Effects of different prone patient positioning on Optic nerve sheath diameter in spine surgery

Proposedstudy procedure**:**

Patients will be screened forinclusion and exclusion criteria on the preoperative day. After a thoroughpreoperative examination, patientswill be randomized to either of the two arms based on the computer generatedrandom numbers. The informed consent will be obtained.

 Patients in both the groups will besubjected to a standardized anesthesia technique comprising of induction withThiopentone sodium (4-5mg/kg i.v); analgesia with fentanyl (2mg/kg i.v);muscle relaxation with vecuronium (0.15mg/kg I.v) and positive pressureventilation with O2 :Nâ‚‚O (1:1) / Sevoflurane. Maintenance ofanesthesia will be titrated with sevoflurane to achieve a target value of0.8-1.0 MAC. Intraoperative analgesia will be provided by boluses dosing ofintravenous fentanyl of 1 mcg/kg i.v. Ventilator settings will be adjusted to maintainend tidal carbon dioxide (EtCO2) between 35-40 mm Hg. Intubation ofpatients will be done by flexometallic endotracheal tube of appropriate sizeand secured properly.

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

 ONSD measurement

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

The ONSD will be measured 3 mm behindthe globe in both the eyes. The ONSD measurements will be obtained averagingthree readings from each eye.  Afterintubation ONSD will be in the following time points

Ø  Supineposition

Ø  After 10minutes of prone positioning

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

Ø  Immediatelyturning the patients to supine

Ø  30 minutespost surgery after giving 30 degree of head elevation