Episode

Key Takeaways

Antiarrhythmic drugs are categorized into four distinct classes based on their target cardiac ion channels.
Understanding the five phases (0-4) of the cardiac action potential is fundamental to antiarrhythmic drug mechanisms.
Mnemonics are frequently utilized to simplify the memorization of specific drug names, their actions, and potential adverse effects.
Class 1 antiarrhythmics, particularly Class 1A agents like Quinidine, pose risks such as QT prolongation and Torsades de Pointes.
Amiodarone, a Class 3 antiarrhythmic, exhibits extensive adverse effects, including thyroid and lung toxicity, attributed to its high lipid solubility.

Class 1A drugs, including Disopyramid, Quinidine, and Procanamide, are noted for prolonging the QT interval and increasing Torsades de Pointes risk.
Quinidine (Class 1A) historically caused drug-induced torsade de pointes and can induce cinchonism, a syndrome of tinnitus and confusion.
Procanamide (Class 1A) is used for Wolf-Parkinson-White syndrome arrhythmias and can cause a lupus-like syndrome with chronic administration.
Class 1B drugs, Lidocaine and Mexiletine, treat ventricular arrhythmias, especially post-myocardial infarction, with potential for CNS toxicity.

Amiodarone is a potent Class 3 antiarrhythmic with a long half-life, up to 100 days, owing to its high lipid solubility.
Its adverse effects (mnemonic: BITCH) include blue-gray skin discoloration and interstitial lung disease, affecting up to 5% of patients.
Thyroid toxicity (hypo- or hyperthyroidism) is common due to Amiodarone's high iodine content, necessitating baseline and periodic monitoring.
Other notable effects include typically benign corneal microdeposits and hepatotoxicity, with transient liver enzyme rises observed in 25% of patients.

Class 4 antiarrhythmics comprise Verapamil and Diltiazem, identified as non-dihydropyridine calcium channel blockers.
These drugs selectively inhibit L-type calcium channels within cardiac pacemaker and non-pacemaker cells.
They decrease calcium influx during phases four and zero of the pacemaker action potential, thereby slowing heart rate and AV node conduction velocity.
Non-dihydropyridine calcium channel blockers primarily affect cardiac activity, in contrast to dihydropyridines which mainly target vascular smooth muscle.

Class 1 (sodium channel blockers) acts on phase zero; 1A prolongs QT, 1C is contraindicated post-MI, and 1B treats post-MI ventricular arrhythmias.
Class 2 (beta blockers) decreases calcium influx in phase 4 for rate control in AFib/Aflutter; contraindications include asthma and diabetes.
Class 3 (potassium channel blockers like Amiodarone, Dofetilide) prolongs phase 3 repolarization; Amiodarone has numerous adverse effects (BITCH mnemonic).
Class 4 (Verapamil, Diltiazem) decreases calcium influx in phases 4 and 0 for rate control in AFib/Aflutter/SVT, with constipation and bradycardia as ADRs (CCB mnemonic).

Class 2 beta blockers, often ending in '-LOL' or '-OLOL', block epinephrine and norepinephrine binding to beta receptors.
They flatten the phase 4 slope in pacemaker cells by slowing calcium influx, decreasing SA and AV node firing rates.
Beta blockers also reduce cardiac contractility and oxygen demand in cardiomyocytes through decreased calcium influx.
Primary indications for beta blockers as antiarrhythmics include rate control for atrial fibrillation and atrial flutter.

Cardioselective beta blockers (beta-1 selective) include Bisoprolol, Esmolol, Atenolol, and Metoprolol, remembered by the mnemonic BEAM.
General beta blocker contraindications (mnemonic: ABCD) include Asthma, Bradycardia, Cardiogenic shock, and Diabetes (due to masked hypoglycemia).
Class 3 antiarrhythmics, also known as potassium channel blockers, prolong phase three repolarization by blocking potassium efflux.
These drugs can prolong the QT interval, increasing the risk of Torsades de Pointes, similar to Class 1A agents.

A 64-year-old man's fatigue, bluish-gray skin, elevated TSH, and increased liver enzymes were likely caused by Amiodarone, referencing the BITCH mnemonic.
For a 70-year-old man with atrial fibrillation experiencing palpitations, Dofetilide, a potassium channel blocker, was identified as suitable for rate control.
A 64-year-old AFib patient with mild, well-controlled asthma, intolerant to calcium channel blockers, was deemed suitable for cardioselective Metoprolol.