The goal of this randomized controlled double blinded study is to compare the effect of the type of fluid theraphy on ultrasonographic upper airway measurements for spinal surgery in prone position longer than two hours. The main question it aims to answer is how do goal directed fluid theraphy and standard fluid theraphy effect ultrasonographic upper airway measurements.

Participants will be American Society of Anesthesiology (ASA) physical status I and II, between 18- 65 ages.

Resarchers will compare goal-directed fluid therapy with standard fluid theraphy to see if upper airway ultrasonography measurements differs between the groups.

In this prospective, randomized, double-blind study, patients aged between18-65 years, American Society of Anesthesiologists (ASA) physical status classification I and II , scheduled to elective spinal surgery longer than 120 minutes in the prone position. Patients will be informed about the study and written consent will be included in the study after their consent has been obtained. All patients will undergo a detailed preoperative anesthetic evaluation. Patients will be monitored standard non-invasively with electrocardiography, peripheral O2 saturation and non-invasive blood pressure. EEG monitoring (Bispectral index (BIS)) for sleep-wake status will be provided with an electrode placed on the forehead of the patient. Two 20 Gauge intravenous cannulas will be placed in the upper extremities of all patients. All patients will be administered general anesthesia with inhalation anesthesia. Patients will be divided into two groups according to the list to be created using a computer based program (http://www.randomizer.org). Numbering of the cases will be done according to the order of arrival in the operating room. Upper airway ultrasonography will be performed on all patients in the premedication room in the supine position using a bedside ultrasound device (Toshiba Aplio 500) and a linear high-frequency (10-13 mHz) probe. Epiglottic thickness (mm) will be recorded in the patient follow-up form.

The patient will be taken to the operating room after the USG measurements. In both groups, premedication will not be applied to the patients before general anesthesia. After preoxygenation (6 L/min oxygen for 5 minutes via a face mask) in the supine position, anesthesia induction will be done. In both groups, intravenous 2 mg/kg propofol and 1µg/kg remifentanil will be used as hypnotic agents in the induction of general anesthesia. Remifentanil infusion of 0.2 µg/kg/min will be started 2 minutes before induction, based on ideal body weight. Muscle relaxation will be performed with 0.6 mg/kg rocuronium bromide based on total body weight. Muscle relaxation will be maintained by administering 0.15 mg/kg IV rocuronium bromide every 30 minutes intraoperatively. An endotracheal tube with an internal diameter of 7/7.5 mm for women and an internal diameter of 8/8.5 mm for men will be used, and the tracheal tube will be adjusted to maintain the cuff pressure at 20 to 25 cm H2O. General anesthesia will be maintained with a mixture of oxygen (40%) and air, sevoflurane (titrated to keep the BIS value between 40 and 60), remifentanil (0.2 µg/kg/min). Lungs will be ventilated intraoperatively with a tidal volume of 6-8 mL/kg, a PEEP of 8 cm H2O, and a fresh gas flow rate of 2 L/min. Respiration rate will be adjusted so that the end tidal carbon dioxide value is kept between 35-40 mm Hg. In addition to non-invasive monitoring, invasive radial arterial pressure, tympanic temperature and urine output will also be monitored. Normothermia (36-37°C) will be maintained using a heater throughout the intraoperative period. Patients will be randomly divided into two groups.

Group 1 (Standard Fluid Therapy Group):

In Group 1, fluid will be administered according to the standard fluid calculation (4-2-1 rule) considering the patient's fasting time, evaporation loss, maintenance fluid requirement, replacement of urine output, and surgical loss. Further fluid administration will be adjusted according to hemodynamic parameters to keep heart rate (HR) and mean arterial pressure (MAP) within 20% of baseline.

Group 2 (Targeted Fluid Therapy Group):

In Group 2, close PPV (pulse pressure variation) monitoring will be performed with intraoperative invasive arterial monitoring, and fluid therapy will be given to keep the PPV target below 13%. Fluids or vasopressors will be administered to maintain PPV<13, and MAP>65 mm Hg targets. If PPV>13, 200 mL of stable crystalloid will be administered over 5 minutes and PPV will be reassessed after 5 minutes. If MAP is <65 mmHg and PPV is <13%, vasopressor (norepinephrine) treatment will be started through the intravenous line. A hemoglobin value below 8 mg/dL will be considered as a transfusion trigger for both groups and total intraoperative input and output including blood loss will be recorded.

In both groups, sevoflurane and remifentanil infusion will be stopped while the skin is closed. After the operation the patient will be placed in the supine position again. Reversal of neuromuscular blockade before extubation will be done with sugammadex (2-4 mg/kg). When the patient is awake (BIS value >80), tracheal extubation will be performed. Ultrasonography will be performed using a bedside ultrasound device (Toshiba Aplio 500) and a linear high-frequency (10-13 mHz) probe. Epiglottic thickness (mm) will be recorded in the patient follow-up form. Patients will be followed up and recorded in terms of stridor, need for re-intubation, hoarseness, and cough in the PACU and within 24 hours postoperatively. All data will be statistically analyzed.

Inclusion Criteria:

• 18-65 years old
• ASA (American Association of Anesthesiologists) I and II
• Elective spine surgery in prone position
• Surgeries longer than 120 minutes

Exclusion Criteria:

• Chronic pulmonary disease
• Renal dysfunction (glomerular filtration rate <30 ml/kg/min)
• Congestive heart failure (ejection fraction ≤35%)
• Serious arrhythmias
• Peripheral vascular disease
• Pregnance
• Patients with coagulation disorders
• Patients who need more than 2 intubation attempts
• Prior cervical surgery
• Patients with neck extension limitation
• Upper respiratory tract infection in the last 3 weeks
• Patients expected to have a difficult airway (Mallampati III-IV)
• Patients with a body mass index (BMI) of 35 and above
• Patient's refusal