Atrial Flutter

ECG features of atrial flutter
  • Narrow complex tachycardia
  • Regular atrial activity at ~300 bpm
  • Loss of the isoelectric baseline
  • “Saw-tooth” pattern of inverted flutter waves in leads II, III, aVF
  • Upright flutter waves in V1 that may resemble P waves
  • Ventricular rate depends on AV conduction ratio (see below)

Note: the above pattern of inverted flutter waves in inferior leads and upright flutter waves in V1 occurs in atrial flutter due to anticlockwise reentry, which makes up 90% of cases.

Fixed AV conduction ratio (“AV block”)

Ventricular rate is a fraction of the atrial rate, for example:

  • 2:1 block = 150 bpm
  • 3:1 block = 100 bpm
  • 4:1 block = 75 bpm
Variable AV conduction ratio
  • The ventricular response is irregular and may mimic atrial fibrillation (AF)
  • On closer inspection, there may be a pattern of alternating 2:1, 3:1 and 4:1 conduction ratios
Atrial flutter with a 3-1 block
Atrial flutter with a 3:1 block

Pathophysiology of atrial flutter

Atrial flutter is a form of supraventricular tachycardia caused by a re-entry circuit within the right atrium. The length of the re-entry circuit corresponds to the size of the right atrium, resulting in a fairly predictable atrial rate of around 300 bpm (range 200-400)

  • Ventricular rate is determined by the AV conduction ratio (“degree of AV block”). The most common AV ratio is 2:1, resulting in a ventricular rate of ~150 bpm
  • Higher-degree blocks can occur — usually due to medications or underlying heart disease — resulting in lower rates of ventricular conduction, e.g. 3:1 or 4:1 block.
  • Atrial flutter with 1:1 conduction can occur due to sympathetic stimulation, or in the presence of an accessory pathway. The administration of AV-nodal blocking agents to a patient with Wolff-Parkinson-White syndrome can precipitate this
  • Atrial flutter with 1:1 conduction is associated with severe haemodynamic instability and progression to ventricular fibrillation

NB. The term “AV block” in the context of atrial flutter is something of a misnomer. AV block is a physiological response to rapid atrial rates and implies a normally functioning AV node.


This is based on the anatomical location and direction of the re-entry circuit.

Typical atrial flutter (common)

Sometimes known as type I flutter, this involves the IVC & tricuspid isthmus in the reentry circuit. Further classified based on the direction of the reentry circuit (anticlockwise or clockwise):

Anticlockwise Reentry: Commonest form of atrial flutter (90% of cases). Retrograde atrial conduction produces:

  • Inverted flutter waves in leads II,III, aVF
  • Positive flutter waves in V1 — may resemble upright P waves

Clockwise Reentry: This uncommon variant produces the opposite pattern:

  • Positive flutter waves in leads II, III, aVF
  • Broad, inverted flutter waves in V1
Learn more about the mechanism behind “anticlockwise” and “clockwise” reentry…
  • Interatrial conduction during normal sinus rhythm occurs via Bachmann’s bundle (BB), the fossa ovalis, and the coronary sinus os
  • Whilst patterns of interatrial conduction in atrial flutter are less well established, they are thought to occur via similar pathways
  • In both forms of typical atrial flutter, the reentrant circuit is formed in the right atrium. The left atrium is passively activated and is not necessary to maintain the tachycardia
Anticlockwise reentry
Anticlockwise or “counterclockwise” atrial flutter
  • Anticlockwise reentry circuit forms in the right atrium
  • Interatrial conduction and subsequent left atrial activation occurs via Bachman’s bundle (BB) and the coronary sinus os
  • The anticlockwise nature of the reentry circuit means that interatrial conduction occurs first via the coronary sinus os, and later via BB
  • The majority of left atrial conduction and activation thus occurs in a retrograde manner, forming negative flutter waves in inferior leads
Clockwise reentry
Clockwise atrial flutter
  • Clockwise reentry circuit forms in the right atrium
  • Interatrial conduction occurs via the same pathways, with left atrial activation now occurring first via BB
  • The majority of left atrial activation occurs in an anterograde manner, forming positive flutter waves in inferior leads

Atypical atrial flutter (uncommon)
  • Sometimes referred to as type II flutter, does not fulfill criteria for typical atrial flutter
  • Often associated with higher atrial rates and rhythm instability
  • Less amenable to treatment with ablation

Handy Tips For Spotting Flutter
Rapid Recognition
  • Narrow complex tachycardia at 150 bpm (range 130-170)? Yes -> Suspect flutter!
  • Turn the ECG upside down and closely examine the inferior leads (II, III + aVF) for flutter waves
Vagal Manoeuvres +/- Adenosine
  • Atrial flutter will not usually cardiovert with these techniques (unlike AVNRT), although typically there will be a transient period of increased AV block during which flutter waves may be unmasked
RR intervals
  • In atrial flutter with variable block the R-R intervals will be multiples of the P-P interval — e.g. assuming an atrial rate of 300bpm (P-P interval of 200 ms), the R-R interval would be 400 ms with 2:1 block, 600 ms with 3:1 block, and 800 ms with 4:1 block
  • Look for identical R-R intervals occurring sporadically along the rhythm strip; then look to see whether there is a mathematical relationship between the various R-R intervals on the ECG
  • In contrast, atrial fibrillation will be completely irregular, with no patterns to be discerned within the R-R intervals

ECG Examples
Example 1
ECG Atrial Flutter with 2:1 Block 1

Atrial Flutter with 2:1 Block

This is the classic appearance of anticlockwise flutter:

  • Inverted flutter waves in II, III + aVF at a rate of 300 bpm (one per big square)
  • Upright flutter waves in V1 simulating P waves
  • 2:1 AV block resulting in a ventricular rate of 150 bpm
  • Note the occasional irregularity, with a 3:1 cycle seen in V1-3

Example 2
ECG Atrial Flutter with Variable Block

Atrial flutter with variable block

  • Inverted flutter waves in II, III + aVF with atrial rate ~ 300 bpm
  • Positive flutter waves in V1 resembling P waves
  • The degree of AV block varies from 2:1 to 4:1

The diagnosis of flutter with variable block could be inferred here from the R-R intervals alone (e.g. if flutter waves were indistinct) — note how the R-R intervals during periods of 4:1 block are approximately double the R-R intervals during 2:1 block.

