BACKGROUND: General anesthesia has been widely used in clinics, while its mechanism is not fully clear.Delayed emergence from general anesthesia happens from time to time, which increases the occurrence of complications and economic burden of patients. The analeptics currently used in clinic have some side effects. Recently,studies have shown that orexin can facilitate emergence from general anesthesia.Because of its complex chemical structure and difficulty of synthetic process, a mountain of works needs to be done before its use in clinic. Doxapram has been widely used as a respiratory stimulant on patients. Recently the investigators found in patient that intravenous injection of doxapram would shorten the emergence time from general anesthesia, while its associated mechanism is unclear. Based on our previous discovery that injection of doxapram in patients could increase the plasma level of orexin - A, the investigators hypothesized that doxapram probably facilitate emergence from general anesthesia by promoting the release of orexin.