Atrial Fibrillation and Anticoagulation - The Basics

What is Atrial Fibrillation

Atrial fibrillation is an abnormal heart rhythm, where the normal electrical signal of the heart – starting in the atria – has been overridden by chaotic rapid signals. In the atria (the top chambers of the heart), atrial fibrillation can reach rates over 500 beats per minute (bpm) at any given site in the chamber. These signals are not all relayed to the bottom chambers (ventricles), but the signals that are sent are done so irregularly. The ventricles are chambers responsible for pumping blood and from which we feel our pulse. So, when in atrial fibrillation, we feel an "irregularly irregular" pulse based on the unpredictable pattern in which signals are conducted from atria to ventricles. Most people have a fast rate to accompany their irregular rhythm (resting rate over 100 bpm without medications), but each individual differs.

Why Does Atrial Fibrillation Have a Risk of Stroke?

As noted, the top chambers of the heart (the atria) experience electrical stimulation over 500 bpm. This results in the tissue fibrillating (essentially quivering) and not contracting appropriately. Blood flowing through the atria to the ventricles is already slow, and this makes flow slower, as it is not being forcibly "pushed" into the ventricles.

Further, there is the presence of a small finger-like pouch that is attached to the left atrium, called the left atrial appendage. This pouch is outside the main current of blood that flows from the left atrium to the left ventricle (which is then pumped to the whole body), and in normal rhythm, the flow of blood into and out of the appendage is slower than the rest of the left atrium. In atrial fibrillation, this flow is slowed even more. As a result, the blood that enters the left atrial appendage can slow and stay in the appendage to a point where its stagnation leads to clot formation.

Once this clot is formed, it can either enter the main left atrium during atrial fibrillation or if the patient converts back to normal rhythm (the result being that there are some patients who suffer a stroke which is later found to be due to atrial fibrillation but they were in normal rhythm when presenting with their stroke). From there it would flow to the left ventricle, and then be ejected from the heart to the aorta. The clot will travel typically in the straightest path possible due to its weight compared to the surrounding blood until it reaches a branching vessel smaller than the clot. This means that most clots will flow into the vessels supplying the brain. Some clots will avoid cerebral vessels and end up elsewhere in the body (legs/toes, arms, liver, kidneys, etc), but a majority will go to the brain and cause a stroke.

What is Your Risk of Stroke with Atrial Fibrillation?

The risk of stroke in the setting of atrial fibrillation is not equal across the board. Certain conditions have been found to increase your risk of stroke in the setting of atrial fibrillation. This has been titled the CHADS-VASc score (of note, I have simplified it slightly, as the study authors separate Age to account for the 2 As in the acronym):

1. C = Congestive heart failure (reduced left ventricular function)
2. H = Hypertension
3. A = Age (1 point for 65 or over, 2 points for 75 or over)
4. D = Diabetes
5. S = Prior Stroke or TIA (2 points)
6. V = Vascular disease (coronary artery disease or peripheral vascular disease)
7. Sc = Sex category = female gender

The maximum is a score of 9, but most patients range from 0-5. In practice, a point is given for any of the above diagnoses, even if the condition is well controlled (someone with history of hypertension on a blood pressure drug with currently well-controlled blood pressure would still be given a point).

Should You Be On a Blood Thinner?

If you have a CHADS-VASc score of 0, then your risk of stroke is very low. Blood thinners can still be used (particularly if there is plans for intervention such as cardioversion or an ablation, or if there is strong patient preference).

If you have one point, then the discussion may also involve aspirin versus blood thinner.
If there are two or more points, then typically a systemic blood thinner (not aspirin) would be recommended. Duration of therapy is variable, but for many patients the recommendation would be for lifelong therapy given the fact that atrial fibrillation tends to recur after it has happened once, even with interventions like suppressive medications (anti-arrhythmic drugs) or ablation.

There may be contraindications for use of a blood thinner, such as prior significant bleeding or other conditions that predispose to bleeding with anticoagulant use.

There are risks and benefits to use of anticoagulation, as well as to withholding of therapy as well. Your presentation is unique, and decision about what therapy to pursue for stroke risk reduction should be made in concerted effort with your doctor after considering options.