Atrial fibrillation

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Atrial fibrillation




Atrial fibrillation


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USMLE® Step 1 questions

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Atrial fibrillation

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USMLE® Step 1 style questions USMLE

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A 72-year-old female presents to the emergency department with shortness of breath and lightheadedness. The patient’s symptoms started an hour ago while she was watching television at home. She had a similar episode that occurred several weeks ago and resolved without medical intervention. Past medical history is notable for hypertension and hyperlipidemia. She is a non-smoker and drinks a glass of wine during the weekend. Temperature is 37.0°C (98.6°F), pulse is 144/min, respirations are 15/min, blood pressure is 122/62 mmHg, and oxygen saturation is 97% on room air. Physical exam is otherwise unremarkable, and initial electrocardiogram is demonstrated below:  

Image reproduced from Wikimedia Commons  

If left untreated, this patient is at greatest risk for which of the following complications?  

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Anticoagulant drugs p. 420

atrial fibrillation p. 300


atrial fibrillation p. 728

Atrial fibrillation

β -blockers for p. 329

calcium channel blockers for p. 363

cardiac glycosides for p. NaN

ECG tracing of p. 300

embolic risk with p. 728

embolic stroke p. 529

hypertension as cause p. 306

jugular venous pulse in p. 294

potassium channel blockers for p. 329

sleep apnea p. 703

Coronary artery disease

atrial fibrillation and p. 300


atrial fibrillation p. 300

Hypertension p. 306

atrial fibrillation and p. 298

Stroke p. 529

atrial fibrillation and p. 300

Thromboembolic event

atrial fibrillation p. 300


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.


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  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)

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