What is the role of magnesium in the heart?

Role of Magnesium in the Heart

Magnesium acts as a vasodilator and is an important cofactor in regulating sodium, potassium, and calcium flow across cell membranes, playing a critical role in stabilizing excitable membranes and maintaining normal cardiac function. 1

Physiological Functions of Magnesium in the Heart

Magnesium serves several essential functions in cardiac physiology:

• Ion Channel Regulation: Magnesium is necessary for the movement of sodium, potassium, and calcium into and out of cardiac cells 1

• Membrane Stabilization: Acts as an important cofactor for multiple enzymes, including ATPase, helping to stabilize excitable membranes 1, 2

• Neuromuscular Transmission: Blocks excessive neuromuscular transmission by decreasing acetylcholine release at the motor nerve end-plate 2

• Enzymatic Reactions: Functions as a cofactor for numerous enzymes involved in energy generation and cellular metabolism 3

• Vascular Function: Regulates vascular tone, atherogenesis, thrombosis, vascular calcification, and the proliferation and migration of endothelial and vascular smooth muscle cells 3

Effects on Cardiac Electrophysiology

Magnesium has significant effects on cardiac conduction and rhythm:

• Antiarrhythmic Properties: Increases the ventricular threshold for fibrillation and prolongs sinus node refractoriness and AV node conduction 4

• ECG Changes with Abnormal Levels:

  • Hypomagnesemia: Can lead to prolonged PR and QT intervals
  • Hypermagnesemia: Can cause prolonged PR, QRS, and QT intervals; severe elevations (6-10 mmol/L) may result in AV block, bradycardia, and cardiac arrest 1

Clinical Implications of Magnesium Abnormalities

Hypomagnesemia (< 1.3 mEq/L)

• Cardiac Effects: Associated with various arrhythmias including:

  • Polymorphic ventricular tachycardia (torsades de pointes)
  • Ventricular fibrillation
  • Atrial fibrillation
  • PVCs and other ventricular arrhythmias 1

• Treatment: For cardiotoxicity and cardiac arrest associated with hypomagnesemia, IV magnesium 1-2 g of MgSO₄ bolus is recommended (Class I, LOE C) 1

Hypermagnesemia (> 2.2 mEq/L)

• Cardiac Effects: Can produce vasodilation, hypotension, bradycardia, cardiac arrhythmias, and cardiorespiratory arrest at extremely high levels 1

• Treatment: Administration of calcium (calcium chloride [10%] 5-10 mL or calcium gluconate [10%] 15-30 mL IV over 2-5 minutes) may be considered during cardiac arrest associated with hypermagnesemia (Class IIb, LOE C) 1

Therapeutic Applications

Torsades de Pointes

• Magnesium is the treatment of choice for torsades de pointes (polymorphic VT associated with long-QT interval) 1

• The American Heart Association recommends magnesium for torsades de pointes (Class IIb, Level of Evidence C-LD) 1

• Dosage: 1-2 g IV/IO bolus for VF/pulseless VT cardiac arrest associated with torsades de pointes 5

Cardiac Arrest

• The routine use of magnesium for cardiac arrest is not recommended in adult patients (Class III: No Benefit; Level of Evidence C-LD) 1

• Multiple randomized trials showed no benefit of routine magnesium administration in cardiac arrest for increasing ROSC or survival to hospital discharge 1

Other Cardiovascular Applications

• Used in severe asthma exacerbations with cardiovascular effects (standard dose: 2 grams over 20 minutes) 5

• May have benefits in patients with heart failure and ventricular arrhythmias when hypomagnesemia is present 1, 3

Monitoring and Safety

• Normal plasma magnesium levels: 1.5-2.5 mEq/L 2

• Clinical effects based on levels:

  • At >4 mEq/L: Deep tendon reflexes decrease
  • At ~10 mEq/L: Respiratory paralysis may occur

  • 12 mEq/L: May be fatal 2

• Careful monitoring is essential during magnesium administration, especially in patients with renal disease, as magnesium is excreted solely by the kidneys 2

Pitfalls and Caveats

• Despite its theoretical benefits, there is currently no firm recommendation for routine magnesium supplementation except when hypomagnesemia has been proven or suspected as a cause for cardiac arrhythmias 3

• Magnesium levels should be monitored in critically ill cardiac patients, as hypomagnesemia is common in hospitalized patients and may contribute significantly to cardiac morbidity and mortality 6

• The calcium-antagonistic effects of magnesium can be counteracted by IV administration of calcium if toxicity occurs 2