Comparison of Changes of ONSD Between Groups of Laparoscopic Surgery With and Without Epidural Saline Injection

This study targets patients undergoing laparoscopic gastric cancer surgery. The investigators would like to compare two factors that influence intracranial pressure through optic nerve sheath diameter measurement. First factor is intraperitoneal pressure. Second factor is epidural pressure. The investigators divided the patients into three groups. Group A is patients with laparoscopic gastric surgery who received epidural injection through an thoracic epidural catheter. Group B is patients with laparoscopic gastric surgery who don't received epidural injection through an thoracic epidural catheter. Group C is patients with open gastric surgery.