The exact cause of atrial fibrillation is unknown, but it's more common with age and affects certain groups of people more than others.

Atrial fibrillation is common in people with other heart conditions, such as:

• high blood pressure (hypertension)
• atherosclerosis
• heart valve disease
• congenital heart disease
• cardiomyopathy
• pericarditis

It's also associated with other medical conditions, including:

• an overactive thyroid gland
• pneumonia
• asthma
• chronic obstructive pulmonary disease (COPD)
• lung cancer
• diabetes
• pulmonary embolism
• carbon monoxide poisoning

But not everyone with atrial fibrillation has one of the conditions above. It can sometimes affect people who are physically very fit, such as athletes.