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The human heart has four chambers. The upper chambers, known as atria, are shaped like a pyramid. The lower chambers are called ventricles. Each heartbeat is initiated by the sinoatrial node (SA node), which is the heart’s intrinsic pacemaker and is located in the atrium. The electrical impulse generated by the SA node causes the atria to beat first, which pushes blood into the ventricles. The electrical impulse is carried by fibers to the atrioventricular node (AV node) located in the wall separating the upper and lower chambers. The AV node sends the impulse into the ventricles causing them to beat, which then sends blood into the aorta and pulmonary artery as well as the rest of the body. This electrical activity can be mapped on an EKG. There are many heart disorders that can disrupt this electrical circuit. By far the most common arrhythmia, or abnormal heart rhythm, is atrial fibrillation (AF).

AF is caused by an abnormal pacemaker often located just outside of the heart (usually in the pulmonary vein). This abnormal pacemaker sends out electrical pulses, often as fast as 300 beats per minute, which disrupts the functioning of the SA node. Because of this disruption the atria do not beat, or contract, but rather just quiver or fibrillate. These rapid impulses get through to the AV node, but because they don’t all get through, the ventricle beats at an irregular and often rapid rate. Because in AF the atria quiver and do not contract, blood can pool in its corners (remember the atrium is shaped like a pyramid). This pooled blood can clot and when the clot is dislodged it is sent out into circulation. The most common effect of this clot getting out of the heart is a stroke.

Risk factors for AF include hypertension, obesity, diabetes, sleep apnea, alcohol abuse, and congestive heart failure. Because the formation of a clot in the heart can have deleterious effects including a stroke, blood thinners are often prescribed to those with AF. The blood thinners, aka anticoagulants, make clot formation less likely albeit with significant potential side effects like gastrointestinal bleeding. But not everyone with AF is given blood thinners. The decision to give blood thinners is made by assessing a person’s risk of a stroke. The risk-assessment acronym is CHADS-VAS.

CHADS-VAS stands for Congestive heart failure, Hypertension, Age (75 and older), Diabetes, Stroke or TIA history, Vascular disease, Age (65 to 75), and Sex (female). Age over 75 and history of stroke are given two points each, all the others are one point each. Someone with a score of zero to one will usually be placed on aspirin alone whereas a score of two or over will usually mean full anticoagulation. Smoking and a history of kidney disease are also significant factors that figure in the decision-making process. There is a separate acronym for bleeding risk called HAS-BLED but I won’t get into that today. A high HAS-BLED score (three or higher) will be a factor to consider against blood thinners.

If you have AF you should discuss your treatment as it pertains to your risk factors with your doctor to be sure that your treatment is appropriate. Next week I will discuss treatment options for AF.

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