What is Atrial Fibrillation? Everything You Need to Know

You have just been told you have AF. Here is what that actually means.

Every week I sit across from patients who have just been handed a diagnosis of atrial fibrillation — or AF — and I can see the mixture of confusion, concern and, for many, a kind of relief that a name has finally been put to how they have been feeling. Over 1.4 million people in the UK are currently living with AF, and many more remain undiagnosed. It is the most common sustained heart rhythm disorder we encounter in cardiology.

I have spent my entire career at the intersection of research and clinical care, performing over 2,500 AF ablation procedures at Imperial College London and OneWelbeck Heart Health. I want to give you the kind of clear, honest explanation that I give my own patients — without jargon, without alarm, and with all the hope that modern medicine genuinely justifies.

What Actually Happens During Atrial Fibrillation?

In a healthy heartbeat, a single electrical signal travels in an orderly sequence from the sinoatrial node — the heart’s natural pacemaker — through the upper chambers (the atria) and down to the lower chambers (the ventricles). This coordinated electrical wave produces a smooth, rhythmic pumping action.

In AF, this organised electrical system breaks down. Instead of one clean signal, hundreds of chaotic, competing electrical impulses fire simultaneously from multiple sites within the atria — most commonly from the pulmonary veins, the blood vessels that return oxygenated blood from the lungs into the left atrium. The result is that the atria quiver rapidly and irregularly rather than contracting in a purposeful way. This chaotic activity is then transmitted through the AV node to the ventricles, producing the irregular and often fast pulse that is the hallmark of AF.

When I explain this to patients in clinic, I often describe it as the difference between an orchestra playing in perfect time — every instrument responding to a single conductor — and the same musicians all playing at once with no conductor at all. The sound is chaotic, and the heart, like that orchestra, cannot do its best work.

What Does AF Feel Like?

The symptoms of AF vary considerably between individuals. Some patients describe a very vivid awareness of the heart beating irregularly or rapidly. Others notice they have become breathless doing things that never troubled them before. Many feel an unusual fatigue — a heaviness that sleep does not seem to resolve.

The most common symptoms include:

• Palpitations — an awareness of an irregular, fluttering, or racing heartbeat
• Shortness of breath, particularly on exertion
• Fatigue and reduced exercise tolerance
• Lightheadedness or dizziness
• Chest tightness or discomfort

However — and this is something I always stress — a significant number of patients have no symptoms at all. This is what we call silent AF. Many people first discover they have AF when a GP performs a routine ECG, or when a wearable device such as an Apple Watch or a Kardia Mobile ECG alerts them to an irregular rhythm. This is one of the most significant developments in cardiology over the past decade: consumer technology detecting a condition that was previously going unnoticed for years.

Why Does AF Matter? Understanding the Stroke Risk

The most serious consequence of AF is stroke, and I want to be direct about this — not to alarm you, but because understanding the risk is the first step towards managing it.

During AF, blood can pool in a small finger-shaped pouch in the left atrium called the left atrial appendage. Because the atrium is not contracting properly, blood stagnates in this pouch and can form a clot. If that clot breaks free and travels to the brain, the result is a stroke. AF-related strokes tend to be particularly severe, because the clots are larger than those arising from other causes.

AF increases the risk of stroke by approximately five times compared with the general population. This is why, when I see a new patient with AF, one of my very first priorities is calculating their individual stroke risk using the CHA₂DS₂-VA score — the updated scoring system from the 2024 European Society of Cardiology guidelines — and deciding whether anticoagulation, or blood-thinning medication, is appropriate.

The good news is that this risk is highly treatable. Modern anticoagulants — the direct oral anticoagulants (DOACs) — are highly effective and much more convenient than the older warfarin-based treatments. For the right patient, they reduce the risk of AF-related stroke by approximately two thirds.

The Different Types of AF

AF is not one single condition — it exists on a spectrum, and understanding where you sit on that spectrum matters enormously for treatment planning.

• Paroxysmal AF: episodes that start and stop on their own, by definition within seven days. Many episodes last only a few minutes to a few hours.
• Persistent AF: episodes lasting more than seven days, which require intervention — either medication or an electrical cardioversion — to restore normal rhythm.
• Long-standing persistent AF: continuous AF for more than twelve months.
• Permanent AF: a designation used when both patient and doctor have agreed not to pursue rhythm control strategies, focusing instead on rate control and anticoagulation.

Paroxysmal AF, in my experience, is often the point at which patients are most motivated to seek definitive treatment — and it is also the stage at which catheter ablation is most effective. The evidence strongly supports early intervention: the longer AF is left untreated, the more the atria remodel and the more difficult rhythm control becomes. As we say in electrophysiology, AF begets AF.

Can AF Be Treated?

Absolutely — and this is what I want every patient reading this to carry with them. AF is highly treatable, and for many patients, catheter ablation offers a genuine prospect of long-term freedom from the rhythm disorder.

At OneWelbeck Heart Health and Imperial College London, I offer a full spectrum of AF management, from lifestyle optimisation and medication through to the very latest in catheter ablation technology, including pulsed field ablation (PFA). Across more than 2,500 procedures, I have seen the difference that effective treatment makes — not just in heart rhythm, but in energy levels, quality of life, and peace of mind.

If you have been diagnosed with AF, or if you have symptoms that concern you, I would encourage you to seek specialist review. The journey towards a better heart rhythm begins with a proper assessment.

To book a consultation with Dr Boon Lim at OneWelbeck Heart Health, visit drboonlim.co.uk or call 0208 187 9888