Atrial fibrillation is an irregular heart rhythm, which starts in the upper chambers (atria) of the heart. It is caused by disorganised electrical activity from different places in the atria. This makes the atria flutter or quiver (fibrillate) instead of contracting regularly.

Non-invasive cardiology

Non-invasive cardiology is used to diagnose arrhythmias, coronary heart disease and heart failure and includes ECH holter monitoring, cardiopulmonary exercise testing and 24-hour blood pressure ...

Atrial fibrillation ablation

An ablation procedure is a treatment for a heart condition called atrial fibrillation . This is an irregular heart rhythm, which starts in the upper chambers (atria) of the heart.


A cardioversion is a treatment which delivers electrical energy (shock) to the heart using an external defibrillator to get the heart back into sinus rhythm.

Anticoagulant medications

Warfarin is the most widely-used anticoagulant (or blood-thinning medication) to reduce the risk of having a stroke.

On this page

What is normal (sinus) rhythm?

The heart is divided into four chambers: two at the top, called the atria, and two at the bottom, called the ventricles. A normal heartbeat is very coordinated, where the atria is followed by the ventricles in each heartbeat. The average resting heart rate is usually between 60 and 100 bpm (beats per minute). This is called normal sinus rhythm. 

The heartbeat needs an electrical conduction system, rather like electrical wiring. This is made up of cells in the heart that send electrical messages or impulses to the heart muscle. These electrical impulses stimulate the heart to contract. 

In a normal heart, the electrical impulse starts from our natural pacemaker, the sinoatrial node (SA node). You can find this at the top of the right atrium. This electrical impulse then spreads very quickly throughout the right and left atria, making them contract.

It then goes through a gateway from the atria to the ventricles called the atrioventricular node (AV node). Once it is past the AV node, the electrical impulse speeds its way into the ventricles making them contract and push the blood out of the heart.

You can find the AV node at the lower right atrium, and it acts as an electrical junction between the atria and ventricles. During normal sinus rhythm, the AV node's main function is to delay how quickly the electrical impulse travels between the atria and the ventricles. This means the heart pumps blood more efficiently.

Stroke risk and anticoagulation

Atrial fibrillation increases the risk of stroke.

During atrial fibrillation, the risk of stroke increases because the atria do not contract / pump normally, which means the flow of blood is slowed. This can cause blood inside the heart to stagnate (or pool), which may cause a thrombus (or clot) to form. If the blood clot leaves the heart it can travel to any blood vessels, including those in the brain, resulting in a stroke.

Research has shown that strokes caused by AF are often more severe than non AF-related strokes.

If you have AF, you may be at risk of a stroke. To safely reduce the risk of stroke, we must assess your risk factors.

The risk of having a stroke can be calculated using an international scoring system, called the CHA2DS2VASc score.

 Congestive heart failure  1
 Hypertension 1
 Age (>75yrs)  2
 Diabetic 1
 Stroke or TIA  2
 Vascular heart disease  1
 Age (65-74 years)  1
 Sex (female)  1

Each condition is given a score of either 1 or 2. 

You will get a score of 0 if you do not have any of the above risk factors. You will be considered low risk (less than 1 per cent risk of stroke per year) and an anticoagulant will not be needed.  

A score of 1 is considered moderate risk and your doctor may recommend an anticoagulant (such as warfarin or a direct oral anticoagulant). Anticoagulants are effective at reducing the risk of clots by around 50-60%.

Previously, moderate risk patients were offered antiplatelet drugs, such as aspirin and clopidogrel. However, recent research has shown that antiplatelets are ineffective in reducing the risk of stroke while increasing the risk of bleeding.

Although anticoagulants can reduce the presence of clots and risk of stroke, they can increase the risk of bleeding. Therefore, your risk of bleeding is also assessed to weigh the benefits and risks of taking an anticoagulant. 

The table below shows your stroke risk percentage per year according to your CHADSVASC score. A score of 2 or greater means there is a high risk of having a stroke and an anticoagulant is usually recommended. For example, if you have a CHADSVASc score of 2 you have a 2.2 per cent risk each year of having a stroke without anticoagulation. So, two people in every 100 people over a year will have a stroke.

CHADSVASc scoreAnnual stroke risk percentage 
0 0
1 1.3
2 2.2
 3 3.2
 4  4 

Risk of bleeding

To help identify people at a high risk of bleeding, we use another scoring system called HAS-BLED. 

 Hypertension (uncontrolled BP, systolic >160 mmhg)  1
 Abnormal renal or liver function  1 or 2 
 Stroke  1
 Bleeding 1
 Labile INR (<60 per cent in range)  1
 Elderly  1
 Drugs or alcohol  1 or 2 

A score above 3 is considered high risk for bleeding and needs to be carefully monitored. It does not necessarily mean that you will not be prescribed an anticoagulant, as the risk of having a stroke may still be higher than the risk of bleeding. 

Your healthcare team will discuss this with you and will also look at factors, such as controlling blood pressure.

Arrhythmia team

The arrhythmia team includes: 

  • consultants
  • clinical nurse specialists
  • an arrhythmia pharmacist 
  • catheter laboratory technicians.  


Clinical nurse specialists

  • Beatrice Moloce, Royal Brompton Hospital
  • Harriet Fisher, Royal Brompton Hospital
  • Sue King, Harefield Hospital
  • Sally Deane, Harefield Hospital

Arrhythmia pharmacist

  • Carol Hayes


Harefield Hospital
01895 828979

Royal Brompton Hospital
020 7351 8364 

Useful links

If you want to know more about arrhythmia, here are some helpful organisations and websites: