Atrial Fibrillation: The Irregular Rhythm

Atrial fibrillation can be surprisingly quiet. One person may notice a pounding, unpredictable heartbeat; another may feel only more tired or breathless than usual; someone else may have no symptoms at all. Yet the same rhythm can change ventricular filling, worsen cardiac function and increase the risk of thromboembolic stroke.

Atrial fibrillation, usually shortened to AF, begins in the atria. Instead of one organised wave of depolarisation producing a coordinated atrial contraction, electrical activity becomes rapid and disorganised. The atria no longer contract effectively, and impulses reach the atrioventricular node in an irregular pattern. Following that physiology from atria to ventricles makes the irregular pulse, electrocardiogram changes, symptoms and need for stroke prevention much easier to understand.

Begin with normal atrial conduction

In sinus rhythm, the electrical impulse normally begins in the sinoatrial node, high in the right atrium. It spreads across both atria, producing coordinated atrial depolarisation and contraction. The impulse then reaches the atrioventricular node, where conduction slows briefly before travelling through the His-Purkinje system to depolarise the ventricles.

On an electrocardiogram, organised atrial depolarisation is represented by a P wave before each QRS complex. The atria contract before the ventricles, helping complete ventricular filling. This additional atrial contribution is sometimes called the atrial kick. It becomes particularly important when ventricular filling is already limited, such as in a stiff left ventricle.

In AF, the atria are activated by rapid, disorganised electrical wavefronts rather than one coordinated impulse. The atrial myocardium quivers instead of producing an effective contraction. The atrioventricular node is bombarded with impulses, but it does not conduct every one to the ventricles. The impulses that do pass through arrive unpredictably, producing an irregularly irregular ventricular rhythm.

This protective filtering by the atrioventricular node prevents the ventricles from following the full atrial rate. Even so, the ventricular response may be too fast, appropriately controlled or occasionally slow, depending on conduction, medicines and coexisting disease.

What you see on the ECG

The classic electrocardiogram pattern is an irregularly irregular R-R interval with no consistent, discrete P waves. The baseline may contain fibrillatory activity, but this varies and can be subtle. The QRS complexes are usually narrow unless there is bundle branch block, ventricular pre-excitation or another reason for abnormal ventricular conduction.

A pulse that feels irregular raises suspicion, but it does not confirm AF. Ectopic beats and other arrhythmias can also produce irregularity. NICE recommends a 12-lead electrocardiogram to diagnose AF when an irregular pulse has been detected, with ambulatory monitoring considered when paroxysmal AF is suspected but not captured on the initial trace.

The pulse and monitor can also disagree during rapid AF because not every ventricular contraction produces a pulse wave strong enough to be felt peripherally. This is sometimes described as a pulse deficit. If the patient is unwell, assessment should not depend on one automated reading. Look at the patient, assess the pulse and rhythm, review the ECG and use the wider clinical picture.

Why some patients feel unwell and others feel nothing

Symptoms depend on the ventricular rate, the loss of coordinated atrial contraction, the patient's cardiac reserve and how abruptly the rhythm began. A fast ventricular response shortens diastole, giving the ventricles less time to fill. Myocardial oxygen demand rises while stroke volume may fall. The irregular cycle length also means that filling and ejection vary from beat to beat.

Patients may describe palpitations, breathlessness, fatigue, reduced exercise tolerance, dizziness, chest discomfort or near-syncope. AF can precipitate or worsen heart failure, particularly when the rate is persistently rapid or the ventricle is already impaired. In some people, prolonged tachycardia contributes to a potentially reversible form of ventricular dysfunction called tachycardia-mediated cardiomyopathy.

Not everyone is symptomatic. AF may be found during routine observations, pre-operative assessment or investigation of another problem. The absence of palpitations does not mean the rhythm is harmless. Stroke risk is assessed from clinical risk factors, not from how strongly the patient can feel the arrhythmia.

AF is described by its pattern, not its importance

Paroxysmal AF begins and ends spontaneously or after intervention, with episodes that are not continuously present. Persistent AF continues and does not terminate on its own within the usual clinical timeframe, so cardioversion may be considered if restoration of sinus rhythm is appropriate. Permanent AF describes a shared clinical decision to accept AF rather than continue attempts to restore and maintain sinus rhythm.

