[Small tidal volume hyperventilation relieves intraocular and intracranial pressure elevation in prone spinal surgery: a randomized controlled trial].

Objective: To investigate the effects of different ventilation strategies on intraocular pressure (IOP) and intracranial pressure in patients undergoing spinal surgery in the prone position under general anesthesia.

Methods: Seventy-two patients undergoing prone spinal surgery under general anesthesia between November, 2022 and June, 2023 were equally randomized into two groups to receive routine ventilation (with Vt of 8mL/kg, Fr of 12-15/min, and etCO₂ maintained at 35-40 mmHg) or small tidal volume hyperventilation (Vt of 6 mL/kg, Fr of18-20/min, and etCO₂ maintained at 30-35 mmHg) during the surgery. IOP of both eyes (measured with a handheld tonometer), optic nerve sheath diameter (ONSD; measured at 3 mm behind the eyeball with bedside real-time ultrasound), circulatory and respiratory parameters of the patients were recorded before anesthesia (T₀), immediately after anesthesia induction (T₁), immediately after prone positioning (T₂), at 2 h during operation (T₃), immediately after supine positioning after surgery (T₄) and 30 min after the operation (T₅).

Results: Compared with those at T₁, IOP and ONSD in both groups increased significantly at T₃ and T₄(P < 0.05). IOP was significantly lower in hyperventilation group than in routine ventilation group at T₃ and T₄(P < 0.05), and ONSD was significantly lower in hyperventilation group at T₄(P < 0.05). IOP was positively correlated with the length of operative time (r=0.779, P < 0.001) and inversely with intraoperative etCO₂ at T₃(r=-0.248, P < 0.001) and T₄(r=-0.251, P < 0.001).ONSD was correlated only with operation time (r=0.561, P < 0.05) and not with IOP (r=0.178, P>0.05 at T₃; r=0.165, P>0.05 at T₄).

Conclusion: Small tidal volume hyperventilation can relieve the increase of IOP and ONSD during prone spinal surgery under general anesthesia.