Clinical Research Directory

Inclusion Criteria: * Adults aged 18 years or older * Patients scheduled for elective gastrointestinal tumor surgery * Surgery planned under general anesthesia * Body mass index (BMI) \<40 kg/m² * Written informed consent provided Exclusion Criteria: * Refusal to participate or inability to obtain written informed consent * Body mass index (BMI) ≥40 kg/m² * Pregnancy * Emergency surgery * Difficult intubation * ASA physical status ≥IV * Pediatric patients (\<18 years) * Inability to perform carotid Doppler measurements * Arrhythmia other than continuous sinus rhythm or uncontrolled cardiac arrhythmia * Severe cardiac dysfunction or left ventricular ejection fraction \<40% * Severe valvular heart disease * Clinically significant autonomic nervous system disorders * Inadequate noninvasive blood pressure monitoring or inability to obtain the required hemodynamic measurements * Active sepsis or systemic inflammatory syndrome * Severe peripheral vascular disease or significant carotid artery stenosis (\>50%) * Severe pulmonary disease or advanced respiratory failure * Implanted cardiac device (pacemaker/ICD) * Baseline mean arterial pressure (MAP) \<65 mmHg * Preoperative systolic blood pressure \>190 mmHg or diastolic blood pressure \>110 mmHg