Antiarrhythmic Drugs: List of Medication Classes & Examples
Antiarrhythmic Drugs: List of antiarrhythmic medication classes and examples! Memory tricks and cardiac action potentials included!
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Antiarrhythmic Drugs
What are Antiarrhythmic Drugs?
Antiarrhythmic drugs are medications used to treat abnormal heart rhythms (arrhythmias), such as:
Atrial fibrillation
Atrial flutter
Supraventricular tachycardia
Ventricular tachycardia
Ventricular fibrillation, etc.
What are the 4 Classes of Antiarrhythmic Drugs?
Class I: Sodium Channel Blockers
Class II: Beta Blockers
Class III: Potassium Channel Blockers
Class IV: Calcium Channel Blockers
How do Antiarrhythmic Drugs work? (Mechanism of Action)
Antiarrhythmic drugs work to slow or normalize the heart rate and rhythm by:
Blocking various ion channels in the heart
Sodium channel blockers (class I)
Potassium channel blockers (class III)
Calcium channel blockers (class IV)
Blocking the sympathetic nervous system effects on the heart
Beta blockers (class II)
The above actions affect different phases of the cardiac action potential to slow or normalize the heart rate and rhythm.
Memory Trick!
This lecture includes:
Memory tricks to remember the different classes of antiarrhythmics
Mechanism of action
Examples
How each class affects the cardiac action potential
Diagrams and more!
Comment below if this lecture was useful!
Antiarrhythmic Drugs: List of antiarrhythmic medication classes and where they act on cardiac action potential phases.
Cardiac Action Potentials
Let’s briefly review cardiac action potentials, as this will help us understand antiarrhythmics.
If you would like more detail on cardiac action potentials, please visit our full lecture below:
There are 2 main types of cardiac cells:
Pacemaker cells (conduction)
Non-pacemaker cells (contraction)
The cardiac action potentials of pacemaker cells and non-pacemaker cells are slightly different.
Let’s review each one below.
1. Pacemaker Cells (conduction)
What are Pacemaker Cells?
Pacemaker cells are specialized heart muscle cells (cardiac myocytes) capable of generating their own spontaneous action potentials.
This is called automaticity.
These action potentials lead to cardiac conduction and contraction.
Of note: Cardiac (heart) muscle cells are also called cardiac myocytes, cardiomyocytes, or myocardiocytes.
Where are Pacemaker Cells Located?
Pacemaker cells are primarily located in the:
Sinoatrial node (SA node)
Atrioventricular node (AV node)
Pacemaker cells are also present in other parts of the conduction system including:
Bundle of His
Purkinje fibers
What is the Action Potential of Pacemaker Cells?
In the cardiac action potential lecture, we used the following memory trick to remember the action potentials of pacemaker cells:
Memory Trick: “Climb and Plummet”
The above memory trick helped us remember the action potential phases of pacemaker cells:
Phase 0: Climb (depolarization) = Calcium in
Phase 3: Plummet (repolarization) = Potassium out
Phase 4: “Resting” Phase
Reminder: Pacemaker cells do not have a phase 1 or phase 2 in their action potentials because they do not contract.
Instead, they are conduction cells, and simply need to depolarize and repolarize over and over to generate and conduct electrical impulses.
The electrical impulses depolarize the contractile non-pacemaker cells (see below), which in turn leads to cardiac contraction.
Reminder: Pacemaker cells do not have a true “resting phase”.
During phase 4, the pacemaker cells slowly become more positive on their own.
Once a threshold voltage (-40 mV) is met, a spontaneous action potential is generated.
Cardiac Action Potentials (Pacemaker Cells): Diagram of pacemaker cell action potentials and their ions explained.
2. Non-Pacemaker Cells (contraction)
What are Non-Pacemaker Cells?
Non-pacemaker cells are heart muscle cells (cardiac myocytes) that physically contract the heart and pump blood.
Unlike the pacemaker cells, they do not generate their own action potentials.
Instead, their action potentials are generated when neighboring cardiac myocytes are depolarized or when pacemaker cells stimulate them.
Where are Non-Pacemaker Cells Located?
Non-pacemaker cells are primary located in the:
Atria (atrial myocytes)
Ventricles (ventricular myocytes)
Non-pacemaker cells are responsible for atrial and ventricular contraction.
