Antiarrhythmics: Magnesium and Procainamide
When it comes to antiarrhythmic use, there are several that we must be familiar with to be a competent ACLS provider. Lidocaine and Amiodarone are discussed most frequently because they are the two primarily given to cardiac arrest patients in a shockable rhythm. Today, we’re reviewing two additional antiarrhythmics: Magnesium and Procainamide, as our pass ACLS tip topic.
Magnesium Sulfate
Magnesium can be given for a variety of medical conditions. For the purposes of ACLS, there are two primary indications for its use: Torsades de Pointes and hypomagnesemia. Depending on your level of training and organization’s protocols, magnesium may also be indicated for the treatment of eclampsia of pregnancy, severe asthma, and digitalis toxicity.
Torsades is a distinctive form of polymorphic ventricular tachycardia. The QRS complexes go through “cycles” of getting larger and smaller along the isoelectric line. Like other forms of V-Tach, patients in Torsades may have a pulse or be pulseless. As such, cardioversion with a synchronized shock should be administered to unstable patients with a pulse or defibrillation for those without a pulse.
For Torsades patients with a pulse, we’ll generally be in the tachycardia algorithm. Torsades is a polymorphic wide-complex rhythm so we should start an antiarrhythmic infusion and get an expert consultation. If we’re unsure if the rhythm is Torsades or just a funny looking V-Tach we may consider starting with Lidocaine to see if it resolves. Amiodarone should not be given to patients with a wide QT segment. Instead, start a magnesium drip. You can review the antiarrhythmics Lidocaine and Amiodarone in another Pass ACLS tip of the day.
For patients in Torsades with a pulse or patients with confirmed hypomagnesemia we should consider administration of 1-2 grams of Magnesium Sulfate given slowly via IV. For patients with a pulse, magnesium is diluted in D5 or normal saline and given over 5 to 60 minutes. Magnesium relaxes smooth muscle and can cause widespread blood vessel dilation resulting in hypotension so it should be given slowly to patients with a pulse and requires close monitoring to maintain a systolic BP above 90.
For pulseless patients with suspected Torsades refractory to defibrillation, magnesium is given as 1 to 2 grams slow IV push over 5-20 minutes. Once Torsades has resolved, magnesium may be continued as a drip at 500 mg to 1 gram per hour.
Procainamide
Let’s go back to the tachycardia algorithm and this time assume a stable patient has a monomorphic wide-complex rhythm on the ECG. In this case Procainamide would be preferred over magnesium and may be considered for use after, or in place of, Lidocaine.
Procainamide is given slowly as an infusion at 20-50 mg/min. We will start at the low dose and titrate up slowly. The patient must be monitored closely to determine when enough Procainamide has been given. We’ll stop the drip once we identify any of the following:
- The V-Tach rhythm has changed;
- The QRS widens by 50% from baseline;
- Hypotension occurs (in which case we should prepare to cardiovert); or
- We reach the maximum dose of 17 mg/kg.
Summary
Stable patients with polymorphic V-Tach suspected of Torsades should be given Magnesium while stable patients with monomorphic V-Tach should receive Procainamide. Both are administered slowly as a drip and can result in hypotension, so close monitoring is required.
If a patient with monomorphic or polymorphic V-Tach becomes unstable, immediate cardioversion with a synchronized shock should be given. Pulseless patients in V-Tach should be defibrillated following the Adult Cardiac Arrest algorithm.
