The purpose of this study is to evaluate the change of the optic nerve sheath diameter after deflation of pneumatic tourniquet.

Deflation of pneumatic tourniquet could induces many changes in various organ systems. It has been shown to affect intracranial pressure. Thus, the investigators evaluate the change of optic nerve sheath diameter using ultrasound after deflation of pneumatic tourniquet in patients undergoing knee surgery.

Inclusion Criteria:

• ASA physical status 1-4
• patients undergoing knee surgery using pneumatic tourniquet

Exclusion Criteria:

• patients who refuse to participate in this study
• patients with history of cerebrovascular disease
• patients who are judged ineligible by the medical staff to participate in the study for other reasons

Patients undergoing knee surgery using pneumatic tourniquet