Olanzapine has been used as prophylactic antiemetic for chemotherapy induced nausea and vomiting.

The project aims to evaluate the efficacy of olanzapine in combination with ondansetron and dexamethasone in patients at high risk of postoperative nausea and vomiting, with previous history of nausea and vomiting induced by prior chemotherapy, submitted to medium and large surgery.

Double blinded randomized clinical trial. Fifty patients will be selected for each group. Group A will receive 10mg olanzapine in the preoperative and group B will receive placebo in the preoperative period. Both groups will receive intravenous general anesthesia combined with epidural. All patients will receive epidural PCA.

The primary outcome of this study is the incidence of post-operative nausea and vomiting. The power of analysis was based on the following parameters: type error I (α = 0.05), error II (β = 0.8), 95% confidence interval and hypothesis test two-tailed. Thus, 42 patients are indicated per group to obtain reduction of the incidence of nausea and / or vomiting of the population risk from 60% to 30%. The population will be composed of 100 patients because it is assumed that percentage of loss of 10% per group.

The statistical analysis will be performed in program R version 3.5.1. The results obtained will be presented as mean + standard deviation for continuous data or absolute frequency for described data.

For quantitative variables, the Student's t-test will be used for with normal distribution or Mann-Whitney for non-distributed data normal.

For qualitative variables, the chi-square test will be used or Fisher's exact test. Significant difference is defined as p, 0.05.

Data will be collected and managed using REDCap data capture tools

Inclusion criteria:

• Patients submitted to medium and large surgeries in the thoracic or abdominal region (mastectomies, breast plastic, hysterectomies, annexectomies, gynecological pelvic surgeries, colectomies, rettosigmoidectomies) under general anesthesia
• Patients aged between 18 and 60 years
• Patients considered to be at high risk for PONV according to the Apfel scale (Apfel 3 or 4)
• Patients with a history of chemotherapy-induced nausea and vomiting.

Exclusion criteria:

• Pregnancy or Lactation
• Current use of typical anti-psychotic medications or atypical
• History of allergy to olanzapine
• Myocardial infarction or unstable angina in the 6 months prior to the day of surgery planning
• History of severe ventricular arrhythmia (eg, VT or VF)
• Heart Failure Class II or greater second NYHA
• Postural hypotension or vasovagal syncope in the 6 months prior to the day of surgery planning
• Narrow angle glaucoma
• Parkinson's disease
• Dementia
• Inability to swallow medicines
• QT interval history greater than 450ms or torsades de pointes
• Patient does not want to participate in the study
• Videolaparoscopy surgery
• Contraindication for neuraxial block