What is the recommended dose of magnesium sulfate (Mg sulphate) for a patient with ventricular tachycardia?

Magnesium Sulfate Dosing for Ventricular Tachycardia

For polymorphic VT with QT prolongation (torsades de pointes), administer magnesium sulfate 1-2 g IV over 15 minutes, which can be repeated as needed; for pulseless VT/VF associated with torsades, give 1-2 g IV/IO bolus diluted in 10 mL D5W. 1

Specific Dosing by Clinical Scenario

Torsades de Pointes (Polymorphic VT with Prolonged QT)

This is the primary indication where magnesium is highly effective and considered first-line therapy:

• Initial dose: 1-2 g IV over 15 minutes 1
• Cardiac arrest setting: 1-2 g IV/IO bolus diluted in 10 mL D5W (Class IIb, LOE C) 1
• Non-arrest setting: 2 g IV infusion as first-line agent, regardless of serum magnesium level (Class IIa, LOE B) 1
• Repeat dosing: If torsades persists, repeat 2 g infusions as necessary 1
• Alternative regimen: 8 mmol bolus followed by 2.5 mmol/h continuous infusion 2, 3

Monomorphic Ventricular Tachycardia

Magnesium is NOT routinely recommended for monomorphic VT with normal QT interval:

• Magnesium sulfate is not likely to be effective in terminating polymorphic VT in patients with a normal QT interval 1
• Three randomized controlled trials showed no significant benefit from magnesium compared to placebo in VF arrest without torsades 1
• Routine administration of magnesium sulfate in cardiac arrest is not recommended (Class III, LOE A) unless torsades de pointes is present 1
• Research evidence shows only 20-40% termination rates for monomorphic VT, which is not statistically significant compared to placebo 4

VT Storm or Refractory Cases

• For suspected hypomagnesemia in VT storm: 8 mmol IV (Class IIa) 2
• In acute myocardial infarction with VF/VT: 8 mmol bolus followed by 2.5 mmol/h infusion 3, 5

Administration Guidelines

Rate and Preparation

• Standard rate: Do not exceed 150 mg/minute (1.5 mL of 10% concentration) except in severe eclampsia with seizures 6
• Emergency bolus: Can be given over 1 minute for life-threatening arrhythmias (2-3 g over 1 minute has been used) 7
• Dilution: Solutions for IV infusion must be diluted to 20% concentration or less prior to administration 6
• Typical preparation: Dilute in 10 mL D5W for cardiac arrest, or 250 mL normal saline for continuous infusion 1, 6

Monitoring Requirements

• Follow magnesium levels if frequent or prolonged dosing is required, particularly in patients with impaired renal function 1
• Maximum dosing: In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent serum magnesium monitoring 6
• Monitor for side effects: hypotension, CNS toxicity, respiratory depression 1

Critical Clinical Distinctions

When Magnesium IS Indicated

• Polymorphic VT with prolonged QT interval (torsades de pointes) - this is the key indication 1
• Drug-induced QT prolongation with ventricular arrhythmias 1
• VT/VF in setting of acute myocardial infarction 3, 5
• Suspected or documented hypomagnesemia 2

When Magnesium is NOT Indicated

• Monomorphic VT with normal QT interval 1
• Routine use in all cardiac arrest situations 1
• As first-line therapy before amiodarone or lidocaine in stable monomorphic VT 1, 2

Common Pitfalls to Avoid

• Do not delay cardioversion in unstable patients to give magnesium - immediate synchronized cardioversion takes priority 3, 5
• Do not use magnesium as routine therapy for all VT - it is specifically effective for torsades de pointes 1, 4
• Do not assume efficacy in monomorphic VT - evidence shows minimal benefit in this setting 4
• Do not forget to correct underlying causes - stop QT-prolonging drugs, correct electrolyte abnormalities (target potassium 4.5-5 mmol/L), and address bradycardia 1