Class II antiarrhythmics: Beta blockers

Antiarrhythmic medications help control arrhythmias or abnormal heartbeats.

There are four main groups of antiarrhythmic medications: class I, sodium-channel blockers; class II, beta-blockers; class III, potassium-channel blockers; class IV, calcium-channel blockers; and miscellaneous antiarrhythmics, or unclassified antiarrhythmics. Now, we’ll focus on class II antiarrhythmics in this video.

First, let’s start with the two main types of cells within the heart; pacemaker cells and non-pacemaker cells.

Pacemaker cells build the electrical conduction system of the heart, which consists of the sinoatrial node, or SA node; the atrioventricular node, or AV node; the bundle of His; and the Purkinje fibers.

Pacemaker cells have a special property called automaticity, which is the ability to spontaneously depolarize and fire action potentials.

On the other hand, non-pacemaker cells, also known as cardiomyocytes, make up the atria and ventricles; and they give the heart its ability to contract and pump blood throughout the body.

Now, in contrast to non-pacemaker cells, whose action potential has 5 phases, an action potential in pacemaker cells has only 3 phases.

Here’s a graph of the membrane potential vs. time. Phase 4, also known as the pacemaker potential, starts with the opening of the pacemaker channels.

The current through these channels is called pacemaker current or funny current (If), and it mainly consists of sodium ions.

These sodium ions cause the membrane potential to begin to spontaneously depolarize and as the membrane potential depolarizes, voltage-dependent T-type calcium channels open up, thereby further depolarizing the pacemaker cell.