What is the protocol for administering magnesium (Mg) perioperatively?

Perioperative Magnesium Administration

Magnesium supplementation perioperatively does not provide clinically meaningful benefits for preventing postoperative atrial fibrillation or improving major outcomes (mortality, morbidity, length of stay), though serum magnesium levels should be monitored and corrected if deficient. 1

Evidence Quality and Recommendations

Cardiac Surgery Context

The most recent and highest-quality evidence from the 2019 EACTS/EACTA/EBCP guidelines definitively addresses magnesium use in cardiac surgery 1:

• A meta-analysis of 20 RCTs (3,696 patients) showed magnesium might reduce supraventricular arrhythmias, but this effect disappeared when only high-quality studies were analyzed 1
• A more recent RCT of 389 patients demonstrated intraoperative magnesium did NOT reduce postoperative atrial fibrillation incidence 1
• Magnesium had no effect on death, major morbidity, or length of stay 1

When to Consider Magnesium

Scottish recommendations suggest magnesium MAY be used when prophylaxis for AF and ventricular arrhythmias is specifically indicated following CABG surgery 1, though this is a weak recommendation given the conflicting evidence.

The 2005 ACCP guidelines reinforce this limited role 1:

• 14 trials with 1,853 patients showed only 1 of 14 trials found statistically significant reduction in postoperative AF 1
• Propranolol was superior to magnesium sulfate for AF prevention in direct comparison 1
• Serum magnesium levels should be maintained, perhaps with empiric supplementation, but magnesium does not provide significant prophylaxis 1

Monitoring and Correction Protocol

Preoperative Assessment

Potassium and magnesium homeostasis should be evaluated preoperatively, with special attention to patients on diuretics and those prone to arrhythmias 1:

• Any electrolyte disturbance—especially hypomagnesemia—should be corrected in due time before surgery 1
• Diuretics increase renal excretion of both K and Mg 1
• Dietary advice to increase Mg intake should be given; depleting drugs reduced if possible; sparing diuretics may be added 1

Intraoperative Management

Hypocalcemia (not hypomagnesemia) should be promptly treated during CPB due to its critical role in coagulation, cardiac rhythm, positive inotropy, and vascular tone 1

If magnesium is administered despite limited evidence:

• 2 g after induction of anesthesia until cardiopulmonary bypass, then 8 g upon ICU arrival infused over 24 hours 2
• Alternatively, 2 g IV intraoperatively after termination of cardiopulmonary bypass 3

Dosing Specifications (FDA Label)

For magnesium deficiency treatment 4:

• Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses 4
• Severe hypomagnesemia: Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours, or 5 g (40 mEq) added to 1 liter IV fluid infused over 3 hours 4
• IV injection rate should generally not exceed 150 mg/minute (1.5 mL of 10% concentration) 4
• Solutions for IV infusion must be diluted to 20% concentration or less 4

Monitoring Parameters

Therapeutic serum magnesium levels range from 3 to 6 mg/100 mL (2.5 to 5 mEq/L) 4:

• Normal serum level is 1.5 to 2.5 mEq/L 4
• Deep tendon reflexes begin to diminish when levels exceed 4 mEq/L 4
• Reflexes may be absent at 10 mEq/L, where respiratory paralysis is a potential hazard 4

Clinical safety indicators 4:

• Presence of patellar reflex (knee jerk)
• Absence of respiratory depression (approximately 16 breaths/min or more)
• Urine output maintained at 100 mL or more during the 4 hours preceding each dose

Critical Precautions

Renal Function

Magnesium is removed solely by the kidneys; use with extreme caution in renal impairment 4:

• In severe renal impairment, maximum dosage is 20 g/48 hours 4
• Frequent serum magnesium concentrations must be obtained 4
• Geriatric patients often require reduced dosage due to impaired renal function 4

Pregnancy Considerations

Continuous maternal administration beyond 5 to 7 days can cause fetal abnormalities including hypocalcemia, skeletal demineralization, and osteopenia 4:

• For pre-eclampsia/eclampsia: 4-5 g IV in 250 mL fluid, with simultaneous IM doses up to 10 g (5 g in each buttock), then 4-5 g IM every 4 hours as needed 4
• Magnesium sulfate with concurrent nifedipine carries risk of hypotension 1

Drug Interactions

Additive CNS depression occurs with barbiturates, narcotics, hypnotics, and systemic anesthetics—adjust dosages cautiously 4:

• Excessive neuromuscular block with neuromuscular blocking agents 4
• Serious cardiac conduction changes (heart block) in digitalized patients if calcium is required to treat magnesium toxicity 4

Antidote

An injectable calcium salt should be immediately available to counteract magnesium intoxication 4

Alternative Analgesic Role

Perioperative magnesium has been shown to reduce postoperative pain and opioid consumption through NMDA receptor antagonism 1, though this represents a different indication than arrhythmia prophylaxis.

Oral magnesium oxide promoted postoperative bowel function in some enhanced recovery protocols 1, though one RCT within an established ERAS protocol showed no significant effect on bowel function, nausea, or length of stay 1.

Bottom Line Algorithm

1. Preoperatively: Check and correct magnesium deficiency if present, especially in patients on diuretics 1
2. Intraoperatively: Do NOT routinely administer magnesium for arrhythmia prophylaxis based on current high-quality evidence 1
3. Consider magnesium only if specifically indicated for CABG patients at high risk for AF/ventricular arrhythmias AND beta-blockers are contraindicated 1
4. Postoperatively: Monitor levels and correct deficiency if it develops; maintain normal physiological magnesium status 1
5. Prioritize beta-blockers (class II agents, sotalol, or amiodarone) over magnesium for arrhythmia prophylaxis 1