Atrial fibrillation (AF) is the most frequently occurring arrhythmia in the world. After reviewing critical clinical aspects concerning AF ECG in the previous part, let's have a look at the significance of atrial flutter.

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ECG characteristics of atrial flutter

• Narrow complex tachycardia
• Regular atrial activity at 300 beats per minute
• "Saw-tooth" pattern of inverted flutter waves in leads II, III, and aVF
• Upright flutter waves in V1 that may mimic P waves
• Loss of the isoelectric baseline
• The ventricular rate is influenced by the AV conduction ratio (refer to the image below)

It is important to note that the aforementioned pattern of inverted flutter waves in inferior leads and upright flutter waves in V1 occurs in 90% of instances of atrial flutter caused by anticlockwise re-entry.

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Fixed AV conduction ratio (“AV block”)

In comparison to the heart's atrial pace, the ventricular rate is just a small portion like:

• 2:1 block equals 150 beats per minute
• 3:1 block equals 100 beats per minute
• 4:1 block equals 75 beats per minute

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Variable AV conduction ratio

1. The ventricular response is erratic, resembling AF.
2. Upon closer examination, a pattern of alternating 2:1, 3:1, and 4:1 conductor ratios may be seen.

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Atrial flutter pathophysiologic mechanisms

1. Atrial flutter is a kind of supraventricular tachycardia generated by a right atrial re-entry circuit. The length of the re-entry circuit is proportional to the size of the right atrium, resulting in a rather predictable atrial rhythm of around 300 beats per minute (range 200-400).
2. The AV conduction ratio ("degree of AV block") is used to calculate the ventricular rate. The most often used AV ratio is 2:1, which results in a ventricular pace of 150 beats per minute.
3. Higher-degree blocks, often caused by drugs or underlying cardiac disease, may develop, resulting in decreased ventricular conduction rates, e.g. a 3:1 or 4:1 block.
4. Atrial flutter with 1:1 conduction may develop as a result of sympathetic stimulation or when an auxiliary route is present. Treatment with AV-nodal blocking medications in a patient with WPW may trigger this.
5. Atrial flutter with 1:1 conduction is linked with significant hemodynamic instability and ventricular fibrillation development.

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Categorisation

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

1. Typical atrial flutter (common, or type I atrial flutter)

The reentry circuit is comprised of the IVC and tricuspid isthmus. It can be categorised further on the direction of the reentry circuit (anticlockwise or clockwise).

Anticlockwise re-entry

This atrial flutter is the most common kind (90 per cent of cases). Atrial retrograde conduction results in:

• Flutter waves inverted in leads II, III, and aVF
• In V1, positive flutter waves resemble upright P waves

Clockwise re-entry

This unusual variation results in the inverse pattern :

• Flutter waves that are positive in leads II, III, and aVF
• V1 has broad, inverted flutter waves

2. Atypical atrial flutter (uncommon, or type II atrial flutter)

• Does not meet the criteria for typical atrial flutter
• Is often linked with increased atrial rates and rhythm instability
• Is less responsive to ablation therapy

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Top tips for recognising flutter

Recognised immediately

1. Is there a narrow complex tachycardia at 150 beats per minute (range 130-170)? If yes-> Flutter is a possibility.
2. Invert the ECG and carefully inspect the inferior leads (II, III + aVF) for flutter waves.

Vagal manoeuvres +/- adenosine

Unlike AVNRT, these procedures do not often cardiovert atrial flutter, but a transitory period of enhanced AV block may reveal flutter waves.

RR intervals

1. R-R intervals in an atrial flutter with variable block will be multiples of the P-P interval - for example, given an atrial rate of 300 bpm (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.
2. Look for similar R-R intervals that occur intermittently throughout the rhythm strip; then, determine if the various R-R intervals on the ECG have a mathematical connection.
3. By contrast, atrial fibrillation is characterised by total irregularity, with no discernible patterns within the R-R intervals.

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Important practice notes

1. Suspect atrial flutter with 2:1 block anytime a regular narrow-complex tachycardia at 150 bpm occurs - especially when the rate is exceedingly constant.
2. In comparison, the rate in sinus tachycardia normally fluctuates somewhat from beat to beat, but the rate in AVNRT/AVRT is frequently quicker (170-250 bpm).
3. To distinguish between these rhythms, do some vagal movements or administer a test dose of adenosine - AVNRT/AVRT often return to sinus rhythm, but decreasing the ventricular rate in sinus tachycardia or atrial flutter reveals the underlying atrial rhythm.

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Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

The author is a practising super specialist from New Delhi.