Atrial Fibrillation (AFib)
Atrial Fibrillation (AFib)
An irregularly irregular rhythm with no P waves and a restless fibrillatory baseline, at whatever rate the AV node lets through.
On the trace
Read the strip in this order.
- Look for a P wave.
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There is none, anywhere. In its place the baseline is never still: a low, irregular tremor of about 0.1 mV that changes shape from beat to beat and never repeats. Measure it in two different gaps and you will get two different answers, anywhere from 0.06 to 0.15 mV. - Measure several R-R intervals.
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Here they run from 396 ms to over 1.8 s, and consecutive intervals differ by 380 ms on average. No two gaps are alike. - Check that the irregularity has no pattern.
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This is what "irregularly irregular" means. Group beating, or intervals that land on multiples of one number, belong to a different rhythm. - Measure the QRS.
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Narrow, 76 ms. The chaos is entirely above the AV node; what gets through is conducted normally. - Take the rate as an average.
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About 79 bpm across this strip, but no single beat is at 79. In AFib the rate is a statistic, not a measurement.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| P waves | Absent | Replaced by fibrillatory waves. Their coarseness varies and means nothing diagnostically. |
| Baseline | Restless, non-repeating | Flutter's baseline also never rests, but it repeats exactly. |
| Rhythm | Irregularly irregular | No pattern over any number of beats. |
| Ventricular rate | Whatever the node passes | Under 100 here. Over 100 is the rapid variant, under 60 the slow one. |
| QRS | Narrow, 76 ms here | A wide, fast, irregular rhythm is pre-excited AF and is handled differently. |
| Duration | Paroxysmal, persistent or permanent | Under 7 days, over 7 days or needing cardioversion, and accepted respectively. |
The strip
Mechanism
The atria are not being driven by one thing. Rapidly firing foci, multiple simultaneous re-entrant wavelets and rotors all coexist, sustained by an atrium that has remodelled electrically and structurally to support them. No single wavefront ever crosses the whole atrium, so there is no P wave to inscribe, and the mechanical result is that the atria quiver rather than contract. The AV node is bombarded far faster than it can conduct, and passes impulses through at intervals that depend on when it last recovered. That is where the irregularity comes from: not from the atria being random, but from the node sampling chaos.
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Background
No sustained arrhythmia is more common: atrial fibrillation affects 1 to 2% of the population.
Why the stroke risk is the main event
Atrial fibrillation multiplies stroke risk three to five fold and accounts for about a fifth of all strokes. That risk climbs steeply with age, from roughly 1.5% a year in the fifties to over 20% a year past eighty, and AF-related strokes tend to be larger, more disabling and more likely to recur than others. It follows that the rate on the monitor is rarely the most important thing about the rhythm.
Why the score, not the rhythm, decides anticoagulation
Because the absolute risk varies so much between patients, it is estimated rather than assumed. The CHA2DS2-VASc score and its successors exist to identify who is genuinely low risk, so anticoagulation can be withheld from them and given to everyone else. What the score does not depend on is whether the patient is in sinus rhythm today, how fast they are, or whether the AF is paroxysmal: a restored rhythm does not restore a low-risk atrium.
Rate control, and its limits
Beta-blockers and the non-dihydropyridine calcium channel blockers slow the node directly. Digoxin is weaker on its own, with a slow onset, but useful added to one of the others or where they are not tolerated. Amiodarone and dronedarone slow the rate as well as controlling rhythm, which helps in heart failure. The risk at the other end is real: over-slowing produces bradycardia and block, and some patients end up needing a pacemaker before the rate can be controlled at all. All of these are avoided or used with great care in pre-excited AF, where blocking the node pushes conduction down the accessory pathway instead.
Rhythm control
Rhythm control suits younger patients, recent-onset AF, limited structural disease and anyone still symptomatic despite an adequate rate. Class Ic drugs such as flecainide and propafenone work in paroxysmal AF but are contraindicated with structural heart disease, and can organise the atrium into flutter. The class III drugs, ibutilide, dofetilide, sotalol and amiodarone, are usable with structural disease at the cost of QT prolongation and torsades risk. Amiodarone is the most effective and the most toxic, and it potentiates warfarin.
Clinical impact
The rhythm itself is often well tolerated. What harms patients is the embolic risk and, separately, an uncontrolled rate sustained over months.
Management
Unstable: synchronised cardioversion
Hypotension, ischaemic chest pain, acute heart failure or altered mental state. In practice this is almost always the rapid variant, since it is the rate that decompensates the patient.
Decide about anticoagulation first, and separately
Score the stroke risk and treat it on its own terms. It is not settled by the rate, by symptoms, or by whether sinus rhythm is restored today.
Rate control
A beta-blocker or a non-dihydropyridine calcium channel blocker, with digoxin added or substituted where those are not tolerated. Watch for over-slowing.
Rhythm control where it will help
Cardioversion, antiarrhythmic drugs or ablation, chosen for younger patients, recent onset, limited structural disease, or symptoms that persist despite a controlled rate.
Differential
The one that matters. Flutter with a changing ratio is also irregular and narrow, but its baseline is a repeating sawtooth and its R-R intervals land on whole multiples of the flutter cycle.
A fixed ratio makes flutter regular. If the rhythm is regular and narrow near 150, look at the baseline before calling it an SVT.
Frequent atrial ectopics make sinus rhythm look irregular, but each ectopic has a P wave and the underlying sinus beats keep their own regular spacing.
Regular, with a discrete P before every QRS and a flat baseline between. AFib has neither.
References
- 2024 ESC Guidelines for the management of atrial fibrillation developed in collaboration with the EACTS — European Heart Journal, 2024
- 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation — Circulation, 2023
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020