Premature atrial contraction

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Premature atrial contraction




Premature atrial contraction


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Premature atrial contraction

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USMLE® Step 1 style questions USMLE

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A 26-year-old female presents to the emergency department for evaluation of sudden onset palpitations. She is otherwise healthy and does not take any medications. The patient has had similar episodes in the past, and they self-resolved without treatment. However, the symptoms today have persisted for longer than usual. Temperature is 37.0°C (98.6°F), pulse is 172/min, respirations are 21/min, and blood pressure is 110/71 mmHg. Initial electrocardiogram is demonstrated below:  

Image reproduced from Wikimedia Commons  

Which of the following features of this patient's electrocardiogram can be used to localize the anatomic origin of this arrhythmia?  

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The heart has two upper chambers, which are called the atria, and a premature atrial contraction, or PAC, is when the atria contract earlier than normal in the cardiac cycle.

So normally, the sinoatrial, or SA, node sends an electrical signal called a depolarization that propagates out through the walls of the heart and causes both upper chambers to contract. Then, that signal moves to the atrioventricular, or AV, node, where it’s delayed for a split second. Then, the signal travels down into the ventricles, or lower chambers, where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing them to contract as well. This trip is called a depolarization wave, and in a healthy heart, it makes sure that the upper chambers contract before the lower chambers contract. On an electrocardiogram, or ECG, which measures the electrical activity of the heart via electrodes that are placed on the skin, the atrial depolarization and its contraction are seen as a P-wave, the ventricular contraction is seen as a QRS complex, and the ventricular repolarization and its relaxation are seen as a T-wave.

If an atrial cell outside of the SA node initiates a depolarization, that’s called an atrial ectopic focus. Now, this could be initiated by a cell that’s part of the conduction system, or it could just be initiated by another cardiac muscle cell. This typically happens when atrial cells are irritated and stressed by electrolyte imbalances, drugs like cocaine or methamphetamines, ischemic damage like a heart attack, or anything that increases sympathetic activity, like anxiety. Another type of atrial ectopic focus is a reentrant loop, which is when there’s some tissue that doesn’t depolarize properly, which could happen in scar tissue after a heart attack. As a result of this damage, the depolarization wave ends up circling around and around that tissue. A reentrant loop basically starts sending out depolarization waves to the rest of the heart tissue each time the wave goes around.


Premature atrial contraction is when the atria contract too early because of depolarization from an ectopic focus. While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, premature atrial contractions occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat. The ECG will show an early and abnormal P-wave, as well as one of three patterns, depending on when the PAC occurs: normal ventricular conduction, aberrant ventricular conduction, or a conduction block.


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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. "Premature atrial contraction-induced cardiomyopathy" EP Europace (2013)
  5. "Excessive Premature Atrial Complexes and the Risk of Recurrent Stroke or Death in an Ischemic Stroke Population" Journal of Stroke and Cerebrovascular Diseases (2017)

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