- Atrial flutter is a type 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 commonest AV ratio is 2:1, resulting in a ventricular rate of ~150 bpm.
- Higher-degree AV 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 — especially if AV-nodal blocking agents are administered to a patient with WPW.
- 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, or Type I Atrial Flutter)
Involves the IVC & tricuspid isthmus in the reentry circuit. Can be further classified based on the direction of the circuit:
Anticlockwise Reentry. This is the 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
Atypical Atrial flutter (Uncommon, or Type II Atrial Flutter)
- Does not fulfil criteria for typical atrial flutter.
- Often associated with higher atrial rates and rhythm instability.
- Less amenable to treatment with ablation.
- Narrow complex tachycardia
- Regular atrial activity at ~300 bpm
- Flutter waves (“saw-tooth” pattern) best seen in leads II, III, aVF — may be more easily spotted by turning the ECG upside down!
- Flutter waves in V1 may resemble P waves
- Loss of the isoelectric baseline
Fixed AV blocks
Ventricular rate is a fraction of the atrial rate, e.g.
- 2:1 block = 150 bpm
- 3:1 block = 100 bpm
- 4:1 block = 75 bpm
Variable AV block
- The ventricular response is irregular and may mimic AF
- On closer inspection, there may be a pattern of alternating 2:1, 3:1 and 4:1 conduction ratios
Handy Tips For Spotting Flutter
- Narrow complex tachycardia at 150 bpm (range 130-170)? Yes -> Suspect flutter!
- Turn the ECG upside down and scrutinise 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.
- 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.
Atrial Flutter with 2:1 Block
- There are inverted flutter waves in II, III + aVF at a rate of 300 bpm (one per big square)
- There are upright flutter waves in V1 simulating P waves
- There is a 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
This is the classic appearance of anticlockwise flutter.
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.
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.
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).
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).
Tip: The combination of new-onset atrial flutter with high-grade AV block is very suspicious for digoxin toxicity.
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.
Atrial Flutter with 2:1 Block
- There is a narrow complex tachycardia at 150 bpm.
- There are no visible P waves.
- There is a 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 tell the difference 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.
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.
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