Inclusion Criteria:  1. Age ≥18 years old; 2. Scheduled for elective digestive tract surgery, gynecological surgery, urological surgery, thoracic surgery, orthopedic limb surgery, orthopedic spine surgery or body surface surgery (thyroid surgery or breast surgery), head and neck surgery; 3. ASA score Ⅰ～Ⅲ; 4. The informed consent form was signed by the patients.  Exclusion Criteria:  1. The expected length of hospital stay of the patient is less than 48h; 2. Patients expected to be admitted to the ICU after surgery; 3. Patients expected to return to the ward with tracheal catheter after surgery; 4. Be allergic to S (+) - ketamine; 5. Patients with severe disorder of consciousness or mental system diseases (schizophrenia, mania, bipolar disorder, psychosis, etc.) or cognitive dysfunction; 6. Patients with a history of severe cardiovascular disease (congestive heart failure, severe angina attack, or unstable angina or myocardial infarction within 6 months); 7. Patients during pregnancy or lactation; 8. Patients with MMSE score <18 points;  9. Patients with any of the following contraindications of S (+) - ketamine:  Patients with risk of serious rise of blood pressure or intracranial pressure; Patients with high intraocular pressure (glaucoma) or penetrating ocular trauma; Patients with poorly controlled or untreated hypertension (Resting systolic blood pressure greater than 180 mmHg, or resting diastolic blood pressure greater than 100mmHg); Patients with untreated or undertreated hyperthyroidism.