<qa>  What is the best dose of ketamine?Ketamine and opioid are known to improve the intubating condition, respectively. Combination of ketamine and opioid potentiate analgesic effect and give hemodynamic stability in complementary manner. Therefore, the combination use of ketamine and remifentanil could be useful for acceptable intubating condition in the general anesthesia without the use of neuromuscular blocking agent. Generally, induction dose of ketamine ranges between 1 and 2 mg. There was no report about the optimal dose of remifentanil with 1 or 2 mg ketamine induction dose for intubation without neuromuscular blocking agent. The investigators focus on remifentanil dose using Dixons up and down method. 