What is the Action Potential of Non-Pacemaker Cells?
In the cardiac action potential lecture, we used the following memory trick to remember the action potentials of non-pacemaker cells:
Memory Trick: “Summit, Plummet, Continue, Plummet”
The above memory trick helped us remember the action potential phases of non-pacemaker cells:
Phase 0: Summit (depolarization) = Sodium in
Phase 1: Plummet (slight repolarization) = Potassium out
Phase 2: Continue (contraction) = Calcium in
Phase 3: Plummet (repolarization) = Potassium out
Phase 4: Resting Phase
Cardiac Action Potentials (Non-Pacemaker Cells): Diagram of non-pacemaker cell action potentials and their ions explained.
Antiarrhythmic Drugs: Mnemonic
Now that we understand cardiac action potentials, let’s discuss how antiarrhythmics work by blocking different phases of these action potentials.
The mnemonic to remember the different classes of antiarrhythmic medications is:
“Some Block Potassium Channels”.
“Some” = Sodium channel blockers = Class I antiarrhythmics
“Block” = Beta blockers = Class II antiarrhythmics
“Potassium” = Potassium channel blockers = Class III antiarrhythmics
“Channels” = Calcium channel blockers = Class IV antiarrhythmics
Antiarrhythmic Drug Class List: Use the mnemonic “Some Block Potassium Channels” to remember the classification of antiarrhythmic medications.
Class I: Sodium Channel Blockers
As the name suggests, sodium channel blockers (class I antiarrhythmics) block sodium channels.
What part of the action potential did we say sodium is involved?
We said it was involved in the “summit” phase (phase 0) of atrial and ventricular myocyte action potentials, in which the influx of sodium led to depolarization of the cell.
If sodium is blocked from entering the cell, then the myocyte will have a harder time becoming more positive and depolarizing.
Therefore, sodium channel blockers decrease the slope of phase 0, and the depolarization rate and amplitude will be reduced.
Cardiac myocyte conduction velocity and transmission will be decreased as a result.
This will decrease atrial and ventricular myocyte excitability and serve to suppress abnormal conduction rhythms.
Since pacemaker cells use calcium ions to depolarize, sodium channel blockers have little effect on the SA node and AV node (pacemaker cells).
For this reason, sodium channel blockers are useful in re-entry tachyarrhythmias where blocking the AV node could be detrimental.
Sodium Channel Blocker Subclasses
It is important to note that there are 3 main classes to sodium channel blockers:
Class IA
Class IB
Class IC
The different class types are based on their effects on repolarization.
Some sodium channels increase action potential duration and effective refractory period by having a prolonged repolarization phase (class IA), some decrease action potential duration and effective refractory period by having a shortened repolarization phase (class IB), and some have no effect on either action potential duration, effective refractory period, or repolarization phase (class IC).
Examples of Class I Antiarrhythmics
Common examples of sodium channel blockers include:
Procainamide (class IA)
Lidocaine (class IB)
Phenytoin (class IB)
Flecainide (class IC)
Class I Antiarrhythmic Drugs: Sodium channel blockers are class I antiarrhythmic medications that act on phase 0 of non-pacemaker cells (red line - mechanism of action).
Class II: Beta Blockers
We’re going to skip beta blockers briefly as they do not act directly on ion channels. We will first discuss class III and IV antiarrhythmics and finish with beta blockers.
Class III: Potassium Channel Blockers
As the name suggests, potassium channel blockers (class III antiarrhythmics) block potassium channels.
What part of the action potential did we say potassium was involved?
We said it was involved in the “plummet” phase (phase 3) of both atrial/ventricular myocyte and pacemaker cell action potentials, in which the efflux of potassium led to repolarization of the cell.
If potassium is blocked from exiting the cell, then the repolarization phase will be prolonged. This will increase action potential duration and effective refractory period.
Since pacemaker cells and atrial/ventricular myocytes both use potassium ions to repolarize, potassium channel blockers will act on both types of action potentials.
They will reduce the excitability of atrial and ventricular myocytes as well as reduce conduction velocity through the conduction system.
This will serve to suppress tachyarrhythmias.
