We hypothesized that patients with mild hypercapnia during anesthetic emergence after TIVA had a shorter recovery time than patients with normal blood carbonate levels. We will select patients undergoing transurethral lithotripsy, who were expected to have mild postoperative pain, compare tracheal extubation time in patients with normal blood carbonic acid level and mild hypercapnia, to evaluate the effect of blood carbonic acid level during anesthetic emergence on recovery time from TIVA by. We also examined the changes of cerebral blood flow by TCD to investigate the possible mechanism of mild hypercapnia affecting the recovery time from TIVA.

At present, there is no effective intervention method for the recovery of general anesthesia, and the elimination of anesthetic agents, especially from the brain is a decisive factor in the emergence from general anesthesia. Many studies have reported the effect of hypercapnia on recovery time from volatile anesthetics. Hypercapnia can increase cardiac output and cerebral blood flow, increasing the ability to remove volatile anesthetics from the brain and shortening the recovery time from anesthesia.

Propofol is becoming the most common intravenous agent used for induction as well as maintenance of total intravenous anesthesia (TIVA). Its rapid elimination from the blood and brain is expected to shorten the recovery time from TIVA. Only a few clinical studies have investigated the relationship between hypercapnia and emergence time with propofol anesthesia. But those studies may have been influenced by postoperative pain, so the recovery time cannot reflect propofol metabolism alone. Moreover, intraoperative hypercapnia may increase the need for intraoperative anesthetic agents by increasing the clearance of agents from the brain.

As a non-invasive method, Transcranial Doppler ultrasonography（TCD）can be used to monitor blood flow changes in different parts of cerebral arterial circle(Willis circle）, and its safety and effectiveness in perioperative related fields have been proved.

We hypothesized that patients with mild hypercapnia during anesthetic emergence after TIVA had a shorter recovery time than patients with normal blood carbonate levels. We will select patients undergoing transurethral lithotripsy, who were expected to have mild postoperative pain, compare tracheal extubation time in patients with normal blood carbonic acid level and mild hypercapnia, to evaluate the effect of blood carbonic acid level during anesthetic emergence on recovery time from TIVA by. We also examined the changes of cerebral blood flow by TCD to investigate the possible mechanism of mild hypercapnia affecting the recovery time from TIVA.

Inclusion Criteria:

• Adult patients aged 20-60 years, with American Society of Anesthesiology physical status 1 to 2, undergoing transurethral lithotripsy under TIVA with endotracheal intubation; voluntarily signed informed consent forms.

Exclusion Criteria:

• Patients with mental, pulmonary, cardiac, endocrine, neuromuscular, liver and nervous system diseases or a history of such diseases; Lung CT showed atelectasis, pulmonary infection and pleural effusion. Patients taking sedatives or other drugs that might interfere with the study; Alcohol or drug dependence; Patients who have had a history of general anesthesia within the past month; Patients with body mass index (BMI) of 30 kg/m2 or above; Patients who are expected to be under anesthesia for less than 30 minutes; Patients with difficult airways.