This is a randomized, interventional prospective study. Patients (n=66) undergoing mastectomies with or without tissue expander will be randomized to receive a supra-serratus or sub-serratus regional plane block prior to surgery. The main effect to be measured is total opioid consumption 24 hours after the operation. Secondary endpoints include measuring a change between pre-and post-operative pain scores, patient satisfaction of pain control during first 24 hours after the operation, presence of postoperative nausea and vomiting, duration of sleep on first postoperative night, and block performance time and length of stay, between the two treatment arms.

Subjects will be randomized to one of two groups: the supra-serratus anterior plane block (depositing local anesthetic superficial to the serratus anterior muscle) or the sub-serratus anterior plane block (depositing local anesthetic deep to the serratus anterior muscle). Randomization will be performed using a computer generated randomization program. The results of the randomization will be revealed to the physician performing the block just prior to its performance. The site of the block will be marked and a safety timeout will be performed with the block nurse, regional anesthesiologist, and patient present and participating.

Once the above is completed, patients will receive a pre-operative ultrasound-guided single-injection serratus anterior plane block using either the supra-serratus or sub-serratus techniques according to their randomization. The block nurse will record the time when the timeout is performed, the "needle insertion" time when the block needle enters the skin, and the "block complete" time when the needle exits the skin. If the block is bilateral, the block nurse will record the time when the timeout is performed, then will record two sets of "needle in" times and "needle out" times, one for each side. In the case of a bilateral block, the same method of serratus anterior plane block will be utilized for both sides.

After the block, the patient will proceed to surgery. The operating room anesthesia team will be notified that the patient received a serratus anterior plane block. An appropriate multimodal general anesthetic will be employed at the operating room anesthesiologist's discretion. Upon completion of the surgery and emergence from general anesthesia, the patient will be seen by study personnel in the Post Anesthesia Care Unit and pain will be evaluated using a 0-10 Likert scale (0=no pain, 10= worst imaginable pain) about 1 hour post-operatively. Pain will be evaluated again by study personnel using the same scale on post-operative day #1, about 24 hours after initial block placement. If the patient is discharged home prior to the post-operative day #1 assessment, a phone interview will be performed 24 hours following the block to evaluate the post-operative day #1 scores and monitor for adverse events. Additional pain scores will be documented in the electronic medical record by nursing staff per protocol, and will be collected by study personnel.

During the intraoperative and post-operative period, the patient will continue to receive an appropriate multimodal analgesic regimen as necessary to adequately control the pain. No pain control method will be withheld from the patient as a result of participating in the study.

Patient satisfaction scores will also be used as a measure to evaluate quality of perioperative pain management. These scores will be assessed on post-operative day #1 on a scale of 1 to 5 (Poor=1, Fair=2, Good=3 Very good=4, Excellent=5).

During follow-up on post-operative day #1, the patients will also be asked for an estimate on duration and quality of sleep to help assess their general comfort level during their first post-operative night.

During the routine post-operative visit, an assessment of post-operative nausea and vomiting will be made by asking the patient directly. Chart review looking for post-operative antiemetic usage will also be evaluated to assess the presence of post-operative nausea and vomiting.

The decision as to whether or not to perform a serratus anterior plane block (SAPB) will be made independent from the introduction of the patient to the study. Therefore, the patient's choice to receive a SAPB will not be affected by the introduction of the study or their desire to participate in a clinical investigation. A multimodal intraoperative anesthetic technique will be employed for all patients, and post-operative pain control will be managed by the patient's primary surgical team.

Inclusion Criteria:

1. Adult patients 18 years or older
2. Individuals undergoing either a mastectomy or mastectomy with tissue expander for which the anesthetic plan includes a serratus anterior plane block.

Exclusion Criteria:

1. Any known sensory deficit of the anterolateral chest wall.
2. Any local disorder of the skin or otherwise where blockade is to be performed which would prevent safe performance of the block
3. Pregnancy
4. American Society of Anesthesiology classification greater than 3
5. Allergy to amide local anesthetic medications
6. Chronic pain conditions
7. Preoperative opioid use greater than 20 oral morphine equivalents per day
8. Any coagulation abnormality which would be a contraindication for block placement
9. Preoperative chronic renal dysfunction requiring renal replacement therapy or a serum creatinine greater than 1.4 mg/dL
10. Body mass index >50
11. Incarceration
12. Inability to understand study procedures including inability to understand the English language
13. Inability to provide adequate informed consent
14. Refusal to participate in the study

Women over the age of 18 who underwent a mastectomy, either unilateral or bilateral, with or without tissue expander placement, and with or without axillary node dissection were recruited.

24 hours