Magnesium Benefit May Be Overestimated

Much like ketamine and tranexamic acid, magnesium is a darling of emergency medicine. Use of magnesium to augment rate and, to some extent, rhythm control in AFRVR has had some support.⁶ The new AHA/ACC guideline states that in patients with AFRVR, the addition of IV magnesium to standard measures is reasonable to achieve rate control. The committee gave this a class 2A (moderate) recommendation, citing the highest level of evidence (A) from more than one randomized control trial (RCT) or meta-analysis of high quality RCTs. The AHA/ACC cites a meta-analysis including five RCTs of 745 participants. On the surface, this meta-analysis found that the IV magnesium had greater odds of rate control (OR 2.49; 95 percent CI 1.80-3.5) as well as conversion to sinus rhythm (OR 1.75; 95 percent CI 1.08-2.84).7 Digging into the meta-analysis, a few important details emerge. First, a single study by Bouida and colleagues drove the meta-analysis, contributing over 60 percent of patients. The study compared four grams and 9.5 grams of IV magnesium to placebo, doses considerably higher than the two grams of magnesium often administered in the emergency department (ED). Further, the patients in this trial are not representative of U.S.-based care of ED patients with AFRVR. Over one-third of patients in this study received digoxin as their rate control agent, and about half received either diltiazem or a beta blocker. In fact, in the meta-analysis, digoxin was the most commonly administered agent. Consistent with guideline recommendations, digoxin is not commonly administered as a rate control agent in the U.S. due to the lengthy onset of action and higher risk profile compared with CCBs and beta blockers. Second, the outcomes definitions rate and rhythm control at four and 24 hours are far beyond times relevant in emergency medicine in the U.S. The evidence suggests that large doses of magnesium (4.5 g) may improve rate control in patients receiving digoxin.

Summary

The AHA/ACC guideline is a mixed bag, mostly centered on limited data for a bread-and-butter disease process. Cardiovert when it’s indicated or in concordance with patients’ wishes and the AFRVR is the cause. Try to avoid CCBs in patients with systolic dysfunction, understanding that there is limited evidence. And magnesium? Sure, add if you’re giving digoxin and/or failing at other rate control attempts with primary AFRVR.

Dr. Westafer (@Lwestafer) is an attending physician and research fellow at Baystate Medical Center, clinical instructor at the University of Massachusetts Medical School in Worcester, and co-host of FOAMcast.