These descriptions can change over time. They help clinicians discuss the rhythm's pattern and treatment strategy, but they do not create a hierarchy in which paroxysmal AF can be ignored. Thromboembolic risk still requires assessment.

How AF can lead to stroke

Without coordinated atrial contraction, blood can move more slowly within parts of the atria, particularly the left atrial appendage. AF is also associated with structural and endothelial changes and a tendency towards thrombosis. These processes fit the broader principles of Virchow's triad: abnormal blood flow, vessel-wall or endothelial change and altered coagulation.

A thrombus that forms in the left atrium can dislodge, enter the systemic circulation and obstruct a cerebral artery. This is why stroke prevention is a central part of AF care. The risk is not identical for every patient, so validated clinical tools are used alongside judgement.

For UK practice, NICE recommends using the CHA₂DS₂-VASc score to assess stroke risk in people with symptomatic or asymptomatic paroxysmal, persistent or permanent AF, and in atrial flutter. The letters are a memory aid for the clinical factors being counted:

C = congestive heart failure or left ventricular dysfunction — 1 point
H = hypertension — 1 point
A₂ = age 75 years or older — 2 points
D = diabetes mellitus — 1 point
S₂ = previous stroke, transient ischaemic attack or systemic thromboembolism — 2 points
V = vascular disease — 1 point
A = age 65–74 years — 1 point
Sc = sex category (female) — 1 point

The small 2 matters: age 75 or over and previous stroke, transient ischaemic attack or systemic thromboembolism each contribute 2 points rather than 1. The two age categories are alternatives, so a person does not receive points for both.

You may also encounter CHA₂DS₂-VA in newer European literature. The 2024 European Society of Cardiology guideline uses this version, which removes the sex-category component (Sc). The remaining factors and weighting are the same, giving a maximum score of 8 rather than 9. This is an important difference between current ESC guidance and NICE NG196, not simply a shortened spelling of the same score. In a UK nursing context, use the risk assessment specified by the relevant national and local pathway.

Anticoagulation decisions are then made according to the person's thromboembolic risk, bleeding risk, preferences and clinical circumstances. For most people with AF who need anticoagulation, NICE recommends a direct-acting oral anticoagulant (DOAC), taking account of clinical circumstances and patient preference. A vitamin K antagonist such as warfarin is used when a DOAC is contraindicated, not tolerated or otherwise unsuitable. People with mechanical prosthetic heart valves require vitamin K antagonist anticoagulation rather than a DOAC.

Bleeding risk should also be assessed and modifiable factors addressed. NICE recommends the ORBIT tool for bleeding-risk assessment in people taking or being considered for anticoagulation. A high bleeding-risk score is not, by itself, a reason to leave stroke risk untreated. It should prompt careful review of factors such as uncontrolled hypertension, interacting medicines, alcohol use, anaemia, renal function and follow-up.

Antiplatelet medicines are not an equivalent substitute for anticoagulation for stroke prevention in AF. The patient may have another indication for antiplatelet therapy, but that is a separate clinical decision.

Rate control and rhythm control answer different questions

Rate control accepts that AF is present and aims to control the ventricular response. NICE generally recommends rate control as the first-line strategy for most people with AF, except when there is a reversible cause, heart failure thought to be primarily caused by AF, new-onset AF where rhythm control is appropriate, or atrial flutter suitable for ablation. Medicines may include a standard beta-blocker or a rate-limiting calcium-channel blocker such as diltiazem or verapamil, with digoxin considered in selected people. Drug choice depends on symptoms, ventricular function, comorbidities and contraindications.

Rhythm control aims to restore and, where possible, maintain sinus rhythm. Options include electrical cardioversion, antiarrhythmic medicines and catheter ablation. NICE recommends considering rhythm control when symptoms continue after heart rate has been controlled, or when a rate-control strategy has not been successful. In acute AF, the timing of onset and the patient's haemodynamic stability change what happens next.

Cardioversion requires careful attention to the duration of AF and thromboembolic protection. If AF has lasted more than 48 hours, or the onset is uncertain, anticoagulation is central to the pathway; when long-term rhythm control is planned, NICE recommends delaying cardioversion until therapeutic anticoagulation has been maintained for at least 3 weeks. Transoesophageal echocardiography may be used as part of a TOE-guided strategy in selected patients. The important exception is life-threatening haemodynamic instability caused by new-onset AF: emergency electrical cardioversion should not be delayed simply to achieve anticoagulation.

