Dexmedetomidine, might attenuate the sympathetic activation and contribute to stable hemodynamics. The investigator hypothesized that continuous infusion of dexmedetomidine during laparoscopic lower abdominal surgery, might help to attenuate the increment of intracranial pressure and the investigator would observe the optic nerve sheath diameter using ultrasonography.

Laparoscopic lower abdominal surgery requires the Trendelenburg position with pneumoperitoneum for a prolonged time, but this can cause significant increase in intracranial pressure. Dexmedetomidine, a selective alpha2 receptor agonist might attenuate the sympathetic activation and contribute to stable hemodynamics. The investigator hypothesized that continuous infusion of dexmedetomidine during laparoscopic lower abdominal surgery, might help to attenuate the increment of intracranial pressure and the investigator would observe the optic nerve sheath diameter using ultrasonography.

Inclusion Criteria:

• clinical diagnosis of colorectal cancer
• planed for laparoscopic low anterior resection

Exclusion Criteria:

• morbid obesity (body mass index > 30 kg.m-2)
• history of cerebrovascular disease
• history of respiratory disease
• uncontrolled cardiovascular disease