HEALTH TOPICS 
Atrial Fibrillation
What Is It?
Atrial fibrillation is a heart rhythm disorder that causes a rapid and irregular heartbeat.
The heart is a muscle that has four chambers through which blood flows. The two upper chambers are the atria. The two lower chambers are the ventricles.
To continue reading this article, you must login.
Subscribe to Harvard Health Online for immediate access to health news and information from Harvard Medical School.
- Research health conditions
- Check your symptoms
- Prepare for a doctor's visit or test
- Find the best treatments and procedures for you
- Explore options for better nutrition and exercise