Atrial Fibrillation (AFib)

By John Chorba, MD - Posted in Clinical Education
Summary: Read an overview of atrial fibrillation, which is the most common treated chronic arrhythmia.
Drawing of cardiac electrical system

Overview of atrial fibrillation (AFib)

Atrial fibrillation (AFib) is the most common treated chronic arrhythmia, with a lifetime risk of around 1 in 4 people older than 40.1 In AFib, rapid and disorganized electrical activity in the atria causes them to contract erratically (“fibrillate”) and ineffectively. This electrical activity bombards the AV node with inputs, prompting irregular ventricular contractions. Thus, AFib promotes atrio-ventricular asynchrony and generally results in an irregular heartbeat. AFib can be intermittent (“paroxysmal”) but can also become permanent, especially if left untreated. The major complications are tachycardia and intra-cardiac blood clots, which can travel to the brain and cause a stroke.2

Drawing of blood flow pattern in an anatomical heart

Pathophysiology

Though well studied, the mechanisms that cause AFib are still poorly understood. Many AFib drivers have been identified such as cellular level changes (calcium handling and automaticity) and abnormal cardiac physiology (atrial pressures and structure). AFib is associated with abnormal atria due to underlying cardiac diseases, such as ischemic heart disease, hypertension, valvular heart disease, and cardiomyopathies. It can also be driven by non-cardiac causes such as electrolyte disturbances, pulmonary embolism, toxic substances, and acute infections. These factors can also interact to drive feedback loops. For example, fibrosis and inflammation that cause morphological changes to the atria can alter the refractory period and promote a continuously running electrical circuit within the atrial tissue known as “re-entry.”

Sound recorded by Eko digital stethoscope technology:

Audio

When present, AFib produces an “irregularly irregular” heart rhythm. The erratic electrical activity in the atria results in contractions of varying strength, which causes S1 to vary in intensity (heard best at the cardiac apex).

ECG 

In AFib, the focus of atrial depolarization moves continuously around the atrium and is not fixed, resulting in an ECG with no clear P waves. Instead, irregular, low-amplitude fibrillatory waves may be seen, though these may be difficult to identify. The spontaneous electrical activity in the atria causes irregular ventricular depolarizations, which can be seen on the ECG as inconsistently spaced QRS complexes. The variable time between ventricular depolarizations also means that the strength of each ventricular contraction can vary. Some QRS complexes are associated with contractions that are so weak that they do not produce an audible heart sound. This is known as “electrical-mechanical dissociation.”

References

1. Kornej J, Börschel CS, Benjamin EJ, Schnabel RB. Epidemiology of Atrial Fibrillation in the 21st Century: Novel Methods and New Insights. American Heart Association. 

2. January CT, Wann LS, Alpert JS, et al. 2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation. Circulation. 2014;130(23).

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About the Author

John Chorba, MD
John Chorba, MD - Assistant Professor of Medicine, UCSF

John Chorba, MD, is a physician-scientist and cardiologist focusing on developing chemical tools to investigate the mechanistic basis of cardiovascular disease and develop novel therapies. He graduated from Harvard Medical School, completed his internal medicine residency at Massachusetts General Hospital, and his clinical cardiology fellowship at University of California, San Francisco. As an Assistant Professor in Residence at UCSF, Dr. Chorba now leads an NIH-funded chemical biology-focused research program that spans discovery and translational science.