Example 3
Atrial flutter with 4:1 block

Atrial flutter with 4:1 block

  • There are inverted flutter waves in II, III + aVF at a rate of 260 bpm
  • There are upright flutter waves in V1-2 (= anticlockwise circuit)
  • There is 4:1 block, resulting in a ventricular rate of 65 bpm
  • The relatively slow ventricular response suggests treatment with an AV nodal blocking agent

Example 4
ECG Atrial Flutter variable block 2:1 4:1

Atrial Flutter with Variable Block

  • The block varies between 2:1 and 4:1
  • The presence of positive flutter waves in lead II suggests a clockwise re-entry circuit (= uncommon variant)

Example 5
ECG Atrial Flutter with High-Grade AV Block

Atrial Flutter with High-Grade AV Block

  • There is anticlockwise flutter with marked AV block (varying from 5:1 up to 8:1)
  • The very low ventricular rate suggests treatment with AV nodal blocking drugs (e.g. digoxin, beta-blockers)
  • Other possibilities could include intrinsic conducting system disease (true “AV block”) or electrolyte abnormality (e.g. hyperkalaemia)

Note: The combination of new-onset atrial flutter with high-grade AV block is very suspicious for digoxin toxicity

Example 6
ECG Atrial flutter 1:1 block

Atrial Flutter with 1:1 Block

  • There is a very rapid, regular narrow-complex tachycardia at 250-300 bpm
  • Flutter waves are not clearly seen, but there is an undulation to the baseline in the inferior leads suggestive of flutter with a 1:1 block
  • Alternatively, this may just be rapid SVT (AVNRT/AVRT) with rate-related ST depression

With ventricular rates as rapid as this, spending any further time evaluating the ECG is unwise! Resuscitation is the priority… This patient will almost certainly be haemodynamically unstable, requiring emergent DC cardioversion.

Example 7
ECG Atrial Flutter 2:1 block

Atrial Flutter with 2:1 Block

  • Narrow complex tachycardia at 150 bpm
  • There are no visible P waves
  • Sawtooth baseline in V1 with flutter waves visible at 300 bpm
  • Elsewhere, flutter waves are concealed in the T waves and QRS complexes
  • The heart rate of 150 bpm makes this flutter with a 2:1 block

NB. Flutter waves are often very difficult to see when 2:1 block is present.

  • Suspect atrial flutter with 2:1 block whenever there is a regular narrow-complex tachycardia at 150 bpm — particularly when the rate is extremely consistent
  • In contrast, the rate in sinus tachycardia typically varies slightly from beat to beat, while in AVNRT/AVRT the rate is usually faster (170-250 bpm)
  • To differentiate between these rhythms, try some vagal manoeuvres or give a test dose of adenosine — AVNRT/AVRT will often revert to sinus rhythm, whereas slowing of the ventricular rate will unmask the underlying atrial rhythm in sinus tachycardia or atrial flutter
ECG Strip Flutter waves adenosine 2
Flutter waves unmasked by adenosine

AVNRT reverts to sinus rhythm Adenosine
AVNRT reverts to sinus rhythm after an adenosine bolus

Example 8
ECG Atrial flutter with Negative flutter waves

Atrial flutter with 3:1 Block

  • Negative flutter waves at ~ 300bpm are best seen in the inferior leads II, III and aVF (= anticlockwise pattern)
  • There is a 3:1 relationship between the flutter waves and the QRS complexes, resulting in a ventricular rate of 100 bpm

Related Topics


  • Dr Smith’s ECG Blog – Atrial Flutter
  • Rob Orman @ ERCast – Atrial Flutter, Fibrillation and Ablation (podcast)
  • Saoudi N, Cosío F, Waldo A, Chen SA, Iesaka Y, Lesh M, Saksena S, Salerno J, Schoels W; Working Group of Arrhythmias of the European of Cardiology and the North American Society of Pacing and Electrophysiology. A classification of atrial flutter and regular atrial tachycardia according to electrophysiological mechanisms and anatomical bases; a Statement from a Joint Expert Group from The Working Group of Arrhythmias of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Eur Heart J. 2001 Jul;22(14):1162-82. PMID: 11440490 Full Text
  • Wells JL Jr, MacLean WA, James TN, Waldo AL. Characterization of atrial flutter. Studies in man after open heart surgery using fixed atrial electrodes. Circulation. 1979 Sep;60(3):665-73. PMID: 455626 Full Text
  • Marine et al. Different Patterns of Interatrial Conduction in Clockwise and Counterclockwise Atrial Flutter. Circulation. 2001;104:1153–1157

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MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |


  1. This was a great article. I’ve got a Zoll on my desk with an EKG trainer and have been scratching my head at Flutter for awhile now – thank you for this contribution! -Jesse

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