Episode 87- Should we give IV magnesium to all patients in AF?

Hello and welcome to Episode 87 of ER-Rx- a podcast tailored to your clinical needs. I’m your host, Adis Keric, and in this Pharmacy Consult episode I’ll discuss whether or not we should be giving IV magnesium (Mg) to every patient in atrial fibrillation (AF) in your emergency department.

Stable, rapid AF is usually defined as a HR > 110 without signs of hemodynamic instability. In the ER setting, these patients are typically treated with beta-blockers or calcium channel blockers, and sometimes even digoxin or amiodarone. Lately I’ve been hearing more and more that adding IV Mg to the mix could help achieve rate control and maybe even rhythm control. This makes sense because Mg blocks calcium channels in the heart, reducing the sinus node rate while also prolonging AV conduction time and refractory periods. It also has some amiodarone-like actions on cardiac potassium channels.

We have data from two previous meta-analyses published in 2007 that looked at giving Mg in addition to standard of care for the treatment of rapid AF. And what they found was that Mg usually helped achieve rate control, and sometimes even rhythm control, when it was given as an adjunct agent. There’s also an updated, more powerful meta-analysis published in 2021 that’s going to be the focus of this episode.

In this newer meta-analysis, they excluded studies comparing Mg to other antiarrhythmic drugs or electrical cardioversion—remember we want to know its adjunct effects specifically- not if Mg should or could be used as a sole agent. They also excluded post-cardiac surgery patients given the unique, self-limited nature of AF in this setting, patients in AF without RVR, those who got their Mg orally, or those with other arrhythmias. What we’re left is looking at randomized, controlled trials that compared IV Mg to placebo in addition to standard-of-care to treat rapid AF in adults. The primary outcome included the ability to achieve rate and rhythm control as defined by the individual trial- typically this meant a pretty conservative HR <90-100 for the rate control or the ability to maintain sinus rhythm for the rhythm control outcome.

They included data from 6 trials, ending with 448 patients in the Mg group and 297 in the control group. The dose of Mg ranged from 3-10 grams, almost all patients had normal Mg levels at baseline, and the drugs used as standard-of-care varied between the studies, as expected. They found that IV Mg worked significantly better than placebo at achieving rate control (63% vs 40%; OR 2.49; CI 1.80-3.45) and unlike most previous trials, they found significantly more patients converted to normal sinus rhythm (21% vs 14%; OR 1.75: CI 1.08-2.84)- which can be both a good or bad thing depending on whether or not you actually wanted to convert your specific patient.

I know what you’re thinking: 3-10 g is a lot of Mg. And I really wish the individual trials would have used more reasonable doses, like the typical 1,2, or even 4 grams we usually give. But thankfully, in a subgroup analysis they found that lower doses of Mg ( 5 g for achieving rhythm control- and although it’s kind of a mystery as to why this was the case- I’m super glad it was because now we don’t have to worry about giving 6+ g of Mg in the ER.

Side effects included more flushing in those who got Mg (9% vs 0.4%; OR 19.79; CI 4.30-91.21), but there were no differences in rates of bradycardia (1% vs 0.4%) or hypotension (2% vs 0.8%). And of course, the meta-analysis itself wasn’t perfect; it included small, old studies, gave more digoxin than we do today, and didn’t mention potassium levels- which we know can also affect rates of conversion.

To wrap up- giving Mg with your rate-control agent of choice probably helps reduce the heart rate, and we have both a plausible mechanistic reason for this along with decent data from a few meta-analyses. But there are some caveats and cautions. We know that low baseline Mg levels are associated with developing AF, and that giving Mg may work better in patients with low Mg at baseline. But in all three meta-analyses, most patients had normal Mg levels- so it’s possible that the effect of Mg would be even more pronounced if we only gave it to patients with low Mg levels at baseline.

Also, in two meta-analyses, including the newest one, Mg actually helped convert patients to normal sinus rhythm, which as I mentioned before may be harmful depending on the scenario. If the rhythm started > 48 hours ago or patients aren’t anticoagulated already, converting to normal sinus rhythm isn’t a good idea.

My personal take is that Mg is a good adjunct to give to healthy, anticoagulated patients or those with AF that started within the previous 48 hours- especially if their Mg is low. So the answer to the question is, no, not every patient in AF should get Mg- especially those you don’t want to convert. If you do decide to give it, I recommend 1-2 grams over 30 minutes (as I talked about in Episode 30) prior to giving the rate-control agent. Along with Mg, remember to check and replace potassium as well if you have the time. And keep in mind that Mg shouldn’t be given alone and it probably doesn’t help when given prior to planned electrical or chemical cardioversion.

As always, thank you so much for your time, and thank you for wanting to learn more about pharmacotherapy. If you have any comments or anything you’d like to add to this episode, please give me a shout out on the @errxpodcast Instagram page, or reach out to me personally on errxpodcast.com- I’d love to respond to all comments and criticisms.

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