Atrial fibrillation (AF) is a disorder of how your heart conducts the electrical signals that lead to contraction. Specifically it is a supraventricular tachyarrhythmia which is a fancy term that described where in the heart the problem starts.
Using a very simple illustration of the heart let’s first deconstruct this medical terminology.
The 2 chambers at the top of the heart are the atrias, the 2 chambers are the bottom of the heart are the ventricles.
“Supra” in medical terminology means above, therefore supraventricular means “above the ventricles”
“Tachy” in medical terminology means fast.
Simply put, atrial fibrillation, is a fast irregular rhythm occurring above the ventricles (the atria).
Normal Sinus Rhythm
To understand the pathology of AF and any other arrhythmia we must understand the basics of normal conduction of impulses through the heart. These impulses are what stimulates the contraction of the heart muscles to pump blood out to the body.
Coordination of impulse generation, impulse conduction and muscle contraction is what maintains normal sinus rhythm.
To understand normal cardiac conduction we will add the sinoatrial (SA) node and atrioventricular (AV) node to our illustration of the heart.
In normal conduction, the SA node generates an impulse that hits both atria and the AV node.
From the AV node the impulse continues down conduction pathway of the heart to stimulate the ventricles.
coordinated stimulation of cardiac muscles = strong contraction of cardiac muscles
In normal conduction this is all a coordinated relay of impulses and contractions that moves in waves across the heart.
This allows the atria to contract and empty blood in its chambers into the ventricles before the ventricles contract and pump blood into systemic circulation.
Conduction in Atrial Fibrillation
In AF, the impulse generated by the SA node is not coordinated. It is erratic and basically ricochets throughout the heart.
In AF, the atria may not be done contracting when the ventricles get the impulse to contract. This means the atria cannot fully empty its chambers. This can result in pools of stagnant blood. Whenever blood is stagnant it is likely to clot.
Most patients are not in AF all the time. They convert in and out of NSR and AF. This conversion between weak, uncoordinated and then strong, coordinated contractions creates opportunities for clots to become dislodged from the atria and enter systemic circulation where they can block off blood flow to critical sites like the brain and lungs.
Blockage of flow to the brain by a clot leads to ischemic strokes.
Blockage of flow to the lungs leads to a pulmonary embolism.
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Categorization of Atrial Fibrillation
Valvular versus Non Valvular AF (NVAF)
This is the most important categorization of AF.
Please note, valvular AF does NOT include all types of valvular disease, only moderate to severe mitral valve stenosis and artificial heart valves.
Other valvular conditions like aortic stenosis or regurgitation or mitral regurgitation would be considered NVAF.
In this regard the term “valvular” AF is a bit misleading but the exceptions are few so this remains the accepted terminology.
The distinction between valvular and non valvular AF is crucial to determine how the patient will be assessed and what therapies can and cannot be used.
From this point forward when AF is used I am referring to NVAF.
Of note, the guidelines do recommend that the use of the terms “valvular and non valvular” should be made obsolete because there has been inconsistencies in how patients have been placed in these categories and included in trials.
They note that recent trials assessing therapies in non-valvular AF have included patients with moderate to severe mitral stenosis.
As an example, although dabigatran is approved from the treatment of non-vavular AF, the manufacturer includes only patients with mechanical prosthetic heart valves among their listed contraindications.
Atrial Fibrillation as a Progressive Disease
The 2023 Guidelines for the Diagnosis and Management of Atrial Fibrillation introduced a new way of categorizing AF. The goal is to emphasize that AF is a progressive disease. This highlights the need for addressing modifiable risk factors and preventative care.
The 4 stages of AF are:
Classification Based on Duration or Pattern of AF
Traditionally, AF has been classified based on the duration/pattern of symptoms only. This classification is limiting because the risk of stroke is consistent across the patterns of AF.
Selection of stroke prevention therapy should be based on the risk of stroke which is not dependent on the pattern/duration of AF.
The timeframes used:
Now that we’ve out the basics of atrial fibrillation in terms of pathology and classification, the next step will be to how we treat atrial fibrillation. This is covered in its own unit.
If you’ve found this unit helpful I would love to hear from you! Leave a comment or question below.
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The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.
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