near-infrared spectroscopy (NIRS). A NIRS sensor will be placed on the submental region, just inferior to the mandibular border and near the midline, after appropriate skin preparation. The sensor will be secured to minimize ambient light interference and motion artifacts. Baseline submental tissue oxygenation will be defined as the values obtained during a hemodynamically stable period of at least five minutes following anesthesia induction, without active surgical stimulation or significant fluctuations in mean arterial pressure.

Submental NIRS measurements will be recorded at 15-minute intervals throughout the intraoperative period, and hourly average and minimum values will be documented. A NIRS desaturation event will be defined as a decrease of 10% or more from baseline tissue oxygenation lasting for at least three minutes.

Microvascular reactivity will be assessed using the vascular occlusion test (VOT) applied to the thenar eminence of the non-dominant hand. A NIRS probe will be positioned on the thenar region following skin preparation. Transient arterial occlusion will be achieved using a non-invasive blood pressure cuff placed on the same upper extremity. The reoxygenation slope (ReO₂ slope) obtained during the reperfusion phase will be recorded as an indicator of microvascular reactivity. VOT measurements will be performed hourly during surgery and, whenever possible, synchronized with arterial blood gas sampling.

Arterial blood gas samples will be obtained hourly through an existing arterial catheter as part of routine intraoperative monitoring. Recorded parameters will include arterial lactate concentration, pH, base excess, hemoglobin, and hematocrit. Arterial lactate level will be considered the primary outcome measure of the study.

Intraoperative clinical data, including mean arterial pressure, vasopressor use, estimated blood loss, administered crystalloid and colloid fluids, and blood product transfusions, will be recorded on an hourly basis. For patients receiving blood transfusions, the indication for transfusion and timing of administration will be documented.

The intraoperative observation period will extend from anesthesia induction to the completion of surgery. All collected data will be analyzed to assess associations between submental tissue oxygenation, microvascular reactivity parameters, and arterial lactate levels over time. The study aims to explore whether non-invasive monitoring of tissue oxygenation and microvascular function may provide early insight into intraoperative tissue perfusion disturbances during major surgery.

Inclusion Criteria:

• Adults aged 18 years and older
• Scheduled for major surgical procedures under general anesthesia
• Major surgery defined as procedures associated with a risk of significant blood loss and/or impaired tissue perfusion, requiring invasive arterial blood pressure monitoring and hourly arterial blood gas analysis
• Planned intraoperative arterial catheter placement as part of routine clinical care
• Ability to provide written informed consent

Exclusion Criteria:

• Age younger than 18 years
• Refusal or inability to provide informed consent
• Absence of invasive arterial blood pressure monitoring
• Known pregnancy
• Severe peripheral vascular disease affecting upper extremities
• Anatomical abnormalities or skin conditions preventing placement of NIRS sensors on the submental or thenar regions
• Emergency surgery requiring immediate intervention
• Preoperative severe metabolic acidosis (arterial pH < 7.20)

The study population consists of adult patients aged 18 years and older undergoing major surgical procedures under general anesthesia at a single tertiary care center. Major surgery includes abdominal, neurosurgical, and major orthopedic procedures associated with a risk of significant blood loss and/or impaired tissue perfusion, requiring invasive arterial blood pressure monitoring and hourly arterial blood gas analysis as part of routine clinical care. All participants are enrolled prospectively and monitored intraoperatively without any alteration to standard anesthetic or surgical management.