This study aims to compare the effectiveness of paravertebral block and erector spinae plane block after mastectomy.

The primary objective of this randomized controlled trial is to demonstrate equivalent dermatomal spread for ultrasound-guided single-injection Erector Spinae Plane (ESP) block performed at T4-T5 level and ultrasound-guided single-injection (paravertebral) PVB block at the same level.

Secondary objectives are Numerical Rating Scale (NRS) pain scores in the first 24 post-operative hours, opioid analgesia use intra-operatively and in the first 24 post-operative hours, block procedural time and patient discomfort during block insertion.

The investigators hypothesize that ESP block efficacy is not inferior to PVB with reference to dermatomal sensory spread and analgesic efficacy, while being easier to perform with less associated discomfort during block insertion.

Paravertebral block (PVB) has been shown to provide excellent analgesia for major breast surgery. Advantages of a PVB technique include reduced postoperative pain, analgesic consumption, opioid-related side effects, and shorter post anesthesia care unit (PACU) stay. There is also evidence to suggest that PVB may have a favorable impact on cancer recurrence after mastectomy. Paravertebral blockade results in somatosensory and sympathetic blockade after injection of local anesthetic solution to the paravertebral space posterior to the pleura. Erector spinae plane (ESP) block is a recently described technique which may be an alternative to PVB for providing thoracic analgesia. Numerous case reports and case series describe ESP block for the management of acute and chronic thoracic pain. It involves injection of local anesthetic into the fascial plane deep to erector spinae muscle. Radiological imaging in a cadaver model has demonstrated that a single injection at the level of the T5 transverse process produced cranio-caudal spread between C7 and T8. This accounts for the extensive sensory block that has been observed in case reports and is at least as extensive as the spread seen with PVB.

ESP is a more superficial block with a better defined end-point - injection between the bony transverse process and erector spinae muscle. A more superficial ultrasound-guided block will be faster to perform and less painful for the patient. Furthermore, ESP does not have the same risk of pneumothorax as PVB.There have been no randomized controlled trials involving ESP to date. All descriptions of the technique have been in case report / series format.

The investigators hypothesize that ESP block efficacy is not inferior to PVB with reference to dermatomal sensory spread and analgesic efficacy, while being easier to perform, has less associated discomfort and fewer complication risks.

All eligible participants will be randomized to the PVB arm or the ESP arm after fully informed consent has been obtained. Subjects randomized to the PVB arm will have an ultrasound-guided paravertebral block performed pre-operatively in the anesthesia block room with 20ml of a ropivacaine / bupivacaine mixture. Subjects randomized to the ESP arm will have an ultrasound-guided erector spinae plane block performed pre-operatively in the anesthesia block room with 20ml of a ropivacaine / bupivacaine mixture. Conduct of intra-operative anesthesia will be at the discretion of the attending anesthesiologist.

The primary outcome is equivalent dermatomal spread for ultrasound-guided single-injection ESP block performed at T4-T5 level and ultrasound-guided single-injection PVB block at the same level.

Secondary outcomes are NRS pain scores in the first 24 post-operative hours, opioid analgesia use intra-operatively and in the first 24 post-operative hours, block procedural time and patient discomfort during block insertion.

The primary outcome will be collected by a research associate who is blinded to the study group allocation. The secondary outcomes will be collected by the study team from the patient chart and through patient interview. Continuous variables will be analyzed using t-tests and categorical variables will be analyzed using chi-squared test. Markers of intervention safety will be monitored included local anesthetic toxicity, nerve injury, infection and pneumothorax.

A test of equivalence will be performed for the primary outcome of dermatomal spread between the 2 groups. If the equivalence is not established for the block extent, a 2-way superiority will be conducted. Secondary outcomes will be analyzed using Mann-Whitney U test and the Hodges-Lehmann method for calculating 95% confidence intervals (CIs) around the median difference. Assuming a median difference of 1.5 dermatomal segments, and a pooled standard deviation (SD) of 1.65 segments, α of 0.05, and 90% power, a total sample size of 42 patients is estimated (21 patients in each group).

Inclusion Criteria:

• Adult patients over 18 years
• Consented to a regional anesthesia technique for post-operative analgesia
• Partial or complete mastectomy

Exclusion Criteria:

• Subjects <18 years
• those who refuse consent for PVB or ESP blockade,
• opioid tolerant patients
• psychiatric illness
• allergy to local anesthetic
• Local or systemic infection