The primary objective of the study is to measure plasma levels of bupivacaine following erector spinae (ESP) regional block in patients undergoing mastectomy.

Breast cancer is one of the most frequent types of cancer in women. Mastectomy is important in treating these cases; however, sometimes it is associated with acute and chronic pain. Multimodal analgesia, combining drug therapy and regional anesthesia, can help in preventing acute and perhaps chronic pain in breast cancer patients undergoing mastectomy.

Erector spinae block is a new regional anesthesia technique that has emerged to treat thoracic pain following thoracic and breast surgery. It consists of injecting local anesthetics in the space located between the erector and paravertebral muscles using ultrasound guidance. The injection can be done at the level of T5 allowing distribution of the drug to upper and lower dermatomes.

The dose of local anesthetic injected after erector spinae block should aim to maximize analgesia while minimizing the chance of toxic systemic concentrations. Defining the rate of absorption of local anesthetics into the blood after an erector spinae block will therefore help anesthesiologists determinate optimal analgesic doses, in terms of both safety and effectiveness.

This observational study will determine bupivacaine pharmacokinetics after single shot erector spinae block with bupivacaine, to further define the right dose and duration of surveillance in post-anesthesia care.

Methods:

For the erector spinae block, the patient will be placed in the sitting position. Using an ultrasound machine with a high frequency linear probe (Sonosite, HFL50 15-6MHz) placed in the parasagittal plane, 3cm away from the midline, the anesthesiologist will position an insulated hyperechoic needle (50-80 mm, 22 gauge, SonoPlex STIM, Nanoline, Pajun, Germany) at the level of the 5th thoracic vertebrae, between the erector spinae and the paravertebral muscles. The anesthesiologist will confirm the correct position of the needle with the injection of 1 mL of 5% dextrose. Then, after negative aspiration, he will inject bupivacaine 0.5% with epinephrine 5 mcg/mL in 5 ml aliquots for a total dose of 2mg/kg of ideal body weight (maximum of 150mg).

The end of injection will be considered as T0. Collection of 4.5mL of blood will be performed at T10min, T20min, T30min, T45min, T60min, T90min, T120min, T180min, and T240min.

Blood tubes will be immediately placed on ice to be ultimately sent to the laboratory for centrifugation and measurement of bupivacaine level using liquid chromatography-tandem mass spectrometry (LC-MS/MS) for each of the samples.

General anesthesia will then be induced in the operating room with no additional bupivacaine allowed by the anesthesiologist or the surgeon. After surgery, in the Post-Anesthesia Care Unit, the level of the sensory block will be identified by pinprick and the quality of analgesia will be evaluated using a verbal numerical rating scale, and opioid consumption will be noted.

Inclusion Criteria:

• ASA I-III
• Undergoing unilateral mastectomy under erector spinae block and general anesthesia

Exclusion Criteria:

• Patient's refusal or inability to consent
• Allergy, hypersensibility or resistance to local anesthetic
• Contra-indication to regional anesthesia: infection in the designated area, acquired or congenital coagulopathy
• Severe hepatic or renal insufficiency (GFR<30 mL/min)

Patients undergoing unilateral mastectomy under general anesthesia and erector spinae block for post-operative pain management.