The goal of this observational study is to determine the correlation between cerebral oxygenation values measured by near-infrared spectroscopy (NIRS) and other routine monitoring parameters in patients placed in the semi-sitting position. The primary questions investigated are : Do NIRS values correlate with heart rate, invasive mean arterial pressure, and end-tidal carbon dioxide? Do NIRS values reflect changes observed in arterial blood gas analysis? Are cerebral perfusion-related parameters associated with patients' comorbidities such as diabetes, obesity, and hypertension? Researchers will compare routine noninvasive monitoring (heart rate, blood pressure, peripheral oxygen saturation) with invasive arterial monitoring and NIRS to assess whether NIRS provides additional information for evaluating cerebral perfusion. Participants will be monitored for: Heart rate Invasive mean arterial pressure End-tidal carbon dioxide Arterial blood gas changes Cerebral oxygenation with NIRS Patients' comorbidities (e.g., diabetes, obesity, hypertension) will also be recorded, and their association with perfusion-related parameters will be analyzed.