Use of Magnesium During Total Hysterectomies
Oral magnesium oxide should be administered postoperatively as part of a multimodal approach to prevent ileus and accelerate return of bowel function, while intravenous magnesium sulfate can be used intraoperatively as an adjuvant analgesic to reduce postoperative pain and opioid consumption.
Postoperative Oral Magnesium for Ileus Prevention
The primary evidence-based role for magnesium in hysterectomy is as a postoperative oral laxative to enhance gastrointestinal recovery. 1
Oral magnesium oxide combined with disodium phosphate in fast-track hysterectomy reduces time to first defecation by approximately 1 day (n=53 randomized trial), though this does not translate to changes in length of hospital stay or other major outcomes. 1
Magnesium hydroxide combined with bisacodyl suppositories has been described in cohort studies of patients undergoing radical hysterectomy as part of enhanced recovery protocols. 1
The ERAS Society guidelines for pelvic surgery provide a weak recommendation (low evidence level) for incorporating oral laxatives including magnesium into a multimodal approach to optimize gut function after pelvic surgery. 1
Important caveat: The evidence shows improved time to defecation but no impact on morbidity, mortality, or length of stay—the outcomes that truly matter. 1
Intravenous Magnesium Sulfate for Perioperative Analgesia
Intravenous magnesium sulfate administered intraoperatively provides meaningful reductions in postoperative pain and opioid requirements, which can improve quality of life in the immediate postoperative period. 1, 2, 3, 4
Dosing Regimens for IV Magnesium
Single bolus approach:
- 50 mg/kg in 100 mL normal saline administered IV over 15 minutes before induction of anesthesia reduces postoperative pain scores at 6,12, and 24 hours and significantly decreases 24-hour pethidine consumption (P=0.0001). 2
Continuous infusion approach:
- 15 mg/kg/hour continuous IV infusion starting 15 minutes before induction and continuing throughout surgery reduces pain scores at 6 and 12 hours postoperatively and decreases total opioid consumption over 24 hours (P=0.0001). 3
Standard infusion approach:
- 50 mg/kg in 500 mL Ringer's solution over 20 minutes after induction reduces pain scores immediately and at 1,2,6, and 12 hours postoperatively with lower morphine consumption. 4
Mechanism and Clinical Effects
Magnesium acts as an NMDA receptor antagonist, reducing central sensitization and postoperative pain. 3, 4
IV magnesium sulfate infusion significantly decreases serum beta-endorphin concentrations at the end of surgery (P=0.04), suggesting modulation of endogenous pain pathways. 3
The 2022 ERAS Society guidelines for low-middle-income countries specifically mention magnesium among agents to consider for multimodal opioid-sparing analgesia during surgery. 1
Regional Anesthesia Adjunct
Magnesium sulfate as an adjuvant to local anesthetics in TAP blocks:
Adding 200 mg magnesium sulfate (2 mL of 10% solution) to 20 mL of 0.25% bupivacaine per side in ultrasound-guided TAP blocks significantly prolongs time to first analgesic request (15.67 vs 7.33 hours, P<0.001) and reduces 24-hour morphine consumption (7.63 mg vs 16.20 mg, P<0.001). 5
However, one study found that adding 1 mL of 50% magnesium sulfate to ropivacaine 0.2% in TAP blocks did not produce statistically significant improvements in pain scores or analgesic consumption. 6
The discrepancy likely relates to magnesium concentration and total dose—200 mg appears more effective than 500 mg when used in TAP blocks, suggesting an optimal dose window. 6, 5
Magnesium is NOT Recommended for Preemptive Analgesia
Notably absent from the 2017 Society of Gynecologic Surgeons systematic review on preemptive analgesia for hysterectomy: The comprehensive guideline examining 69 randomized trials of preemptive medications for total abdominal hysterectomy did not identify magnesium as an effective preemptive agent. 1
- The guideline found that paracetamol, gabapentin, and narcotics (ketamine, morphine, fentanyl) were effective preemptive agents, but magnesium was not among the studied or recommended interventions. 1
Safety Considerations
Magnesium sulfate has an established safety profile when used appropriately: 7
Monitor for signs of hypermagnesemia: loss of deep tendon reflexes (occurs at 10 mEq/L), respiratory depression (rate <16 breaths/min). 7
Patellar reflex should be present before each repeated dose—if absent, hold magnesium until reflexes return. 7
Have IV calcium immediately available to counteract potential magnesium toxicity. 7
Avoid continuous maternal administration beyond 5-7 days in pregnancy due to risk of fetal abnormalities (hypocalcemia, skeletal demineralization). 7
Use with extreme caution in digitalized patients due to risk of cardiac conduction changes. 7
Reduce dosage in elderly patients and those with renal impairment (maximum 20 g/48 hours in severe renal insufficiency with frequent serum level monitoring). 7
Practical Implementation Algorithm
For total abdominal hysterectomy under general anesthesia:
Intraoperatively: Administer IV magnesium sulfate 50 mg/kg in 100-500 mL crystalloid over 15-20 minutes after induction of anesthesia. 2, 3, 4
If using TAP blocks: Add 200 mg magnesium sulfate (2 mL of 10% solution) to bupivacaine 0.25% (20 mL per side) for ultrasound-guided bilateral TAP blocks. 5
Postoperatively: Initiate oral magnesium oxide (specific dose not defined in trials but typically 400-800 mg) combined with bisacodyl suppositories starting postoperative day 1 to accelerate return of bowel function. 1
Common pitfall: Using magnesium as a standalone analgesic rather than as part of multimodal pain management—magnesium should complement, not replace, standard analgesic regimens including epidural analgesia for open procedures. 1