This randomized prospective study aims to compare the effects of two target-controlled infusion models, the Schnider and Eleveld models, on anesthetic depth and hemodynamic responses during anesthesia induction in adult patients undergoing spinal surgery.

Patients scheduled for spinal surgery will be randomized into either the Schnider or Eleveld group. Before induction, standard monitoring will be applied, including electrocardiography, peripheral oxygen saturation, invasive arterial blood pressure monitoring, heart rate monitoring, and bispectral index monitoring. Anesthesia induction will be performed with fentanyl 2 mcg/kg, rocuronium 0.6 mg/kg, and propofol administered by target-controlled infusion with an effect-site target concentration of 3 mcg/mL according to the allocated pharmacokinetic model.

Hemodynamic parameters and bispectral index values will be recorded before induction and at the 1st, 3rd, 5th, and 10th minutes after induction. Additional parameters, including time to BIS below 40, time to delta activity, burst suppression duration, total propofol dose during the first 5 minutes, need for additional propofol, hemodynamic response to intubation, vasopressor requirement within the first 10 minutes, and use of esmolol, will also be documented. The primary aim is to evaluate whether the Schnider and Eleveld models differ in terms of induction-related hemodynamic stability and anesthetic depth during the early induction period.

This study is designed as a randomized prospective clinical study in adult patients undergoing spinal surgery. The study will evaluate and compare two pharmacokinetic models used for target-controlled infusion of propofol: the Schnider model and the Eleveld model. The main focus of the study is the first 10 minutes of anesthesia induction, a period in which rapid changes in anesthetic depth and hemodynamic variables may occur.

Eligible patients scheduled for spinal surgery will be assigned to one of two groups according to the propofol target-controlled infusion model used during induction: the Schnider group or the Eleveld group. Patients younger than 18 years, patients with uncontrolled hypertension, patients with uncontrolled diabetes mellitus, and patients who do not provide consent will be excluded.

Before anesthesia induction, demographic and clinical variables will be recorded, including age, sex, ASA physical status, height, weight, body mass index, diagnosis, type of operation, operation date, and comorbid diseases. Baseline vital signs will be obtained from the preoperative ward follow-up values and from the immediate pre-induction period.

All patients will receive standard monitoring before induction, including peripheral oxygen saturation, electrocardiography, invasive arterial blood pressure monitoring, heart rate monitoring, and bispectral index monitoring. Invasive arterial pressure monitoring will allow continuous assessment of systolic arterial pressure, diastolic arterial pressure, and mean arterial pressure during the induction period.

Anesthesia induction will be performed using fentanyl 2 mcg/kg, rocuronium 0.6 mg/kg, and propofol administered by target-controlled infusion. Propofol will be administered using the pharmacokinetic model assigned by randomization, either Schnider or Eleveld. The target-controlled infusion pump will be set to achieve an effect-site propofol concentration of 3 mcg/mL. The induction period will be evaluated during the first 10 minutes after induction.

Heart rate, mean arterial pressure, systolic arterial pressure, diastolic arterial pressure, and BIS values will be recorded at predefined time points: ward baseline, immediately before induction, and at the 1st, 3rd, 5th, and 10th minutes after induction. The presence of a hemodynamic response to intubation will be assessed according to whether heart rate or mean arterial pressure increases by more than 20% compared with ward measurements. Patient movement or straining during intubation will also be recorded as none, mild, or severe.

In addition to routine hemodynamic and BIS measurements, several induction-related variables will be documented. These include the time required for BIS to decrease below 40, the time until delta activity is observed, burst suppression duration, total propofol dose during the first 5 minutes, need for additional propofol, total fentanyl and rocuronium doses, and whether esmolol was used, including its dose. The requirement for vasopressor therapy within the first 10 minutes, including the agent and dose used, will also be recorded.

During the first 10 minutes of induction, magnesium administration will not be performed in order to avoid potential confounding effects on hemodynamic parameters. Throughout the operation, the patient's hemodynamic stability will be observed, and any additional drug requirements will be documented.

The primary objective of the study is to compare the Schnider and Eleveld target-controlled infusion models in terms of hemodynamic changes and anesthetic depth during the first 10 minutes of anesthesia induction. Secondary evaluations will include the incidence of intubation-related hemodynamic response, time to BIS below 40, burst suppression duration, need for additional propofol, vasopressor requirement, and total propofol dose during early induction.

Inclusion Criteria:

• Patients scheduled for elective spinal surgery under general anesthesia
• Patients who will undergo surgery at the Neurosurgery Department of Istanbul University, Istanbul Faculty of Medicine
• Patients aged 18 years or older
• Patients who provide written informed consent to participate in the study

Exclusion Criteria:

• Patients with uncontrolled hypertension
• Patients with uncontrolled diabetes mellitus
• Patients younger than 18 years of age
• Patients who do not provide informed consent to participate in the study

The study population will consist of adult patients scheduled for elective spinal surgery under general anesthesia at the Neurosurgery Department of Istanbul University, Istanbul Faculty of Medicine. Eligible patients will be 18 years of age or older and will provide written informed consent before enrollment.

Patients with uncontrolled hypertension, uncontrolled diabetes mellitus, those younger than 18 years of age, and patients who do not provide consent will be excluded from the study. Eligible participants will be randomized to receive propofol target-controlled infusion using either the Schnider or Eleveld model for anesthetic induction. Hemodynamic parameters and bispectral index values will be evaluated during the first 10 minutes of the induction period.

The clinical data of this trial are available upon reasonable request to the corresponding author.