**3. Classification**

Classification for atrial flutter can be based on electrocardiography (ECG) or anatomical and electrophysiological mechanisms [2].

Originally, atrial flutter was classified as types I and II [3]. Type I atrial flutter is the designated classical sawtooth-appearing atrial tachycardia with rate >240 to 360 bpm, lacking an isoelectric baseline between deflections (i.e. continuous flutter wave). Type II atrial flutter was defined on the basis of a rapid rate (>350 bpm) and the inability to be entrained. However, there are no further systematic electrophysiological studies of type II atrial flutter, and the mechanism is unknown. Now, atrial flutter is referred to as being either typical or atypical.

For clinical and practical purposes, atrial flutter can be broadly classified as per Table 1.


**Table 1.** Classification of atrial flutter

anatomical barrier that is confined within the atria. The atrial rate in atrial flutter is approxi‐ mately 240–360 beats per minute (bpm) with no distinct isoelectric period between the flutter 'F' waves. It is generally paroxysmal in nature in a structurally healthy heart. If the tachycardia persists for a prolonged period, it frequently can degenerate into atrial fibrillation, particularly if the patient already has structural heart disease. As such, atrial flutter and atrial fibrillation

Atrial tachycardia is typically characterised by atrial rates >100 bpm but less than 240 bpm with discrete activation sequences and non-sinus P waves including a baseline isoelectric period between these waves on ECG. Its mechanism can be due to triggered activity or increased automaticity of atrial cells. These mechanisms are distinct from that of atrial flutter which is macro-reentrant; however, atrial tachycardia can also be re-entrant in mechanism similar to atrial flutter but on a microscopic level (re-entry around barriers of less than 2 cm).

Atrial fibrillation is due to fibrillatory waves in the atria with rates that are typically greater than 300 bpm in the atria. Currently these waves are considered chaotic and do not behave like the macro-reentry wavefront of atrial flutter. Re-entry however is still thought to play a

In this chapter, we will discuss the classification, pathophysiology, clinical presentation, electrocardiographic characteristics, electrophysiological testing and both the pharmacologi‐

Evidence based on epidemiological studies in the USA suggests that the overall incidence of atrial flutter is about 88/100,000 person-years. When adjusted for age, the incidence of atrial flutter in men is more than 2.5 times that of women. The age-specific incidence of atrial flutter increases exponentially with age from 5/100,000 person-years in those less than 50 years old

The risk factors that are identified as the highest risk for developing atrial flutter include male gender, increasing age, heart failure, chronic obstructive pulmonary disease (COPD) and

Classification for atrial flutter can be based on electrocardiography (ECG) or anatomical and

Originally, atrial flutter was classified as types I and II [3]. Type I atrial flutter is the designated classical sawtooth-appearing atrial tachycardia with rate >240 to 360 bpm, lacking an isoelectric baseline between deflections (i.e. continuous flutter wave). Type II

role in atrial fibrillation, but its exact involvement is unknown.

to 587/100,000 person-years among individuals more than 80 years [1].

cal and ablative management of atrial flutter.

**2. Epidemiology**

diabetes mellitus.

**3. Classification**

electrophysiological mechanisms [2].

often coexist.

4 Abnormal Heart Rhythms

In 2001, the European Society of Cardiology and the North American Society of Pacing and Electrophysiology proposed a classification [4] that takes into consideration both anatomic features and electrophysiological mechanisms.

#### **3.1. Typical atrial flutter (Counterclockwise CTI-Dependant right atrial macro-reentry)**

Counterclockwise re-entry is the most common type of macro-reentrant atrial tachycar‐ dia. The anatomical boundaries for this re-entrant tachycardia are anteriorly the tricuspid orifice and posteriorly the orifices of vena cavae and the eustachian ridge and the region of the crista terminalis [5, 6]. The conduction of macro-reentrant circuit is up the interatrial septum and around the roof towards the crista terminalis and then down the anterolateral wall (RA free wall anterior to the crista terminalis) to the lateral aspect of the tricuspid annulus (Figure 2).

