What is the recommended dosing regimen for magnesium sulphate (magnesium sulfate) and calcium gluconate in a patient with severe hypocalcemia and hypomagnesemia, as well as QT interval prolongation?

Dosing Regimen for Magnesium Sulfate and Calcium Gluconate in Severe Hypocalcemia, Hypomagnesemia, and QT Prolongation

For patients with severe hypocalcemia, hypomagnesemia, and QT prolongation, administer intravenous magnesium sulfate first to correct the magnesium deficiency (which is essential for calcium homeostasis), followed by calcium gluconate, with both medications requiring repeat dosing based on continuous ECG monitoring and serial electrolyte measurements every 1-4 hours until target levels are achieved. 1, 2, 3

Initial Magnesium Sulfate Administration

Magnesium must be corrected first because hypomagnesemia causes refractory hypocalcemia by impairing PTH secretion and blocking calcium channel function. 4, 5

First-Line Magnesium Dosing:

• For severe hypomagnesemia with torsades de pointes or recurrent arrhythmias: Administer 2 g (approximately 16 mEq) IV magnesium sulfate over 5-10 minutes as an initial bolus 1
• For severe hypomagnesemia without immediate life-threatening arrhythmia: Give 1-2 g (8-16 mEq) IV every 6 hours for 4 doses (total 32.5 mEq per 24 hours) 2
• Maximum infusion rate: Do not exceed 150 mg/minute (1.5 mL of 10% concentration) except in severe eclampsia with seizures 2

Repeat Magnesium Dosing:

• Target serum magnesium: ≥2.0 mmol/L (approximately 2.4 mg/dL) 1
• Continuous infusion option: After initial bolus, administer 5 g (40 mEq) in 1 liter of D5W or normal saline over 3 hours 2
• For critically severe deficiency: Up to 250 mg/kg (approximately 2 mEq/kg) may be given IM over 4 hours if necessary 2
• Maximum daily dose: Do not exceed 30-40 g per 24 hours 2

Critical Magnesium Precautions:

• Magnesium produces vasodilation and may cause hypotension if administered rapidly 1
• Monitor blood pressure continuously during rapid infusion 2
• In severe renal insufficiency, maximum dosage is 20 g per 48 hours with frequent serum magnesium monitoring 2

Calcium Gluconate Administration

Begin calcium gluconate only after initiating magnesium replacement, as hypocalcemia will be refractory to treatment without adequate magnesium. 4, 6

Initial Calcium Dosing:

• Adults with severe symptomatic hypocalcemia and QT prolongation: 1,000-2,000 mg (10-20 mL of 10% calcium gluconate) IV over 10-20 minutes 3
• Pediatric patients: 100-200 mg/kg/dose via slow IV infusion with continuous ECG monitoring 7
• Alternative (calcium chloride if central/IO access): 20 mg/kg (0.2 mL/kg of 10% solution) given slowly, which provides more rapid ionized calcium increase than calcium gluconate 1, 7

Repeat Calcium Dosing:

• Measure serum calcium every 4-6 hours during intermittent infusions 3
• During continuous infusion, measure every 1-4 hours 3
• Repeat bolus doses as necessary based on clinical response and calcium levels 1
• Continuous infusion option: After initial bolus, may give continuous infusion with close monitoring 3

Target Electrolyte Levels:

• Potassium: Repleting to ≥4.0 mmol/L is beneficial for patients with torsades de pointes 1
• Magnesium: ≥2.0 mmol/L 1
• Calcium: Normalize ionized calcium (typically 1.15-1.35 mmol/L) 1

Administration Route and Monitoring Requirements

Preferred Access:

• Central venous catheter is preferred over peripheral IV for calcium administration to reduce severe tissue injury risk from extravasation 1, 7
• IO access is equivalent to IV access and should be obtained immediately if peripheral IV attempts fail 7
• Stop injection immediately if symptomatic bradycardia occurs 1, 7

Continuous ECG Monitoring:

• Mandatory during all calcium and magnesium administration 7, 3
• Monitor for QT interval normalization, which indicates therapeutic response 1, 8
• Watch for bradycardia, heart block, or other arrhythmias 1

Management of Recurrent Torsades de Pointes

If torsades de pointes recurs despite magnesium administration, increase heart rate with atrial or ventricular pacing or isoproterenol to suppress the arrhythmia. 1

• Bradycardia-dependent torsades requires rate acceleration as definitive therapy 1
• Continue magnesium and calcium repletion simultaneously 1

Common Pitfalls to Avoid

• Never attempt to correct hypocalcemia before addressing hypomagnesemia—the calcium will not respond and you waste critical time 4, 5
• Do not mix calcium with sodium bicarbonate or phosphate-containing solutions—precipitation will occur 1, 2, 3
• Avoid over-correction of calcium, which can cause iatrogenic hypercalcemia, renal calculi, and renal failure 1
• Do not administer QT-prolonging medications (including amiodarone without expert consultation) in patients with acquired long QT syndrome 1
• Extravasation of calcium can cause severe tissue necrosis and calcinosis cutis—if this occurs, immediately discontinue infusion at that site 3

Duration and Reassessment

• Continue therapy until paroxysms cease and electrolytes normalize 2
• Hypocalcemia and hypomagnesemia often coexist with hypokalemia, requiring simultaneous correction of all three electrolytes 9, 4
• Long-term management requires identifying and treating the underlying cause (e.g., PPI use, malabsorption, hypoparathyroidism) 5, 1