The purpose of this project is to evaluate the analgesic efficacy of a regional anesthesia technique ( interfascial block at the serratus muscle) performed in patient undergoing breast surgery and the effect of this technique on postoperative analgesia.

Regional analgesia techniques have shown to have benefits over conventional analgesia. The lower opioids analgesics requirements decrease side effects associated with the use of this type of medication. It is also increasingly established scientific evidence that reducing opioids administration may play a role in prognosis of cancer patients given the immunosuppressive effect of these drugs.

In patients undergoing oncologic/reconstructive breast surgery paravertebral block has been traditionally performed, however the paravertebral space lacks a clear anatomical barrier from the spinal cord so the drugs administered can diffuse to the intervertebral foramen causing deeper levels of blocks (epidural or spinal blocks) and injections at the paravertebral level are associated with serious complications both neurological and respiratory (meningitis, spinal hematomas abscesses, pneumothorax, respiratory failure. This has led to the development of less invasive technique with an improved safety profile.

The serratus plane block falls within the framework of these newly developed techniques and the present study aims to assess its role in the management of the patient undergoing cancer / reconstructive surgery.

Considering a decrease in postoperative morphine consumption of 40% and the average consumption of opioid after breast surgery under general anesthesia is 15 milligrams ( standard deviation 8 mg ) investigators calculated that with an alpha error 5% power 80 % 56 patients (28 per group) were required to achieve a significant result. In anticipation of possible loss of sample, 60 patients were recruited . Analysis will be performed by intention to treat.

Quantitative variables (opioid consumption, pain estimated by VAS scale) will be analyzed using mixed linear model adding a random variable for interindividual variability for pain threshold. If the data do not meet the criteria of normality the Mann-Whitney U test will be used. If the groups differ in preoperative variables. Investigators will make a multivariable analysis adjusting for these variables. The need for rescue analgesia over time is analyzed by Kaplan-Meier curve where the terminal state is the administration of the first dose of opioid postoperatively. The difference side effects (nausea, pruritus, apnea, urinary retention, ileus ) and complications IPO questionnaire and analyzed by chi-square test and Fisher exact test. Scheffe correction will be applied for multiple comparison. Missing data are excluded from analysis.

The study has planned data monitoring and auditing by the IIS la Fe according to AEMPS guidelines. Registries will be obtained from electronic medical records available for double check procedures.

Inclusion Criteria:

• Patients older than 18 years, undergoing oncological surgery and/or breast reconstruction surgery whose pathology and surgical intervention supose at least a 24 hours hospital admission

Exclusion Criteria:

• ASA Physical Status Classification System- IV.- Morbid obesity (Body mass index >40). -Impossibility of anatomical structures ultrasound identification in a satisfactory way (there can be no distinction in the interfascial plane between serratus and pectoral muscle). -Opioids treatment before surgery. Sepsis and/or infection at the puncture site.
• Haemostasis disorders. - Allergy to any of the drugs used in the study.

24 hours after surgery

The blinding method cannot entirely exclude ascertainment bias.