What is the recommended intravenous magnesium dosing and infusion protocol for an adult ICU patient with documented hypomagnesemia, including adjustments for renal impairment?

Intravenous Magnesium Infusion Protocol for ICU Patients with Hypomagnesemia

Immediate Assessment Before Infusion

Check renal function first—magnesium sulfate is absolutely contraindicated when creatinine clearance falls below 20 mL/min due to risk of fatal hypermagnesemia. 1

• Obtain 12-lead ECG immediately to assess for QTc prolongation (>500 ms), prominent U waves, or active arrhythmias, as severe hypomagnesemia carries significant risk for torsades de pointes 2
• Initiate continuous cardiac monitoring if any ventricular arrhythmias are present or QTc >500 ms 2
• Measure concurrent potassium and calcium levels, as hypomagnesemia causes refractory hypokalemia and hypocalcemia that will not correct until magnesium is normalized 1, 3
• Assess volume status and check urinary sodium—if <10 mEq/L, this indicates volume depletion with secondary hyperaldosteronism that drives renal magnesium wasting 1

IV Magnesium Dosing Protocol

For Life-Threatening Presentations (Torsades de Pointes, Cardiac Arrest, Seizures)

Give 1–2 g magnesium sulfate IV bolus over 5 minutes immediately, regardless of measured serum magnesium level. 4, 3, 2

• This is a Class I recommendation from the American Heart Association for cardiac emergencies 3
• Follow with continuous infusion of 1–4 mg/min if arrhythmias persist 3

For Severe Symptomatic Hypomagnesemia (Mg <0.70 mmol/L or <1.7 mg/dL)

Administer 1–2 g magnesium sulfate IV over 15 minutes as initial bolus, followed by continuous infusion. 3, 2, 5

• For severe hypomagnesemia, up to 5 g (approximately 40 mEq) can be added to one liter of 5% dextrose or 0.9% sodium chloride for slow IV infusion over 3 hours 5
• The rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of 10% solution), except in severe eclampsia with seizures 5

For Mild-Moderate Hypomagnesemia (Asymptomatic)

Give 1 g magnesium sulfate (equivalent to 8.12 mEq) IV every 6 hours for four doses. 5

• Alternatively, add 4–5 g to 250 mL of 5% dextrose or 0.9% sodium chloride and infuse over 3–4 hours 5

Renal Function-Based Dose Adjustments

• Normal renal function (eGFR ≥90 mL/min): Each 1 g IV magnesium sulfate raises serum magnesium by approximately 0.10 mg/dL 6
• Impaired renal function (eGFR 30–89 mL/min): Each 1 g IV magnesium sulfate raises serum magnesium by approximately 0.15 mg/dL—use reduced doses with close monitoring 1, 6
• Severe renal insufficiency (eGFR <30 mL/min but >20 mL/min): Maximum dose is 20 grams over 48 hours with frequent serum magnesium monitoring 1, 5
• eGFR <20 mL/min: Absolute contraindication except for life-threatening arrhythmias, then use only with extreme caution 1, 3

Critical Concurrent Management

Volume Repletion Takes Priority

If the patient is volume depleted, administer IV normal saline (2–4 L/day initially) BEFORE magnesium supplementation to eliminate secondary hyperaldosteronism. 1, 3

• Failure to correct volume depletion first results in continued renal magnesium wasting despite supplementation—this is the most common therapeutic pitfall 1, 3
• Hyperaldosteronism from volume depletion increases renal magnesium and potassium losses, preventing effective repletion 1

Electrolyte Correction Sequence

Replace magnesium FIRST before attempting to correct hypokalemia or hypocalcemia—these abnormalities are refractory until magnesium is normalized. 1, 3, 2

• Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion 1, 3
• Target potassium >4 mEq/L during magnesium repletion 2
• Calcium normalization typically occurs within 24–72 hours after magnesium repletion begins 3

Infusion Preparation and Administration

• Dilute magnesium sulfate to a concentration of 20% or less prior to IV administration 5
• Common diluents are 5% dextrose or 0.9% sodium chloride 5
• Use a central venous catheter when possible to avoid tissue injury from extravasation 3
• Do NOT mix magnesium sulfate with calcium or vasoactive amines in the same solution 3, 5

Monitoring During Infusion

Immediate Monitoring (During and First 24 Hours)

• Continuous cardiac monitoring for arrhythmias, bradycardia, and hypotension 3, 2
• Check patellar reflexes every 2–4 hours—loss of reflexes occurs at plasma concentrations of 3.5–5 mmol/L and is the first warning of toxicity 7
• Monitor respiratory rate (respiratory paralysis occurs at 5–6.5 mmol/L) 7
• Measure urine output (oliguria increases toxicity risk) 3
• Recheck serum magnesium, potassium, calcium, and creatinine every 6–12 hours during IV replacement 1

Signs of Magnesium Toxicity

• Loss of patellar reflexes: 3.5–5 mmol/L 7
• Respiratory depression: 5–6.5 mmol/L 7
• Altered cardiac conduction: >7.5 mmol/L 7
• Cardiac arrest: >12.5 mmol/L 7
• Hypotension and bradycardia can occur with rapid infusion 3

Have calcium chloride 10% (5–10 mL) immediately available at bedside to reverse magnesium toxicity if needed. 3, 2

Transition to Maintenance Therapy

• Once acute symptoms resolve and patient tolerates oral intake, transition to oral magnesium oxide 12–24 mmol daily (approximately 480–960 mg elemental magnesium) 3, 2
• Administer oral magnesium at night when intestinal transit is slowest to improve absorption 1, 3
• For patients with malabsorption or short bowel syndrome, use organic magnesium salts (aspartate, citrate, lactate) for better bioavailability 1

Special Populations

Patients on Continuous Renal Replacement Therapy (CRRT)

• Hypomagnesemia occurs in 60–65% of critically ill patients on CRRT, especially with regional citrate anticoagulation 1, 8
• Use dialysis solutions containing magnesium to prevent ongoing electrolyte derangements 1, 8
• Implement early IV magnesium supplementation and close monitoring as part of the CRRT protocol 8

Pregnancy (Eclampsia/Pre-eclampsia)

• Initial dose: 4–5 g IV in 250 mL over 15–20 minutes, followed by 1–2 g/hour continuous infusion 5, 7
• Therapeutic level for seizure control: 1.8–3.0 mmol/L (approximately 4.3–7.2 mg/dL) 7
• Do NOT continue magnesium sulfate beyond 5–7 days in pregnancy due to risk of fetal abnormalities 5

Common Pitfalls to Avoid

• Never attempt potassium or calcium correction before normalizing magnesium—these will be refractory to treatment 1, 3, 2
• Never start magnesium in volume-depleted patients without IV saline first—secondary hyperaldosteronism will cause continued renal losses 1, 3
• Never give magnesium if creatinine clearance <20 mL/min unless treating life-threatening arrhythmia 1, 3
• Never infuse faster than 150 mg/minute except in eclamptic seizures—rapid infusion causes hypotension and bradycardia 3, 5
• Never assume normal serum magnesium excludes deficiency—less than 1% of total body magnesium is in blood 1