Atrial fibrillation on an EKG

A 6-second lead II strip of atrial fibrillation on ECG (EKG) paper at a mean ventricular rate of 100, drawn by the ECG rhythm simulator’s model, with the ESC criteria and every RR interval measured.

Lead II, 6 seconds at 25 mm/s and 10 mm/mV: each small square is 0.04 s and 0.1 mV, each large square 0.2 s and 0.5 mV, and the step at the left is the 1 mV calibration pulse.

Try it: Open atrial fibrillation in the ECG simulator →

A study tool, not a clinical one

This page exists for learning what defines each rhythm and how it is measured on paper. It is not clinical decision support and must not be used to diagnose or treat anybody.

The strip is a model, not a recording. Every interval holds for the whole minute and every beat has the same shape, where a real rhythm varies with breathing, autonomic tone and drugs, and real complexes vary between people and between leads.
One lead is not a 12-lead ECG. The axis, bundle branch block patterns, ST changes and pre-excitation need all twelve leads, read by somebody trained to read them.
Definitions differ between sources. Guidelines, textbooks and institutions do not all draw the same rate and interval limits, so check which ones your course or institution uses.
Every criterion names its source. If one does not match what you were taught, check it against the source listed below, and tell us if we have it wrong. How the pages are checked is set out in the editorial policy.

What does atrial fibrillation look like on an EKG?

No P waves, only an irregular wavering baseline, and RR intervals that are irregularly irregular, with no pattern to the spacing of the QRS complexes. The QRS is usually narrow. The ventricular rate depends on how many fibrillatory impulses the AV node lets through, so it can be slow, normal or fast.

Defining criteria

No discernible repeating P waves and irregular RR intervals, when AV conduction is not impaired. (Hindricks et al. 2020)
Those features on a standard 12-lead ECG, or on a single-lead tracing of at least 30 seconds, are diagnostic of clinical atrial fibrillation. (Hindricks et al. 2020)
In the model each RR interval is the AV node’s refractory period plus a random wait for the next fibrillatory impulse, arriving at 400 per minute. The simulator allows a mean ventricular rate of 40 to 180. (simulator model)

Rates and intervals on this strip

Rates and intervals of the atrial fibrillation strip above, from the simulator’s model
Ventricular rate	100 bpm (mean over the minute)
Atrial rate	No P waves
PR interval	None, no P waves
QRS duration	80 ms
QT interval	310 ms
QTc (Bazett)	400 ms
RR interval over the minute	450 to 1044 ms
RR interval on this strip	450 to 718 ms
Regularity	Irregularly irregular
Six-second count	11 QRS × 10 = 110 bpm

Measuring the strip

The mean RR interval is 60 / 100 = 600 ms. The model makes the mean wait for the next impulse 60 / 400 = 150 ms, so the AV node’s refractory period is 600 − 150 = 450 ms and no RR interval is shorter. Over the whole minute they run from 450 to 1044 ms.

On this strip alone the RR intervals run from 450 to 718 ms. Six seconds hold 11 QRS complexes, so the six-second count gives 110 for a rate that averages 100 over the minute. In an irregular rhythm no single RR interval stands for the rate, and a longer strip or a full minute gives a better average.

The baseline is the sum of four waves between 350 and 540 per minute and never strays more than 0.06 mV from zero, under one small square. On a real tracing fibrillatory waves can be coarser, or so fine that the baseline looks flat, so the RR intervals are the surer sign.

Settings the simulator allows for this rhythm

Each rhythm limits its settings to the values its definition allows, so the simulator can never draw it as something else.

Settings the ECG simulator allows for atrial fibrillation, and the value the strip above uses
Setting	Allowed	This strip
Mean ventricular rate	40 to 180 bpm	100 bpm
QRS duration	60 to 160 ms	80 ms

Telling it apart

Atrial flutter has regular sawtooth waves at about 300 per minute and usually a regular ventricular rate at a fixed ratio.
Frequent PACs make the RR intervals irregular too, but each early beat has a P wave and the pattern repeats.

Sources

Hindricks G, Potpara T, Dagres N, et al. 2020 ESC Guidelines for the diagnosis and management of atrial fibrillation. European Heart Journal. 2021;42(5):373-498. doi:10.1093/eurheartj/ehaa612
Kligfield P, Gettes LS, Bailey JJ, et al. Recommendations for the Standardization and Interpretation of the Electrocardiogram, part I. Circulation. 2007;115:1306-1324. doi:10.1161/CIRCULATIONAHA.106.180200
McSharry PE, Clifford GD, Tarassenko L, Smith LA. A dynamical model for generating synthetic electrocardiogram signals. IEEE Transactions on Biomedical Engineering. 2003;50(3):289-294. doi:10.1109/TBME.2003.808805
Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.