What is the recommended magnesium sulfate dosing for treating drug‑induced or congenital QTc prolongation at risk for torsades de pointes in adults and children?

Magnesium Sulfate Dosing for QTc Prolongation and Torsades de Pointes

For adults with drug-induced or acquired QTc prolongation at risk for torsades de pointes, administer magnesium sulfate 2 g intravenously over 1-2 minutes as first-line therapy, regardless of serum magnesium level; for children, use 3-12 mg/kg as an initial bolus. 1, 2

Adult Dosing Protocol

Initial Bolus Dose

• Administer 2 g of magnesium sulfate intravenously over 1-2 minutes as the first-line agent to terminate torsades de pointes, even when serum magnesium levels are normal. 1, 2
• This dose is effective in approximately 75% of cases within 1-5 minutes of administration. 3

Repeat Dosing for Persistent Episodes

• If torsades persists after the initial bolus, repeat with an additional 2 g of magnesium sulfate 5-15 minutes after the first dose. 1
• This second bolus achieves complete abolition of torsades in nearly all remaining cases. 3

Continuous Infusion for Recurrent Episodes

• Initiate a continuous infusion at 1-2 g per hour (or 3-20 mg/min) when episodes continue despite bolus therapy. 2, 3
• Continue the infusion for 7-48 hours or until the QTc interval decreases below 500 ms. 3

Pediatric Dosing Protocol

Initial Bolus Dose

• Administer 3-12 mg/kg intravenously over 1-2 minutes as the initial bolus for children with torsades de pointes. 4, 5
• Most children (approximately 85%) respond completely to this initial dose. 5
• Neonates or refractory cases may require up to 30 mg/kg total until complete abolition of torsades. 4, 5

Continuous Infusion

• Use 0.5-1.0 mg/kg per hour as the continuous infusion rate for children. 4, 5
• Maintain this infusion for 2-7 days to prevent recurrence. 5

Target Serum Magnesium Levels

• Aim for serum magnesium concentrations of 3-5 mg/dL (2.9-5.4 mg/dL range) immediately after bolus injection in pediatric patients. 4, 5

Essential Concurrent Interventions

Electrolyte Optimization

• Correct potassium to 4.5-5.0 mmol/L (or at minimum ≥4.0 mmol/L) simultaneously with magnesium administration. 1, 2
• Maintain serum magnesium ≥2.0 mmol/L for effective QT-interval management. 2
• These electrolyte targets help shorten the QT interval and reduce torsades risk, though magnesium itself does not necessarily shorten QTc. 2, 5

Drug Discontinuation

• Immediately discontinue all QT-prolonging medications that may be contributing to the arrhythmia. 2, 6
• Provide the patient with a list of QT-prolonging drugs (available at www.qtdrugs.org) upon discharge. 1

Refractory Cases: Second-Line Therapies

When Magnesium Fails

• Implement temporary transvenous cardiac pacing at rates >70 beats per minute for pause-dependent torsades that persists despite magnesium therapy. 1, 6
• Alternatively, initiate isoproterenol infusion at 2-10 mcg/min when pacing cannot be immediately implemented, titrating to increase heart rate sufficiently to abolish postectopic pauses. 6

Important Contraindication

• Avoid isoproterenol in patients with congenital long QT syndrome, as it can worsen the condition. 6

Critical Clinical Pearls

Mechanism of Action

• Magnesium suppresses torsades de pointes without necessarily shortening the QT interval, likely acting as a calcium channel blocker at the sarcoplasmic reticulum. 2, 5
• The protective effect occurs regardless of baseline serum magnesium levels. 1, 2

ECG Warning Signs Requiring Immediate Magnesium

• QTc prolongation >500 ms (except with amiodarone or verapamil). 1
• Marked QT-U prolongation and distortion after a pause. 1
• Onset of ventricular ectopy, couplets, or macroscopic T-wave alternans. 1
• Short-long-short R-R cycle sequence initiating polymorphic ventricular tachycardia. 1

Safety Considerations

• Magnesium toxicity is rare at standard torsades dosing but can occur at levels of 6-8 mEq/L, manifesting as areflexia progressing to respiratory depression. 2
• No significant side effects were reported in the landmark adult series using 2 g boluses. 3

Specificity for Torsades

• Magnesium is ineffective for polymorphic ventricular tachycardia with normal QT intervals (non-torsades VT), which require conventional antiarrhythmic therapy instead. 3

Cardiac Arrest Context

• Do not use magnesium routinely in cardiac arrest unless torsades de pointes is specifically suspected. 2
• When torsades is suspected during arrest, administer 1-2 g as a rapid IV push after defibrillation attempts. 2