Magnesium for Tachycardia
Intravenous magnesium is the treatment of choice for torsades de pointes (polymorphic VT with long QT), but has limited to no efficacy for monomorphic ventricular tachycardia or most supraventricular arrhythmias. 1
Primary Indication: Torsades de Pointes
Magnesium is definitively indicated for polymorphic wide-complex tachycardia associated with long QT syndrome (torsades de pointes), whether congenital or acquired. 1
Dosing for Torsades de Pointes
- Initial bolus: 8 mmol (approximately 2 grams) IV over 1-2 minutes 1, 2
- Maintenance infusion: 2.5 mmol/hour 1
- Alternative dosing: 2 grams (8 mEq) IV bolus, repeated as needed 3, 4
- This reliably controls life-threatening arrhythmias in torsades de pointes 5
Mechanism-Specific Application
- Acquired long QT (drug-induced): Magnesium is first-line therapy, supported by multiple studies 1
- Congenital long QT: Magnesium may be used alongside pacing and beta-blockers; avoid isoproterenol 1
- Pause-dependent tachycardia: Consider adding overdrive pacing or isoproterenol when polymorphic VT is precipitated by bradycardia or pauses 1
Limited Efficacy: Monomorphic Ventricular Tachycardia
Magnesium has minimal effectiveness for monomorphic VT and cannot be recommended as emergency treatment. 5
Evidence Against Use in Monomorphic VT
- In randomized trials, magnesium terminated monomorphic VT in only 6 of 20 patients versus 3 of 24 with placebo (not statistically significant) 5
- Observational studies show termination in only 25-32% of cases 5
- No significant changes in RR intervals or QRS duration occur with magnesium administration during monomorphic VT 5
- Electrophysiologic studies demonstrate no antiarrhythmic effects in patients with inducible VT 6
Exception: VT Associated with Acute MI
- Magnesium may be effective for VF/VT specifically when associated with acute myocardial infarction 1, 3
- However, polymorphic VT from acute ischemia (short QT) showed no benefit from magnesium in controlled studies 1
Supraventricular Tachycardia
Magnesium has no established role in treating narrow-complex tachycardias. 1
Evidence Against Use in SVT
- Two studies demonstrated poor response to magnesium in narrow-complex tachycardia 1
- No evidence of benefit for conversion to sinus rhythm 1
Exception: Multifocal Atrial Tachycardia (MAT)
- Intravenous magnesium may be helpful in MAT, particularly in patients with hypomagnesemia 1
- Intramuscular magnesium (using pre-eclampsia dosing regimens) converted MAT to sinus rhythm in 1-2 hours 7
- IV infusion achieved conversion in 4-8 hours 7
- Consider in MAT patients with underlying pulmonary disease or heart failure, conditions associated with magnesium deficiency 7
Special Populations
Congestive Heart Failure
- In patients with heart failure and serum magnesium ≤2.0 mg/dL, IV magnesium reduces ventricular ectopy when serum levels increase ≥0.75 mg/dL 8
- Dose: 0.2 mEq/kg over 1 hour 8
- Patients with ≥300 PVCs/hour showed reduction from 794±309 to 369±223 per hour 8
- Couplets decreased from 233±505 to 84±140 8
Dosing in Other Arrhythmias
- Paroxysmal atrial tachycardia: 3-4 grams (30-40 mL of 10% solution) IV over 30 seconds, only if simpler measures fail and no myocardial damage present 9
- Barium poisoning with muscle stimulation: 1-2 grams IV 9
Important Clinical Caveats
When NOT to Use Magnesium
- Polymorphic VT from acute ischemia with short QT interval: Beta-blockers are preferred; magnesium showed no benefit 1
- Brugada syndrome: Use isoproterenol instead 1
- Catecholaminergic VT: Beta-blockers (IV propranolol) are the treatment of choice 1
- Severe renal insufficiency: Maximum dose is 20 grams/48 hours with frequent serum level monitoring 9
Monitoring Requirements
- Target serum magnesium level: 6 mg/100 mL for seizure control in eclampsia 9
- Maximum daily dose: 30-40 grams in 24 hours 9
- Monitor for precipitate formation when mixing with other solutions 9
- Magnesium may reduce antibiotic activity of streptomycin, tetracycline, and tobramycin 9