Female patients with an American Society of Anesthesia (ASA) physical status I-II, scheduled for elective breast cancer surgery will be included in the study. Subjects will be randomly assigned to one of three groups: the placebo group, the low-dose S-ketamine group, and the high-dose S-ketamine group. The primary outcome is the consumption of sufentanil during the surgery.

All patients will be scheduled to a propofol-remifentanil general anesthesia and receive non-steroid anti-inflammatory drug before incision. Bolus sufentanil will be administered according to the blood pressure, heart rate and bispectral index perioperatively. Patients unable to understand the study procedure or unable to give informed consent, with concurrent analgesic or sedative medication, with history of chronic pain, psychiatric disorders, or alcohol or drug abuse, with an allergy to the study medication, who are pregnant or breast feeding, with a BMI >30 and <18 kg/m2, with severe cardiac, pulmonary, hepatic or renal dysfunction, with intracranial hypertension will be excluded.

Subjects will be randomly assigned to one of three groups: the placebo group, the low-dose S-ketamine group, and the high-dose S-ketamine group. The study medication will be administered in a bolus injected over 30 s after induction, followed by continuous infusion, which started after the initial bolus. The placebo group will receive a bolus of 0.9% saline, followed by continuous infusion of 0.9% saline. The low-dose ketamine group will be administered a bolus of 0.5 mg/kg S-ketamine in saline, followed by 2μg/kg/min S-ketamine in saline, whereas the high-dose ketamine group will be administered a bolus of 0.5 mg/kg S-ketamine in saline, followed by continuous infusion of 4μg/kg/min S-ketamine in saline. The syringes and infusions will be identical for all of the groups and will run at the same rate in all subjects to enable blinding of the investigators. Study medication will be terminated 30 min prior to the end of the surgery. After surgery and in the postanesthesia care unit, patients will be asked about their pain level on a numeric rating scale (NRS), and about the presence of intraoperative awareness, nausea, vomiting, and hallucinations. Rescue analgesia of sufentanil will be provided when NRS ≥ 4 or patients require. Pain level will also be assessed at 0.5, 2, 4, 6, 12 and 24 hour postoperatively as well as 3 and 6 month after the surgery. Depression scale will be evaluated before the surgery and in the 7th postoperative day.

Inclusion Criteria:

1. Female patients with an American Society of Anesthesia (ASA) physical status I-II
2. Scheduled for elective breast cancer surgery

Exclusion Criteria:

1. Patients unable to understand the study procedure or unable to give informed consent
2. with concurrent analgesic or sedative medication,
3. with history of chronic pain
4. with history of psychiatric disorders
5. with history of alcohol or drug abuse
6. with an allergy to the study medication
7. who are pregnant or breast feeding
8. with a BMI >30 and <18 kg/m^2
9. with severe cardiac, pulmonary, hepatic or renal dysfunction
10. with intracranial hypertension

All IPD that underlie results in a publication are to be shared with other researchers.

The IPD will become available when summary data are published.

Yun Wu and Ye Zhang will review requests and criteria to share IPD. Requests are to be sent by email to wuyunanyi@163.com or zhangye_hassan@sina.com.

The syringes and infusions will be identical for all of the groups and will run at the same rate in all subjects to enable blinding of the investigators. Outcomes will be evaluated by anesthesiologists who are blinded to the treatment allocation. Patients, nurses, observers, and the statistician will be blinded to patient allocation throughout the study period.