Magnesium to slow a heart going fast AF

Myth: Magnesium cures AF!
Malady: Don’t use IV magnesium as the sole agent for rate or rhythm control. 

Visualise this. MET call for tachycardia. Patient is stable. ECG shows rapid AF. No history of AF. How will you manage this patient?

Reader, if your answer starts and ends with IV magnesium, there’s a special place on my enemies list for you.

Now I’m not out to get Big Electrolyte or anything. I love potassium mini-bags as much as the next guy. But let’s break down some of the so-called “evidence” for the use of IV magnesium for rate and/or rhythm control.

Mg for AF has been studied in the context of rate and rhythm control, and there exists supportive evidence, including several meta-analyses for both [1-2]. These studies however are highly problematic and suffer from a heterogeneous patient population, very small sample sizes (we’re talking sometimes in the teens), and differing methodologies. The forest plots in these meta-analyses are being carried by a single study that demonstrated significance, and confidence intervals so wide, they make a neurologist’s list of differentials concise. Not all meta-analyses are created equal, and a lot of these tend to overemphasise the positive effects of smaller studies. Many of these studies often evaluate the efficacy of magnesium as an adjunct as opposed to standalone treatment, which is perhaps closer to its rightful place in acute AF management. When you then pool a bag of hogwash in a meta-analysis, as has been done a couple times recently, you only end up with malarkey. 

Another context in which IV magnesium is used is prevention of AF after cardiac surgery, where fortunes may be somewhat variable. A 2012 meta-analysis will have you believe it’s the miracle drug [3]. A more selective meta-analysis in 2013 tried to knock some sense into people [4]. There’s a current trial running so watch this space [5]. 

There’s no denying that magnesium as an ion is important in cardiac physiology. So far, there has not been convincing evidence showing a relationship between AF and dietary or serum magnesium levels. One meta-analysis even found that giving less is more with Mg dosage [1]. The thought process is that there is intracellular magnesium deficiency, one which we need more mechanistic studies to investigate. So, until they give me a tiny cellular spoon to feed those cardiac pacemaker cells with a yummy magnesium meal, I’ll stick to my beta-blockers.

Bottom line? We can still be friends if you use IV mag as an adjunctive therapy along with your rate or rhythm control strategy. I know it makes you feel like you’ve done something. I would probably respect you more if you do nothing if the patient is stable – around 70% of new acute AF reverts spontaneously by 72 hours [6]. The brave Dutch have also demonstrated the comparable efficacy of the watch-and-see approach [7]. But if you walk away from that MET call patting yourself on the back for fixing the problem with IV mag, I won’t apologise if I run you down with the MET trolley.

Giants’ Shoulders:
eTG (aka our Bible) – see Urgent Rate Control for Atrial Fibrillation
Lovely editorial from Professor Kotecha in Circulation: Arrhythmia and Electrophysiology

Others:
[1] Ramesh T et al‘s meta-analysis of adjunctive Magnesium use in Journal of Cardiology: https://www.sciencedirect.com/science/article/pii/S0914508721001519
[2] Ho et al’s meta-analysis in Heart (2007) https://heart.bmj.com/content/93/11/1433
[3] Examination of magnesium to prevent post-op AF after CABG: https://trialsjournal.biomedcentral.com/articles/10.1186/1745-6215-13-41
[4] Cook et al’s brave stance in the Annals of Thoracic Surgery: https://www.annalsthoracicsurgery.org/article/S0003-4975(12)02044-9/fulltext
[5] The POMPAE trial: https://trialsjournal.biomedcentral.com/articles/10.1186/s13063-024-08368-3
[6] Major study showing frequent spontaneous reversion to sinus rhythm (later reproduced multiple times, with similar results): https://www.sciencedirect.com/science/article/pii/S0735109797005342
[7] RCT for the watch-and-wait approach in new onset haemodynamically stable AF: https://www.nejm.org/doi/full/10.1056/NEJMoa1900353