Episode 30- How fast can we give IV magnesium?

Hello and welcome to Episode 30 of ER-Rx. Today we’re going to answer a common question pharmacists get from providers and nurses: how fast can we give intravenous magnesium?

First, let’s back up a little bit. Before we can answer this question, we have to know why we are giving magnesium, and what dose we plan on giving. This is an important question because magnesium has many different roles in the ER. The most common being simply replacing magnesium in the setting of hypomagnesemia. But magnesium is also considered first-line for patients in torsades and for patients with pre-eclampsia or eclampsia. As an adjunctive agent, magnesium is also frequently given to treat asthma exacerbations.

So first, in the setting of hypomagnesemia, in the ER we usually give 1-4 grams, depending on how low the magnesium level is, how symptomatic the patient is, and other patient-specific information such as kidney function. In this setting, in general, the rule is that we don’t want to administer magnesium at a rate faster than 150 mg/minute. Calculated out, this gives us a time of 7 minutes for a 1 g dose and 14 minutes for a 2 g dose. But this comes with a caveat: the slower we give magnesium, the more of it is retained by our bodies. This is because up to 50% of an IV dose can be eliminated in the urine. Some people have been taught that magnesium should be given at a rate of 1 gram per hour, and although optimal, this may hinder ER flow. Who wants to wait around for 2 hours while your stable, ready to discharge patient gets a magnesium infusion?

Either way, a reasonable solution is to default 1-2-gram doses to be given over 30 minutes. Of course, if your patient is being admitted or if they aren’t planning to discharge soon, you can discuss with the nurse and the team and give it slower over, 1-2 hours if you really want to optimize retention.

In the setting of torsades or other ventricular arrhythmias with prolonged QTc intervals, magnesium is thought to work by slowing impulse formation and prolonging conduction time. In this setting, the rate of magnesium administration depends on the stability of your patient. Remember that magnesium in this situation won’t terminate these arrhythmias, it simply prevents them from recurring. In patients with a pulse, it’s recommended to give 1-2 grams of magnesium in adults (or 25-50 mg/kg/dose with a maximum of 2 grams) over 15 minutes in pediatric patients. This is because overly rapid administration can cause hypotension and even asystole- risks we don’t want to take in an already tenuous situation. Nursing can run a bag of 1-2 grams of magnesium using a pump, or run the infusion wide-open, without a pump, which can be adjusted by the nurse to run over 15 minutes-ish.

Now, in the setting cardiac arrest in a patient with pulseless ventricular tachycardia or ventricular fibrillation or torsades, magnesium can be given as a bolus over a couple of minutes. One way to do this is to grab a 1 or 2 gram magnesium bag and have a nurse squeeze the bag into the patient—think of this as a manual pressure bag. Some sites, like mine, have magnesium 1 gram vials in code carts or automated dispensing machines located in the ER. These vials can be diluted in 10-20 mL of normal saline and pushed over 1-2 minutes.

As an adjunct agent in the setting of acute severe asthma exacerbations, magnesium works by acting as a smooth muscle relaxant and by reducing mast cell degranulation. In this setting, we can give 2 grams in adults, or 25-50 mg/kg in pediatric patients over 30 minutes. This is obviously after you have already given your conventional, first-line therapies.

And finally, in the setting of OB emergencies such as severe pre-eclampsia or eclampsia, magnesium is thought to work in part through its effects on the NMDA receptor. In this setting, magnesium is given as a 4-6 gram load over 15-30 minutes, then a continuous infusion of 1-2 grams per hour. This will vary slightly from site to site.

Remember that we have to be very diligent in our doses and rates of magnesium administration. This is especially important in the setting of pre-eclampsia/ eclampsia. The Institute for Safe Medication Practices, or ISMP, has described a number of events related to magnesium overdoses in this setting. Most of the time, this happens as a result of mis-programmed pumps, staff being unfamiliar with doses and signs of magnesium toxicity, or inadequate monitoring. Therefore, they recommend that a qualified nurse is at bedside during infusion to monitor vital signs, oxygen saturations, reflexes, and periodic magnesium levels. They also have a few additional, detailed safe practice recommendations which you can find in my references for this episode. At my site, when magnesium is ordered for OB emergencies, our protocol calls for an OB nurse to come down to the ER for 1:1 patient monitoring.

Overall, with proper dosing and monitoring, side effects of magnesium are minimal. Patients can experience facial flushing and possibly transient hypotension, especially with overly-rapid infusions- so feel free to slow the rate down if any of these side effects occur.

In conclusion, be very mindful of each magnesium order’s indication. Doses between 1-6 grams may be ordered in the ER. Some situations call for bolus or rapid administration, whereas others call for slower infusion rates to reduce the risk of hypotension and increase retention. This will vary for each individual scenario.

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