Magnesium Sulfate Dosing for QTc 603 ms
For an adult with a QTc of 603 ms concerning for torsades de pointes risk, administer 2 g of intravenous magnesium sulfate as a rapid bolus over several minutes as first-line therapy, regardless of serum magnesium level. 1, 2
Immediate Administration Protocol
- Give 2 g IV magnesium sulfate as a bolus over several minutes (not 15 minutes, not 1 hour—this is an emergency situation with severe QT prolongation) 1, 2
- Administer through the most proximal IV access available and follow with a normal saline flush 2
- Have calcium gluconate immediately at bedside to reverse potential magnesium toxicity 3, 2
- Maintain continuous ECG monitoring with a defibrillator immediately accessible 1, 2
Repeat Dosing if Needed
- If torsades de pointes develops or persists, repeat 2 g boluses until rhythm converts 1, 2
- The American Heart Association explicitly supports repeated 2 g doses for refractory cases 1
Critical Actions Beyond Magnesium
- Immediately discontinue all QT-prolonging medications (this is Class I evidence) 1
- Replicate potassium to 4.5-5.0 mmol/L even if currently "normal"—higher-normal potassium targets are specifically recommended for QT prolongation 1
- Consider temporary pacing or isoproterenol infusion if torsades becomes refractory to magnesium 1
Common Pitfalls to Avoid
Do not give 4 g over one hour—this exceeds standard recommendations for torsades and increases toxicity risk without additional benefit 3. The landmark adult study that established magnesium for torsades used 2 g boluses, with complete abolition of arrhythmia in 9/12 patients after a single dose and 3/12 after a second dose 4.
Do not wait for serum magnesium levels—magnesium works for torsades regardless of baseline magnesium status, and the original studies showed normal magnesium levels in all tested patients who still responded to treatment 4.
Do not expect QTc shortening—magnesium abolishes torsades without necessarily shortening the QTc interval, so lack of QTc change does not indicate treatment failure 5, 6.
Monitoring During Infusion
- Watch for hypotension and bradycardia, especially if infusing faster than 150 mg/min 2
- Common benign effects include flushing, warmth, and light-headedness 2
- If hemodynamic instability or sustained torsades develops, proceed immediately to direct-current cardioversion 1
Pediatric Dosing Note
If this were a pediatric patient, the dose would be 25-50 mg/kg (maximum 2 g) given rapidly over several minutes 7, 1, but for adults the standard is the fixed 2 g dose 1, 2, 4.