Intravenous Magnesium Sulfate for Torsades de Pointes
Administer 1-2 g of IV magnesium sulfate over 1-2 minutes as first-line therapy for torsades de pointes, regardless of baseline serum magnesium levels. 1, 2, 3
Dosing and Administration
Adult Dosing
- Initial bolus: 1-2 g IV magnesium sulfate over 1-2 minutes 1, 2, 3
- Dilute in 10 mL D5W for administration 1, 3
- If torsades persists or recurs, repeat with a second 2 g bolus 1, 4
- Follow with continuous infusion of 3-20 mg/min for 7-48 hours if needed until QTc <500 ms 1, 4
Pediatric Dosing
- 25-50 mg/kg IV (maximum 2 g) over 10-20 minutes for torsades with pulses 2, 3
- For pulseless torsades, give as a rapid bolus 2, 3
- Optimal dosing range: 3-12 mg/kg bolus, followed by 0.5-1.0 mg/kg/hr infusion 5
- Target serum magnesium concentration: 3-5 mg/dL 5
Critical Management Points
Mechanism and Efficacy
Magnesium works by preventing reinitiation of torsades rather than pharmacologically converting the rhythm 2, 3. Importantly, magnesium is effective even when baseline serum magnesium levels are normal—do not wait for lab results before administering. 1, 2, 3, 4
The drug does not immediately shorten the QT interval, but it rapidly terminates the arrhythmia within 1-5 minutes in most cases 6, 4. In one series of 12 consecutive patients, a single 2 g bolus completely abolished torsades in 9 patients within 1-5 minutes, with the remaining 3 responding to a second bolus 4.
Complete Treatment Algorithm
Step 1: Immediate stabilization
- If hemodynamically unstable, perform immediate DC cardioversion with appropriate sedation 1, 2, 3
- Withdraw all QT-prolonging medications immediately 1, 2, 3
Step 2: Magnesium administration
- Give 1-2 g IV magnesium sulfate over 1-2 minutes 1, 2, 3
- Repeat 2 g bolus if torsades persists after 5-15 minutes 1, 4
Step 3: Electrolyte correction
- Correct potassium to 4.5-5.0 mEq/L to shorten QT interval 1, 2, 3
- Address any hypomagnesemia with continuous infusion 1, 2
Step 4: Rate control for recurrent episodes
- For pause-dependent or bradycardia-associated torsades: initiate temporary transvenous pacing at rates >70 bpm 1, 2, 3, 7
- Alternative if pacing unavailable: isoproterenol infusion 2-10 mcg/min IV, titrated to abolish postectopic pauses 3, 7, 8
- Critical caveat: Do NOT use isoproterenol in patients with congenital long QT syndrome 2, 3, 7, 8
Monitoring During Treatment
Watch for magnesium toxicity, particularly with continuous infusion 2, 3:
- Hypotension and bradycardia during rapid infusion 2, 3
- Loss of deep tendon reflexes 2
- Respiratory depression or paralysis 2, 3
- CNS toxicity 3
Have calcium chloride available to reverse potential magnesium toxicity if needed 2, 3.
Special Clinical Situations
Ischemia-Related Torsades
- Perform urgent coronary angiography with view to revascularization 2, 3
- Administer IV beta-blockers in addition to magnesium 2, 3
LQT3 Patients
Digoxin-Induced Torsades
- Administer digoxin-specific Fab antibody for severe intoxication 3
Common Pitfalls to Avoid
Do not wait for magnesium level results before administering—magnesium is effective regardless of baseline levels, and 8 of 12 patients in one series had normal magnesium levels before treatment 3, 4.
Do not use magnesium for polymorphic VT with normal QT interval—it is ineffective in this setting 1, 4. In one study, 5 patients with polymorphic VT but normal QT intervals received magnesium without benefit 4.
Do not use calcium for torsades—calcium has no role in management and should not be used 2.
Do not use isoproterenol in congenital LQTS—it can worsen the condition 2, 3, 7.
Monitor for increased defibrillation energy requirements if sodium channel blockers were involved, as these may require reprogramming of cardiac devices 3.