The Pathophysiology of Atrial Fibrillation

In a normal heart, the rhythmic contractions of the atria and ventricles are controlled by electrical signals. The SA node is the pacemaker of the heart, and is found in the right atrium; the SA node makes the impulse that begins each heart beat. This impulse goes through the atria, then through the AV node into the ventricles, causing them to contract.

In atrial fibrillation, the impulses that the atria receive are chaotic, so instead of a rhythmic complete contraction, the atria contracts in an uncoordinated, rapid twitching. When this happens, the AV node gets overloaded with impulses, so the ventricles also become out of sync, but not quite as much as the atria. The result of atrial fibrillation is a fast and uneven heart rate; it can range from 100-175 beats per minute. Since the ventricles are also out of sync, there is less time for the blood to fill the ventricles, which leads to a smaller stroke volume. Also, since the atrial kick is lost, less blood is getting to the ventricles during diastole, so the cardiac output is reduced. With a decreased cardiac output, the person can present with signs and symptoms of heart failure.

Some characteristics found in atrial fibrillation are an atrial rate of 300-600, with a ventricular rate of 120-200 in untreated a-fib; this is also marked by a very irregular rhythm of contraction. On an EKG, the QRS is usually normal, but there are no P waves detected. The PR interval cannot be measured.

Atrial fibrillation can be acute or chronic; in chronic a-fib, there is usually another condition associated with it, such as hypertension, diabetes, coronary artery disease, myocardial infarction, heart valve dysfunction (especially mitral or tricuspid valve stenosis), hyperthyroidism, sick sinus syndrome (a problem with the SA node), chronic obstructive pulmonary disease, viral infections, or sleep apnea.

Atrial fibrillation is associated with an increased risk of ischemic strokes, because blood can pool in the atria when they cannot completely empty- this blood can form a clot. There is also an increased risk of heart failure, because the heart is working so hard to keep up with the workload of the quivering atria.

Atrial fibrillation is a condition that will be diagnosed by a physician; treatments are very individualized, and may include pharmacologic treatments and/or surgical procedures.

References:

Porth, C. (2009). Pathophysiology: concepts of altered health states, (8^th ed.). Philadelphia: Lippincott.

Smeltzer, S. C., & Bare, B. G. (2010). Brunner and Suddarth's textbook of medical-surgical nursing, (12^th ed.).

Philadelphia: Lippincott, Williams & Wilkins.