From the Guidelines
Intravenous magnesium can be used to slow down atrial fibrillation (AF) with a fast ventricular rate, although it is not the first-line treatment, and its use is supported by modest evidence. Magnesium acts primarily by blocking calcium channels, decreasing conduction through the AV node, and prolonging the refractory period, which collectively reduces ventricular response rate in AF 1. The typical dosing regimen is 2 grams of magnesium sulfate administered intravenously over 10-15 minutes, which can be followed by a maintenance infusion of 1-2 grams per hour if needed.
Key Points
- Several small studies have demonstrated that IV magnesium can reduce ventricular rate by approximately 20-30 beats per minute within 30 minutes of administration, with effects lasting several hours.
- Magnesium appears most effective in patients with AF who have normal or low serum magnesium levels.
- It's generally well-tolerated with minimal side effects at therapeutic doses, though patients should be monitored for hypotension, flushing, and respiratory depression, particularly those with renal impairment.
- While magnesium can be useful as an adjunct therapy, it should not replace standard rate-control medications like beta-blockers or calcium channel blockers in most clinical scenarios, as stated in the 2010 international consensus on cardiopulmonary resuscitation and emergency cardiovascular care science with treatment recommendations 1.
- The 2017 AHA/ACC/HRS guideline for management of patients with ventricular arrhythmias and the prevention of sudden cardiac death also supports the use of intravenous magnesium in certain arrhythmias, such as torsades de pointes, and digoxin toxicity 1.
Clinical Considerations
- The physiological basis for magnesium's effectiveness relates to its role as a cofactor in the sodium-potassium ATPase pump and its ability to modulate calcium-dependent processes critical to cardiac conduction.
- In clinical practice, intravenous magnesium can be considered as an adjunctive treatment for AF with a fast ventricular rate, particularly in patients with normal or low serum magnesium levels, and when standard rate-control medications are not effective or contraindicated.
From the Research
Mechanism of Action
- Iv magnesium slows down Af with fast ventricular rate by exerting its influence via its effect on cellular automaticity and prolongation of atrial and atrioventricular nodal refractoriness, thus reducing ventricular rate 2.
- The addition of magnesium to standard pharmacotherapy has been shown to be effective in achieving rate control and rhythm control in non-postoperative patients with AF with rapid ventricular response 2, 3.
Efficacy of Iv Magnesium
- Studies have shown that iv magnesium is effective in achieving rate control (odd ratio [OR] 1.87,95% confidence interval [CI] 1.13-3.11, p = .02) and rhythm control (OR 1.45,95% CI 1.04-2.03, p = .03) in non-postoperative patients with AF with rapid ventricular response 2.
- A systematic review and meta-analysis found that iv magnesium given in addition to standard-of-care was superior in achieving rate control (63% vs 40%; OR 2.49,95% CI 1.80-3.45) and rhythm conversion to sinus (21% vs. 14%, OR 1.75,95% CI 1.08-2.84) compared to standard-of-care alone 3.
- A retrospective study found that patients who received magnesium had a lower mean heart rate (85 BPM versus 96 BPM, P<0.05) 24 hours after onset of the episode, and that administration of higher levels of magnesium resulted in statistically lower heart rates 4.
Comparison with Other Agents
- Iv magnesium has been compared to other agents such as beta-blockers and calcium channel blockers, which are also effective in controlling ventricular rate in AF 5, 6.
- However, the use of iv magnesium as an adjunctive treatment has been shown to provide a synergistic effect in acute and prolonged control of AF with RVR 4.
Dosage and Administration
- The optimal dosage of iv magnesium for the management of AF with rapid ventricular rate is not well established, but studies have used doses ranging from 1-5 grams 3, 4.
- A subgroup analysis found that a lower dose of iv magnesium (5 g or lower) was superior to a higher dose (>5 g) in rhythm control when compared to placebo 3.