Skip to content

Atrial fibrillation

Videos

Notes

Cardiovascular system

Pathology

Vascular disorders
Congenital heart defects
Cardiac arrhythmias
Valvular disorders
Cardiomyopathies
Heart failure
Cardiac infections
Pericardial disorders
Cardiac tumors
Cardiovascular system pathology review

Assessments
Atrial fibrillation

Flashcards

0 / 15 complete

Questions

1 / 5 complete
High Yield Notes
8 pages
Flashcards

Atrial fibrillation

15 flashcards
Questions

USMLE® Step 1 style questions USMLE

3 questions

USMLE® Step 2 style questions USMLE

5 questions
Preview

An 80-year-old woman comes to the emergency department because of left sided paralysis for the past two hours. Her husband says her symptoms began suddenly and that she is also unable to speak. Her temperature is 36.8°C (98°F), pulse is 90/min, respirations are 18/min, and blood pressure is 150/98 mm Hg. An ECG is obtained and is shown below. Which of the following is the most likely cause of the patient's paralysis?

External References
Transcript

Contributors:

The heart has four chambers: two upper chambers, the right and left atrium; and two lower chambers, the right and left ventricles. Fibrillation describes when the muscle fibers are all contracting at different times, so the end result is a quivering, or twitching movement.

Normally, an electrical signal is sent out from the sinus node in the right atrium. The signal then propagates out through both atria super fast, which allows them to depolarize at about the same time, so that you end up with a nice, coordinated contraction of the atria. That signal then moves down to the ventricles and causes them to contract shortly after.

With Atrial fibrillation, or A-fib or AF, signals move around the atria in a completely disorganized way that tends to override the sinus node. Instead of one big contraction, you get all these mini contractions that make it look like the atria are just quivering.

On an electrocardiogram, or ECG, normally the “P wave” corresponds to the atrial contraction. The “QRS complex,” which is the ventricular contraction, follows shortly after. During AF, all these small areas contract at different times so that you end up with an electrocardiogram that looks like scribbles, where each little peak corresponds to one spot in the atria twitching. Sometimes, a signal from one of these areas makes it down to the ventricles and cause ventricular contraction; these QRS complexes are interspersed at irregular intervals though, and usually at fairly high rates between 100 and 175 beats per minute.

In the normal heartbeat, a well-coordinated atrial contraction contributes a small amount of blood that’s called the “atrial kick.” People with AF lose this atrial kick; however, this loss isn’t life-threatening.

Okay, but how or why does this happen in the atrium? Why do the cells start depolarizing in a totally uncoordinated way? Well, the answer isn’t super cut-and-dry. There are a ton of risk factors that predispose someone to developing AF, and the exact mechanisms aren’t well understood. AF often happens alongside other cardiovascular diseases, including high blood pressure, coronary artery disease, valvular diseases — essentially anything that can create an inflammatory state or physically stretch out the atria and potentially damage the cells in the atria. Other, non-cardiovascular risk factors include: obesity, diabetes, and excessive alcohol consumption. Additionally, there also seems to be a genetic component.

These factors likely stress the cells in the atria, which can lead to tissue heterogeneity; or in other words, cells start taking on different electrical properties. For example, one cell might start conducting signals faster than its neighbor, and another cell might develop a shorter refractory period — the time following depolarization during which they can’t conduct another signal. These different tissue properties can ultimately cause the conduction in the atria to become unpredictable.

Normally, with tissue that’s the same, you’ll get essentially one wavefront of conduction that moves through the atria. According to the multiple wavelet theory, with different tissue properties, multiple wavelets develop. These wavelets conduct randomly around the atria, sometimes colliding and creating new “daughter wavelets.”

Along with the multiple wavelet theory, there’s also an automatic focus theory. According to the automatic focus theory, there’s a specific origin that is thought to initiate AF by rapid firing of electrical impulses that overtake the sinus node. Combined with risk factors and tissue heterogeneity, this can promote AF. It’s thought that a focused group of cells conduct cells in the cardiac muscle around pulmonary veins — yeah, pulmonary veins! Remember that these veins physically enter the left atrium, and where the pulmonary veins enter there is tissue that has really unique electrical properties.

Summary
Atrial fibrillation (AF or A-fib) is an abnormal supraventricular tachycardia characterized by rapid and irregular beating. ECG tracing shows a variable heart rate, irregularly-irregular rhythm, usually absent P-waves and non-distinct P-R interval, and a normal QRS interval of less than 0.12 seconds. Often it starts as brief periods of abnormal beating which become longer and possibly constant over time. Most episodes have no symptoms but occasionally there may be heart palpitations, fainting, shortness of breath, or chest pain. 
Sources
  1. "Robbins Basic Pathology" Elsevier (2017)
  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "Atrial Fibrillation" Annals of Internal Medicine (2017)
  5. "Atrial fibrillation" Journal of Biomedical Research (2014)
  6. "Risk Factors and Genetics of Atrial Fibrillation" Cardiology Clinics (2014)
  7. "Atrial Fibrillation" Circulation Research (2017)