Catheter ablation commonly targets electrical triggers around the pulmonary veins. It can reduce AF burden and improve symptoms in selected patients, although AF can recur and anticoagulation decisions after ablation still depend on stroke risk rather than an assumption that the problem has permanently disappeared.

Look for what is driving the rhythm

AF is associated with ageing, hypertension, coronary disease, heart failure, valve disease, diabetes, obesity, obstructive sleep apnoea, chronic kidney disease, hyperthyroidism and alcohol exposure. It can also occur during acute illness, including infection, surgery, myocardial ischaemia and electrolyte disturbance.

Assessment therefore reaches beyond the ECG. Blood tests may include full blood count, renal function, electrolytes and thyroid function. Echocardiography can examine ventricular function, atrial size and valve disease. The history should consider chest pain, infection, alcohol, medicines, sleep-related symptoms and previous episodes.

A reversible trigger does not automatically make long-term risk disappear. AF can recur after the acute illness has settled, so rhythm, symptoms and thromboembolic risk need reassessment rather than assuming the episode has been permanently resolved.

What is important during nursing assessment

Begin with the patient, not the monitor. Are they haemodynamically stable? Ask about chest pain, breathlessness, dizziness, collapse, new confusion and the timing of symptoms. Record the rate and rhythm, blood pressure, respiratory rate, oxygen saturation, temperature and level of consciousness. Look for evidence of heart failure, infection, dehydration, bleeding or a new focal neurological deficit.

AF associated with life-threatening haemodynamic instability requires immediate escalation. Features such as shock, severe hypotension, ongoing myocardial ischaemia or acute pulmonary oedema should make the rhythm part of an emergency ABCDE assessment rather than a routine arrhythmia review. NICE recommends emergency electrical cardioversion when new-onset AF is causing life-threatening haemodynamic instability, without delaying treatment to achieve anticoagulation.

Medication checks are central. Confirm whether anticoagulation has been prescribed, what the patient actually takes, the timing of the last dose and whether there have been missed doses. Ask about bleeding, black stools, haematuria, unexplained bruising or recent falls without using falls risk alone as a reason to dismiss anticoagulation. Review rate-controlling and antiarrhythmic medicines for bradycardia, hypotension, interactions and adherence.

If a patient taking warfarin has an international normalised ratio target, check whether monitoring has been stable. For direct-acting oral anticoagulants, renal function, dose, indication and interacting medicines matter. These medicines are often called blood thinners in conversation, but they do not literally thin the blood. They alter the clotting process.

Patient education should explain both sides of treatment: why stroke prevention matters and how to recognise significant bleeding. Advice needs to be specific to the prescribed medicine, local anticoagulation service and individual risk. Patients should know who to contact before procedures, when unwell or if doses have been missed. They should not stop anticoagulation simply because the pulse feels regular again.

Bring it back to the irregular pulse

Return to the patient whose pulse felt uneven. The irregularity is a clue, not the diagnosis. A 12-lead electrocardiogram is needed to identify the rhythm. Once AF is confirmed, the questions widen: Is the patient stable? How fast is the ventricular response? When might the episode have started? Is there an acute trigger? Is the rhythm contributing to heart failure or ischaemia? What is the person's stroke risk, and are they protected?

That is why AF should never be reduced to “an irregularly irregular pulse”. The pulse tells you what the ventricles are doing at the periphery. The ECG shows the rhythm. The loss of atrial contraction explains some of the haemodynamic effect, and the altered flow within the atria explains why stroke prevention remains important even when the patient feels well.

References

Hindricks, G., Potpara, T., Dagres, N. et al. (2021) ‘2020 ESC Guidelines for the diagnosis and management of atrial fibrillation’, European Heart Journal, 42(5), pp. 373–498.

National Institute for Health and Care Excellence (NICE) (2021, updated 2025) Atrial fibrillation: diagnosis and management. NG196. London: NICE.

NHS (2025) Atrial fibrillation.

Van Gelder, I.C., Rienstra, M., Bunting, K.V. et al. (2024) ‘2024 ESC Guidelines for the management of atrial fibrillation’, European Heart Journal, 45(36), pp. 3314–3414.

Educational note: This article supports learning and does not replace clinical assessment, local policy, specialist advice or professional judgement.