We are planning to measure hemodynamic parameters (Heart Rate, Mean Arterial Pressure, Stroke Volume Index, Stroke Volume Variation, and Pulse Pressure Variation) and ventilatory parameters at four times (T1, T2,T3, and T4) during spinal operation at prone position. First measurement (T1, First baseline) will be performed in the prone position after performing tracheal intubation and confirming hemodynamic stability (defined as mean arterial pressure <10% for 3 minutes). After the T1 measurement, additional 5 cmH2O PEEP (Positive end-expiratory pressure) will be applied for 30 seconds which will be called as "short term low PEEP challenge (SLPC)". At the end of the SLPC and prior to PEEP lowering, T2 measurement will be performed and recorded. After the T2 measurement, PEEP will be decreased to the initial value (5 cmH2O) and three minutes later, a second baseline (T3) measurement will be performed. Thereafter, 500 ml isotonic saline will be loaded in 10 minutes. T4 measurement will be performed again three minutes after volume loading. All of the measurements will be completed before surgery start and surgical stimulus. Patients exhibiting an increase in stroke volume index more than 15% after fluid loading (between T3 and T4) will be classified as volume responders. Absolute pulse pressure variation change due to SLPC, Absolute Stroke volume variation change due to SLPC, stroke volume index percentage change due to SLPC, and stroke volume index percentage change after fluid loading will be calculated and compared. Our aim is to observe if short term low PEEP challenge has the ability to predict fluid responsiveness better than pulse pressure variation and stroke volume variation for patients operated in prone position.

Inclusion Criteria:

• Patients undergoing spine surgery in prone position
• Age≥ 18 years
• Elective surgery

Exclusion Criteria:

• American Society of Anesthesiology physical status over III
• Valvular heart disease
• Any ventricular dysfunction
• Preoperative arrythmias
• Preoperative hemodynamic instability
• Preoperative uncontrolled hypertension
• Preoperative uncontrolled diabetes mellitus
• History of lung disease
• Morbid obesity (Body mass index ≥ 40 kg/m2)

Adult patients undergoing spine surgery in prone position