Atrial fibrillation and atrial flutter

What it is

Atrial fibrillation (AF) is the commonest sustained arrhythmia. The atria are activated chaotically at about 350–600 times a minute, so there is no organised P wave, and the AV node lets impulses through at random: the ventricular rhythm is irregularly irregular. Atrial flutter is a single, organised macro-reentrant circuit, in its typical form running around the right atrium through the cavotricuspid isthmus at about 250–350 a minute; it draws regular «sawtooth» flutter waves and the AV node usually conducts every second wave (2:1), giving a regular ventricular rate close to 150 a minute. Both lose the atrial contribution to filling and let blood stagnate, above all in the left atrial appendage, so both carry a risk of thrombus and stroke and are anticoagulated by the same rules. On the ECG the questions are always the same: is the rhythm regular or irregular, are there P waves, flutter waves or fibrillatory waves, how fast is the ventricular response, and is the QRS narrow or broad. The bedside questions follow: is the patient stable, how long has it lasted, what triggered it and how high is the stroke risk.

How to recognise it

• AF rhythm: irregularly irregular, with no repeating pattern of RR intervals; measure several intervals across the long rhythm strip.
• AF atrial activity: no P waves; an undulating baseline of fibrillatory (f) waves, coarse or so fine it looks almost flat, clearest in V1 and II.
• AF ventricular rate: under 60/min is slow AF, 60–100/min controlled, over 100/min AF with rapid ventricular response.
• AF QRS: narrow (under 120 ms) unless there is a bundle branch block, rate-related aberrancy or an accessory pathway.
• Flutter atrial activity: regular flutter (F) waves at about 250–350/min (typically 300), with no flat line between them, best in II, III, aVF and V1.
• Typical (counterclockwise) flutter: F waves negative in the inferior leads and positive in V1; clockwise flutter gives the reverse polarity.
• Flutter ventricular rate: a fixed ratio gives a regular rhythm (2:1 about 150/min, 4:1 about 75/min); variable block makes it irregular.
• Turning the paper upside down or looking at the inferior leads often shows the sawtooth more clearly than the upright view.

Why it happens

• AF is usually triggered by rapid firing from muscle sleeves in the pulmonary veins and kept going by multiple wandering wavelets or rotors in a stretched, fibrosed atrium.
• The AV node conducts some impulses and blocks others in a random way, which is why the ventricular rhythm has no pattern.
• Typical flutter is one large reentrant loop around the tricuspid annulus; its fixed path and timing produce identical, regular F waves.
• The AV node cannot conduct 300 impulses a minute, so it blocks in a ratio; 2:1 is the commonest, giving a rate near 150/min.
• Common substrates and triggers: hypertension, heart failure, valve disease (above all mitral), ischaemic heart disease, hyperthyroidism, alcohol, sepsis, surgery, lung disease and older age.
• Loss of atrial contraction and stasis in the left atrial appendage allow thrombus, which can embolise to the brain or limbs.

Step by step

1. Look at the rhythm strip first — irregularly irregular RR intervals point to AF; regular ones to flutter with fixed block or another rhythm.
2. Search for P waves in II and V1 — none plus a wavy baseline means AF; regular sawtooth waves mean flutter.
3. Count the atrial rate if waves are visible — about 300/min means flutter; 100–250/min with distinct P waves suggests atrial tachycardia.
4. Count the ventricular rate — it sets urgency and tells you whether rate control is needed.
5. Measure the QRS — broad complexes need an explanation: bundle branch block, aberrancy or pre-excitation.
6. In a regular rate near 150/min, look for a second flutter wave hidden in the QRS or the T wave — 2:1 flutter is easily read as sinus tachycardia.
7. Look for other clues — ST changes, left ventricular hypertrophy or a delta wave on a previous sinus ECG change the plan.

Atrial fibrillation and atrial flutter side by side.

Differential and pitfalls

• Multifocal atrial tachycardia is also irregular, but each QRS has a P wave, with at least three different P shapes; it is common in lung disease.
• Sinus rhythm with frequent atrial ectopic beats is irregular but has P waves before most complexes.
• Very regular «AF» with a slow rate suggests complete heart block with an escape rhythm, for example in digoxin toxicity.
• Tremor, shivering or a loose electrode can mimic fibrillatory or flutter waves; look at the patient and at other leads.
• Treat a broad irregular tachycardia with QRS shapes that keep changing as possible pre-excited AF: avoid AV-node blockers such as adenosine, verapamil, diltiazem, beta-blockers and digoxin.
• Rate-related ST depression during fast AF is common and does not by itself mean an acute coronary syndrome; recheck once the rate is controlled.

Next step

• Unstable (shock, syncope, ischaemic chest pain, acute heart failure): call a senior and arrange synchronised cardioversion at once.
• Stable: control the rate with a beta-blocker or a non-dihydropyridine calcium channel blocker (digoxin in heart failure or low activity); a senior decides on rhythm control.
• Cardioversion when onset is over 48 hours or unknown needs prior anticoagulation or a transoesophageal echo to exclude atrial thrombus.
• Score the stroke risk (CHA2DS2-VASc) and the bleeding risk, and start long-term anticoagulation as guidelines set; flutter follows the same rules as AF.
• Look for the cause: electrolytes, thyroid function, kidney function, alcohol, infection and an echocardiogram for valve disease and ventricular function.
• Refer to cardiology for follow-up; typical flutter responds very well to catheter ablation of the cavotricuspid isthmus.

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• How arrhythmias arise
• The cardiac conduction system
• Arterial pulse: rate, rhythm and amplitude
• Cardiac monitoring at the bedside: leads, alarms and rhythm changes to report

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Atrial Fibrillation (StatPearls (NCBI Bookshelf), 2026); Atrial Flutter (StatPearls (NCBI Bookshelf), 2022).

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