#### **3.2. Reverse typical atrial flutter (Clockwise right atrial macro-reentry)**

A reverse direction of rotation of the above circuit in the right atrium (i.e. ascending the lateral wall and descending the posterior and septal walls; see Figure 2) can occur clinically in the typical atrial flutter circuit in 10 % of cases [7]. This is still called typical atrial flutter because the re-entry path is the same, even though the direction of activation is reversed. Reverse typical atrial flutter has also been called clockwise atrial flutter, referring to the direction of endocardial activation from a left anterior oblique fluoroscopic perspective. It is proposed that there is a 9:1 clinical predominance of typical (counterclockwise) atrial flutter compared to clockwise re-entry. This may be related to the localisation of an area with a low safety factor for conduction in the atrial flutter isthmus, close to the atrial septum.

**Figure 1.** ECG of counterclockwise CTI-dependant atrial flutter: Flutter waves are continuous without an isoelectric base‐ line, best seen in the inferior leads. The 'F waves' (Flutter waves) are most commonly conducted in the ventricle in a 2:1 manner, giving a regular ventricular response during the arrhythmia typically 150 beats per minute (bpm); however, oth‐ er multiples of conduction can occur such as 3:1 or 4:1 (Figure 1), giving slow ventricular rates during the arrhythmia. Less commonly, irregular rhythms can be encountered with a variable pattern in conduction to the ventricle

#### **3.3. Lower-Loop Re-entry**

Counterclockwise re-entry around the inferior vena cava (see Figures 2 and 12) where the anterior arm of the circuit is the inferior vena cava. The posterior arm is the low posterior right atrial wall with conduction across the crista terminalis [8]. Electroanatomical or conventional mapping shows activation rotating around areas of low-voltage electrograms in the right atrial free wall, not due to surgical scars.

#### **3.4. Atypical atrial flutter**

#### *3.4.1. Lesion macro-reentrant atrial tachycardia*

In this macro-reentrant atrial tachycardia, the central obstacle of the circuit is an atriotomy scar, a septal prosthetic patch, a suture line or a line of fixed block secondary to radio-frequency ablation or other causes of scar [9]. This can also lead to complicated tracts for the re-entry circuit.

#### *3.4.2. Right atrial free (Lateral) wall atriotomy tachycardia*

The best characterisation of atriotomy macro-reentrant atrial tachycardia is due to activation around an area of low voltage or scar in the lateral right atrial wall, with a main superoinferior axis.

**Figure 2.** Typical Atrial flutter

**3.3. Lower-Loop Re-entry**

6 Abnormal Heart Rhythms

**3.4. Atypical atrial flutter**

circuit.

axis.

free wall, not due to surgical scars.

*3.4.1. Lesion macro-reentrant atrial tachycardia*

*3.4.2. Right atrial free (Lateral) wall atriotomy tachycardia*

Counterclockwise re-entry around the inferior vena cava (see Figures 2 and 12) where the anterior arm of the circuit is the inferior vena cava. The posterior arm is the low posterior right atrial wall with conduction across the crista terminalis [8]. Electroanatomical or conventional mapping shows activation rotating around areas of low-voltage electrograms in the right atrial

**Figure 1.** ECG of counterclockwise CTI-dependant atrial flutter: Flutter waves are continuous without an isoelectric base‐ line, best seen in the inferior leads. The 'F waves' (Flutter waves) are most commonly conducted in the ventricle in a 2:1 manner, giving a regular ventricular response during the arrhythmia typically 150 beats per minute (bpm); however, oth‐ er multiples of conduction can occur such as 3:1 or 4:1 (Figure 1), giving slow ventricular rates during the arrhythmia. Less

commonly, irregular rhythms can be encountered with a variable pattern in conduction to the ventricle

In this macro-reentrant atrial tachycardia, the central obstacle of the circuit is an atriotomy scar, a septal prosthetic patch, a suture line or a line of fixed block secondary to radio-frequency ablation or other causes of scar [9]. This can also lead to complicated tracts for the re-entry

The best characterisation of atriotomy macro-reentrant atrial tachycardia is due to activation around an area of low voltage or scar in the lateral right atrial wall, with a main superoinferior

#### **3.5. Double-wave re-entry**

In this macro-reentrant tachycardia, two wavefronts circulate simultaneously in the same reentrant circuit. A stable macro-reentrant atrial tachycardia can originate in the left atrium. The clinical incidence is not well known but may be 1/10th that of typical atrial flutter. There is still little information on the anatomical bases of left atrial macro-reentry tachycardia, although recent reports have characterised the substrate as showing wide scarred areas with low voltage or absent electrograms [10].
