Pfannenstiel hysterectomies are now rarely performed so cannot recruit eligible patients

Pain control after surgery is important for patient well-being and recovery. We are interested in determining whether we can improve the duration of action of a local anesthetic procedure (transversus abdominis plane block, or TAP block) by adding magnesium sulfate to local anesthetics given to patients after total abdominal hysterectomy with or without salpingo-oophorectomy.

Total abdominal hysterectomy (TAH) with or without salpingo-oophorectomy is a commonly performed major surgical procedure that results in significant postoperative pain. Traditionally, this pain has been treated with a multimodal approach which includes opioids, often administered via a patient-controlled analgesia (PCA) pump. Common side effects of opioids include sedation, nausea, vomiting, constipation, and pruritus. Thus, to overcome such undesirable side effects, other approaches to pain management have been explored, including post-operative transversus abdominis plane (TAP) blocks. TAP blocks are a popular analgesia technique for abdominal surgery with an incision between the sixth thoracic (T10) vertebrae and the first lumbar (L1) vertebrae. The block involves infiltration of local anesthetic (LA) by an anesthesiologist under direct ultrasound guidance. The anesthetic is deposited into a plane between the internal oblique and transversus abdominis muscles, which contain thoracolumbar nerves that originate from the T6 to L1 spinal roots. These nerves supply sensation to the anterolateral abdominal wall, the area responsible for incision-related pain following TAH ± unilateral/bilateral salpingo-oophorectomy. TAP blocks are a low risk procedure with very rare side-effects including bowel or peritoneum perforation, and local anesthetic toxicity. Almost all anesthesiologists are familiar with performing such a block, which is routinely done under ultrasound guidance to minimize the risk of perforation. Previous studies demonstrate TAP block efficacy in various abdominal surgery including hysterectomy, colon resection, Caesarean section, retropubic prostatectomy, laparoscopic cholecystectomy, and open appendectomy. While TAP blocks have been shown to provide patients with a statistically significant increase in the time to first analgesic request, this mean delay is typically less than 3 hours. To overcome the limited duration of action, we are proposing the addition of magnesium as an adjunct to the LA solution infiltrated in the block. Other regional anesthesia techniques have trialed various adjuncts including epinephrine, dexamethasone, clonidine, and recently magnesium. The goal of using such adjuncts is to prolong and enhance analgesia, and potentially even decrease total LA required. Our study will investigate the use of magnesium as an adjunct to LA in bilateral TAP blocks to increase the duration of analgesia. Magnesium is the fourth most abundant cation in the body and the second most abundant intracellular cation after potassium. The minimum recommended daily intake of magnesium for adults is 0.25 mmol (6 mg)/kg body weight. Magnesium is a natural analgesic through antagonism of N-methyl-D-aspartate receptors. When added to LA, magnesium has been shown to improve the quality and duration of analgesia of neuraxial, femoral, and brachial plexus blocks. In fact, brachial plexus analgesia has been produced with magnesium sulfate (MgSO4) alone. Magnesium as an adjunct to TAP blocks has never been previously investigated in any setting, thus we propose a study to specifically investigate its potential effects in prolonging analgesia in patients undergoing elective TAH ± unilateral or bilateral salpingo-oophorectomy. We hypothesize that the quality and duration of analgesia can be improved by adding a moderate amount of MgSO4 to the local anesthetic used in the TAP blocks.

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) rating I-III
• Age 18-75 years old
• Female
• Undergoing elective total abdominal hysterectomy (TAH) with or without uni/bilateral salpingo-oophorectomy (BSO) under general anesthesia with a Pfannenstiel incision
• Admitted to hospital postoperatively (inpatients)
• Competent to provide informed consent

Exclusion Criteria:

• Emergency TAH ± BSO
• ASA IV-V
• Allergy or sensitivity to study-related medications
• Taking any medications that are contraindicated for the use of any of our study drugs
• Midline incision
• Other regional anesthesia technique (e.g. epidural)
• Morbidly Obese (BMI > 40)
• Incompetent to provide informed consent
• eGFR < 50
• Impaired liver function (INR > 1.5)
• Pre-existing chronic pain condition requiring chronic opioid use
• Significant co-existing cardiovascular disease