Class II Antidysrhythmic Therapy (Beta Blockers)

Class II antidysrhythmic medications, also known as beta blockers, are a class of medications that help control dysrhythmias, or abnormal heart rhythms. Now, there are two types of beta blockers: nonselective beta blockers, like propranolol, and cardioselective beta blockers, like metoprolol.

Alright, beta1 and beta2 receptors are adrenergic receptors that bind with catecholamines like norepinephrine and epinephrine, which are neurohormones of the sympathetic nervous system, or SNS. Beta1 receptors are located in the heart and beta2 receptors are mostly in the lungs. You can remember this by thinking we have one heart and two lungs. In the heart, beta1 receptors stimulate cardiac activity, whereas in the lungs beta2 receptors cause smooth muscle and bronchial relaxation.

Nonselective beta blockers, like propranolol, block effects of both beta1 and beta2 receptors of the heart and lungs, while cardioselective beta blockers, like metoprolol, block only the beta1 receptors unless given at higher doses.

Now, by blocking adrenergic receptors, beta blockers decrease the effects of the SNS, and can be used to decrease dysrhythmias that are caused by an excessive amount of SNS stimulation.

For instance, the excessive catecholamine response after a myocardial infarction, or MI, makes the heart hyperirritable, which can lead to dysrhythmias. Beta blockers block the actions of catecholamines on the beta1 receptors, reducing the risk of post-MI ventricular dysrhythmias.

Specifically, beta blockers decrease the heart rate by delaying the conduction through the atrioventricular, or AV, node and decreasing automaticity of the sinoatrial, or SA, node. They also reduce myocardial contractility, which decreases the workload of the heart.

Now, even though antidysrhythmics are indicated to treat dysrhythmias, they tend to have prodysrhythmic properties, meaning they can make the dysrhythmia worse, or even cause new dysrhythmias.  Other effects include bradycardia and AV block due to its effect on cardiac conduction. They also reduce cardiac output, because of reduced cardiac contractility, which can lead to hypotension, postural hypotension, and dizziness. And, since propranolol is nonselective beta blocker, it can cause bronchoconstriction because of its effects on beta2 receptors.