Atrial flutter on an EKG

A 6-second lead II strip of atrial flutter on ECG (EKG) paper, flutter waves at 300 per minute with 4:1 conduction, drawn by the ECG rhythm simulator’s model, with the ventricular rate at every ratio it allows.

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. Filled triangles mark flutter waves that reached the ventricles, open triangles those that did not.

Try it: Open atrial flutter 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 flutter look like on an EKG?

Regular sawtooth flutter waves take the place of P waves, negative in lead II and with no flat baseline between them, at about 300 per minute. The AV node passes a fixed share of them, one in 2, 3 or 4, so the ventricular rhythm is usually regular at a fraction of the flutter rate.

Defining criteria

Flutter waves at 250 to 350 per minute in place of P waves, drawn as the negative sawtooth of typical flutter in lead II: a steep fall, a rapid rise and a slow descent back, with no flat baseline between waves. (simulator model)
In the model the AV node conducts every second, third or fourth flutter wave, and each conducted wave starts 240 ms before its QRS, a drawing choice held fixed. (simulator model)

Rates and intervals on this strip

Rates and intervals of the atrial flutter strip above, from the simulator’s model
Ventricular rate	75 bpm
Flutter wave rate	300 per minute
Flutter waves conducted	1 of every 4
PR interval	Not measured on flutter waves
QRS duration	80 ms
QT interval	358 ms
QTc (Bazett)	400 ms
RR interval	800 ms
Regularity	Regular
Six-second count	7 QRS × 10 = 70 bpm

Measuring the strip

One flutter cycle is 60 / 300 = 200 ms, 5 small squares, so the sawtooth repeats every large square. Every fourth wave is conducted, and the RR interval is 4 × 200 = 800 ms.

At 300 flutter waves per minute the ratio sets the ventricular rate: 2:1 gives 150, 3:1 gives 100, 4:1 gives 75. At 2:1 every flutter wave runs into a QRS or a T wave, which is why a regular narrow rhythm at 150 deserves a second look.

Drugs that slow conduction through the AV node, such as beta blockers, diltiazem, verapamil and digoxin, let fewer flutter waves through: the conduction ratio rises and the ventricular rate falls while the flutter rate stays the same.

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 flutter, and the value the strip above uses
Setting	Allowed	This strip
Flutter wave rate	250 to 350 a minute	300 a minute
QRS duration	60 to 160 ms	80 ms
Flutter conduction	2:1, 3:1 or 4:1	4:1

Telling it apart

Atrial fibrillation has no organized atrial waves and irregularly irregular RR intervals.
At 2:1 conduction of 300 flutter waves the rate is 150, which can pass for sinus tachycardia when the flutter waves hide in the QRS and T waves.

Sources

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.