Proposed study procedure:

Patients will be screened for inclusion and exclusion criteria on the preoperative day. After a thorough preoperative examination, patients will be randomized to either of the two arms based on the computer generated random numbers. The informed consent will be obtained.

Patients in both the groups will be subjected to a standardized anesthesia technique comprising of induction with Thiopentone sodium (4-5mg/kg i.v); analgesia with fentanyl (2mg/kg i.v); muscle relaxation with vecuronium (0.15mg/kg I.v) and positive pressure ventilation with O₂ :Nâ‚‚O (1:1) / Sevoflurane. Maintenance of anesthesia will be titrated with sevoflurane to achieve a target value of 0.8-1.0 MAC. Intraoperative analgesia will be provided by boluses dosing of intravenous fentanyl of 1 mcg/kg i.v. Ventilator settings will be adjusted to maintain end tidal carbon dioxide (EtCO₂) between 35-40 mm Hg. Intubation of patients will be done by flexometallic endotracheal tube of appropriate size and secured properly.

Patients will be placed in prone position in such a way that there should not be any impediment of venous drainage from the head. Head of the patients will be turned to the left and will be kept on gel foam ring to avoid compression on eyes by using gel foam ring. Measurements of ONSD will be done by first on the left side and then on the right side by rotating the head at different time points. Patients randomized to Group A will undergo surgeries in prone position; and to Group B will undergo surgeries in prone position with 10 degree of reverse trendelenberg. Any hypotension will be treated with fluid bolus/mephentermine boluses to maintain normotension.

ONSD measurement

Ocular ultrasounds will be performed by using GE Healthcare LOGIQ e (portable ultrasound system machine) with a 5–10 MHz linear probe. Scans will be performed on a closed eyelid with a conductive gel. A linear probe will be used to obtain an axial cross-sectional image of the optic nerve entering the fundus. The probe will be placed on the superior and lateral margin of the orbit directed slightly inferiorly and medially to help visualize margins of the nerve. For the purpose of safety biomechanical side effects the mechanical index and thermal index will be reduced to 0.2 and 0.0 respectively.

The ONSD will be measured 3 mm behind the globe in both the eyes. The ONSD measurements will be obtained averaging three readings from each eye.  After intubation ONSD will be in the following time points

Ø  Supine position

Ø  After 10 minutes of prone positioning

Ø  After 2 hrs after prone positioning followed by every 1 hr till the completion of surgery

Ø  Immediately turning the patients to supine

Ø  30 minutes post surgery after giving 30 degree of head elevation