Antiarrhythmics, also called antidysrhythmics, are a class of medications that help control arrhythmias or abnormal heartbeats, such as supraventricular tachycardia, ventricular tachycardia and ventricular fibrillation.
In general, antiarrhythmics act by altering electrical conduction pathways through the heart, and can be administered orally, intravenously, or intramuscularly. Now, based on their mechanism of action, they can be divided into four main classes.
Class I antiarrhythmics are sodium channel blockers that inhibit sodium influx into cardiac cells, and can be further subdivided into three groups depending on how they affect conduction and repolarization. Class IA antiarrhythmics, such as quinidine and procainamide, act by slowing conduction and prolonging repolarization. Class IB, like lidocaine and mexiletine, also act by slowing conduction, but they shorten repolarization. And class IC, such as flecainide and propafenone, prolong conduction but have no effect on repolarization.
Next, class II antiarrhythmics are beta-blocker medications, and include propranolol, esmolol, sotalol, and acebutolol. These medications act by preventing the catecholamines norepinephrine and epinephrine from binding and activating them; ultimately decreasing heart contractility and slowing the conduction through the atrioventricular or AV node.
Moving onto class III antiarrhythmics, these are potassium channel blockers. The most important drug in this class is amiodarone, which acts by inhibiting potassium efflux from cardiac cells, ultimately prolonging repolarization and action potential duration.
Finally, class IV antiarrhythmics are calcium channel blockers, and include verapamil and diltiazem. These medications block calcium influx into cardiac cells, which ultimately helps decrease heart contractility and slow conduction velocity.
Unfortunately, antiarrhythmics can have several side effects. First of all, they can surprisingly cause arrhythmias! Additionally, they can cause bradycardia and hypotension as well as gastrointestinal disturbances, like nausea and vomiting.
Diving deeper into each class, class I antiarrhythmics, or sodium channel blockers, may cause hepatotoxicity. Quinidine can cause additional side effects like angioedema, and hematologic side effects like thrombocytopenia, agranulocytosis, and anemia, as well as cinchonism, which involves symptoms like tinnitus, vertigo, headaches, and visual changes.
Procainamide has a boxed warning for agranulocytosis and medication-induced systemic lupus erythematosus, and mexiletine has a boxed warning for acute liver injury. Additionally, flecainide in particular has a boxed warning for ventricular proarrhythmic effects in clients with atrial fibrillation or atrial flutter.
On the other hand, class II antiarrhythmics, or beta blockers, can cause dizziness, fatigue, and bronchospasm, since they also block beta receptors that are in the lungs. In addition, beta blockers have shown to cause certain metabolic changes like hyperglycemia, hypertriglyceridemia, and hyperkalemia; while clients with diabetes may experience hypoglycemia unawareness. Finally, sotalol has a boxed warning for life-threatening proarrhythmia.
Now, class III antiarrhythmics, or potassium channel blockers, can cause neurological side effects, and amiodarone in particular can cause thyroid problems, and a bluish skin discoloration, as well as skin rash and pruritus, and even toxic epidermal necrolysis. In addition, amiodarone has a boxed warning for cardiac arrhythmias, pulmonary toxicity, and hepatotoxicity.
Finally, class IV antiarrhythmics, or calcium channel blockers, can cause hyperprolactinemia, constipation, and peripheral edema. As far as contraindications go, all antiarrhythmics should not be used in clients with pre-existing hypotension, heart failure, bradycardia, heart block, and torsade de pointes. In addition, antiarrhythmics should be used cautiously during pregnancy and breastfeeding, in children and elderly clients, as well as in clients with hepatic or renal disease.
Class IA antiarrhythmics in particular are contraindicated in clients with thrombocytopenia or thrombocytopenic purpura. These medications should also be avoided in clients experiencing heart block that’s greater than first degree; as well as in those with idioventricular conduction delays who don’t have a functioning artificial pacemaker. Other contraindications for class IA antiarrhythmics include myasthenia gravis, as well as clients adversely affected by anticholinergic medications, and those taking certain quinolone antibiotics that can prolong the QT interval.
On the other hand, the class IB antiarrhythmic lidocaine is contraindicated in clients with heart conditions like Adam-Stokes syndrome and Wolff-Parkinson-White syndrome, as well as severe degrees of heart block who don’t have a functioning artificial pacemaker. Finally, lidocaine should be avoided in clients with corn allergy, as its premixed injection may contain corn-derived dextrose.
Next, beta blockers should be used with caution in clients with asthma and chronic obstructive pulmonary disease, or COPD, due to their bronchoconstrictive effect, as well as in clients with diabetes due to their hyperglycemic effect. Finally, clients on potassium or calcium channel blockers must avoid consuming grapefruit, as their interaction can result in toxicity.