Examples of Class III Antiarrhythmics
Common examples of potassium channel blockers include:
Amiodarone
Dofetilide
Ibutilide
Class III Antiarrhythmic Drugs: Potassium channel blockers are class III antiarrhythmic medications that act on phase 3 of pacemaker and non-pacemaker cells (blue line).
Class IV: Calcium Channel Blockers
As the name suggests, calcium channel blockers (class IV antiarrhythmics) block calcium channels.
What part of the action potential did we say calcium was involved?
We said it was involved in the “climb” phase (phase 0) of pacemaker cell action potentials, in which the influx of calcium led to depolarization of the cell.
We also said calcium was involved in the “continue” phase (phase 2) of atrial/ventricular myocyte action potentials, in which the influx of calcium led to contraction.
If calcium is blocked from entering the pacemaker cells, then the depolarization rate and amplitude will be decreased thereby decreasing SA node automaticity and AV node conduction velocity.
This will ultimately decrease heart rate (negative chronotropy).
If calcium is blocked from entering the atrial/ventricular myocytes, then decreased cardiac contraction will occur (negative inotropy).
Both of these effects will help to suppress tachyarrhythmias.
Examples of Class IV Antiarrhythmics
Common examples of calcium channel blockers include:
Verapamil
Diltiazem
Class IV Antiarrhythmic Drugs: Calcium channel blockers are class IV antiarrhythmic medications that act on phase 0 of pacemaker cells and phase 2 of non-pacemaker cells (green).
Class II: Beta Blockers
Beta blockers do not act directly on ion channels.
Rather they act by antagonizing (blocking) beta-adrenergic receptors.
The heart predominately has beta1 adrenergic receptors. These receptors are located on both atrial/ventricular myocytes and pacemaker cells.
Normally catecholamines such as norepinephrine and epinephrine will bind to beta1 adrenergic receptors in the heart to increase heart rate and cardiac contractility, part of the sympathetic response.
Since beta receptors are located on both non-pacemaker myocytes and pacemaker cells, beta blockers will have effects on both the atrial/ventricular myocytes and SA/AV node.
By blocking the beta adrenergic receptors on pacemaker cells, this will prolong and decrease the slope of phase 4 of the action potential leading to decreased heart rate (negative chronotropy).
Furthermore, blocking beta adrenergic receptors on non-pacemaker myocytes will decrease phase 2 cardiac contractility (negative inotropy).
Both of these effects will help to suppress tachyarrhythmias.
Examples of Class II Antiarrhythmics
Common examples of beta blockers include:
Metoprolol
Propranolol
Esmolol
Atenolol
Timolol
Class II Antiarrhythmic Drugs: Beta blockers are class II antiarrhythmic medications that act on phase 4 of pacemaker cells and phase 2 of non-pacemaker cells (purple line).
Summary
Hopefully this helped clarify antiarrhythmic medications.
Use the mnemonic “some block potassium channels” to remember the different classes of antiarrhythmic medications.
Some = Sodium channel blockers (class I)
Block = Beta blockers (class II)
Potassium = Potassium channel blockers (class III)
Channels = Calcium channel blockers (class IV)
Class I sodium channel blockers: Act by reducing the influx of sodium ions during phase 0 of the atrial/ventricular myocyte action potential. This decreases depolarization rate and amplitude thereby decreasing conduction velocity and myocyte excitability.
Class II beta blockers: Act by antagonizing beta-adrenergic receptors. This decreases heart rate (pacemaker cell phase 4) and cardiac contractility (atrial/ventricular myocyte phase 2).
Class III potassium channel blockers: Act by reducing the efflux of potassium ions during phase 3 of the atrial/ventricular myocyte and pacemaker cell action potentials. This prolongs repolarization thereby increasing action potential duration and effective refractory period; both will lead to decreased excitation of the cells.
Class IV calcium channel blockers: Act by reducing calcium influx of calcium ions during phase 0 of the pacemaker cell action potential and phase 2 of the atrial/ventricular myocyte action potential. This will decrease SA/AV node automaticity/conduction velocity and decrease cardiac myocyte contractility respectively.
Last updated 12/08/2025
https://www.cvpharmacology.com/antiarrhy/sodium-blockers
https://www.cvpharmacology.com/antiarrhy/potassium-blockers
https://www.cvpharmacology.com/vasodilator/CCB
https://www.cvpharmacology.com/cardioinhibitory/beta-blockers