Antiarrhythmics

Antiarrhythmics are drugs used to prevent and treat arrhythmias.

Antiarrhythmic agents are used when the heart rhythm is irregular, especially in case of tachycardia characterized by a racing heart rate.

This group of drugs is used to treat:

• Tachycardias (also called tachyarrhythmias)
• Atrial or ventricular fibrillation
• Atrial flutter
• Extrasystole (premature heart beat)

These types of drugs are primarily used to control the heart rate during an arrhythmic episode but can also be used long-term to prevent the occurrence of new episodes.

Determining the dosage for antiarrhythmic medications is often difficult, because it is common to either fall short or even exceed it and trigger a new arrhythmia.

Regular medical monitoring, including dose reviews and side effect assessments, holds great importance when managing arrhythmias with medication.

Antiarrhythmic agents act by blocking or slowing down the electrical impulse of the heart, contributing to the establishment of a stable heart rhythm.

There are many ways to classify antiarrhythmics, but the following classification of antiarrhythmic is the more common:

• Class I: Sodium-channel blockers.
  • Class IA: Quinidine, procainamide and disopyramide.
  • Class IB: Mexiletine, phenytoin and lidocaine.
  • Class IC: Flecainide and propafenone.
• Class II: Beta blockers. Examples: propranolol, metoprolol, nadolol, carvedilol, nebivolol and timolol.
• Class III: Potassium channel blockers: sotalol, amiodarone, dronedarone and ibutilide.
• Class IV: Calcium channel blockers. Examples: verapamil, diltiazem and nifedipine.

Other antiarrhythmic agents are used mainly in emergency services such as atropine, adenosine and digoxin for very specific cases.

Cardiac glycosides (such as digoxin), also known as digitalis, help the heart muscle to have stronger contractions and to control arrhythmias.

They are prescribed to treat atrial fibrillation, since they slow down the heart rate.

These types of drugs are especially useful if the arrhythmia is causing a heart failure because they improve cardiac output.

Atropine increases heart rate and it is the first-line therapy for symptomatic bradycardia (slower than normal heart rate).

Atropine is used in hospital settings and it is very important in Cardiopulmonary Resuscitation (CPR).

Quinidine slows down the electrical impulses responsible for the contraction of the heart muscle needed to pump blood.

It is used in atrial fibrillation, atrial flutter and paroxysmal supraventricular tachycardia (PSVT).

Beta blockers work by blocking the effect of the hormone epinephrine (adrenaline) in the body, mainly by slowing down the heart.

As a result, the heart beats at a reduced rate since the speed of impulses transmitting from the sinus node to the rest of the heart decreases.

In general, they are used in people with coronary artery disease because, in addition to the antiarrhythmic effect, they relieve chest pain associated with angina and reduce blood pressure. For that reason, they protect the heart and improve life expectancy.

As a side effect, they can produce fatigue, tiredness and lethargy.

Beta blockers have also an adverse effect on blood lipids, especially by reducing high-density lipoprotein (HDL) cholesterol, sometimes called “good” cholesterol.

Classification of beta blockers

Calcium channel blockers prevent calcium from entering the cells of the heart and arteries.

They prevent the constriction of small arteries and increase the flow of blood and oxygen to the heart (as a consequence the body requires less oxygen). At the same time, they slow the heart rate.

Verapamil and diltiazem (two types of calcium channel blockers) are useful to treat arrhythmias.

Classification of calcium channel blockers, dosage and duration of action

Some other antiarrhythmic medications are commonly used both in emergency services and outpatient care.

However, they require specialized evaluation due to their potentially dangerous side effects.

Here are some examples:

• Adenosine: Limited to intravenous administration in case of emergency for paroxysmal atrial tachycardia in unstable patients: low blood pressure, syncope.
• Amiodarone: Used to treat tachycardias. Oral use is somewhat uncertain because it may cause thyroid hormone alterations, high blood pressure, vision problems and liver disorders.
• Flecainide: Used in difficult-to-manage tachycardias. Side effects may include blurred vision and, in extreme cases, heart failure and heart block.
• Ibutilide: Primarily used in emergency services to treat atrial fibrillation and flutter.
• Lidocaine: Known as a local anesthetic, it is also administered intravenously for urgent treatment or prevention of ventricular arrhythmias, for example, during a heart attack.
• Magnesium: It is not known exactly how it works, but it is successfully used in specific arrhythmias like those caused by cardiac glycosides poisoning or a specific heart rhythm disorder called torsades de pointe.
• Propafenone: Used in ventricular tachycardias or atrial fibrillation in the emergency services.

You should discuss the advantages and disadvantages of these drugs with your doctor.