3. Definition and classification

AFL is defined as abnormal atrial activity inside a reentrant circuit with a diameter more than 2 cm<sup>2</sup> at a high rate of 240–320 bpm which makes a continuous oscillation without an isoelectric baseline [2]. In contrast, focal atrial tachycardia (AT) is a rapid abnormal atrial rhythm originating from a "point source" with a baseline between P waves on ECG. The most practical classification is based on isthmus versus non-isthmus dependency (Diagram 1). According to the new classification, typical AFL is a macroreentrant atrial tachycardia that usually proceeds up the atrial septum (counterclockwise or CCW), down the lateral atrial wall, and through the CTI between the tricuspid valve annulus and inferior vena cava (IVC). It is also known as "common AFL" or "CTI-dependent AFL." When the circuit rotates in the opposite direction, it is referred to as clockwise (CW) typical AFL or reverse typical AFL (Figure 1). Clockwise AFL is observed in only 10% of clinical cases. However, the flutter wave morphology might change in the presence of underlying atrial disease, prior surgery, or previous ablation which makes the flutter wave morphology not a reliable indicator of AFL type [2, 3].

group, different circuits have been described, including "perimitral flutter" reentry, LA roof dependent flutter and reentry around scars from previous surgery or ablation in atria. Obviously, these flutters are not amenable to ablation of the CTI, but common AFL often coexists

Figure 1. Counterclockwise typical flutter (a), clockwise typical flutter (b), atypical flutter (c), atrial fibrillation (d), atrial flutter 1:1 in a patient on flecainide (e). In tracing d, the atrial waves amplitude in V1 is changing with irregular irregular

Atrial Flutter: Diagnosis and Management Strategies http://dx.doi.org/10.5772/intechopen.74850 25

The CTI is bounded anteriorly by the tricuspid annulus and posteriorly by the ostium of the IVC and the eustachian ridge. The width and muscle thickness of CTI are variable, from several millimeters to around 3 cm in width and depth of 1 cm roughly. The CTI is wider in the lateral portion and thinner in the central portion. The central isthmus is concave and pouch-like in 47 and 45% of patients, respectively. The subeustachian isthmus is the area between the tricuspid annulus and the eustachian ridge which ends in IVC junction. The pectinates, spare the myocardium just in atrial part of the tricuspid valve and makes the smooth portion of the CTI which is referred to as the vestibular portion. Of note, the septal part of the CTI is adjacent to the posterior extensions of the AV node as well as the middle cardiac vein [5, 6]. This anatomic proximity explains the higher risk of AV block if ablation is done in the septal aspect. Also, the smooth vestibular portion around the tricuspid valve lies

The patients with flutter sometimes are asymptomatic or may present with a variety of symptoms including palpitations, dyspnea, fatigue, dizziness or reduced functional class. However, it might be the first presentation of more serious conditions like acute pulmonary embolism, acute coronary syndrome or acute pulmonary edema. The severity of symptoms

with these atypical reentry circuits [4].

very close to the right coronary artery.

4. Clinical manifestation

3.1. Anatomy of the CTI

RR interval.

Atypical flutter, or "non-CTI-dependent macroreentrant atrial tachycardia," is attributed to those flutters that do not use the CTI originating in the right (RA) or left atrium (LA) [3]. In this

Diagram 1. Classification of atrial flutter (see the text for discussion).

Figure 1. Counterclockwise typical flutter (a), clockwise typical flutter (b), atypical flutter (c), atrial fibrillation (d), atrial flutter 1:1 in a patient on flecainide (e). In tracing d, the atrial waves amplitude in V1 is changing with irregular irregular RR interval.

group, different circuits have been described, including "perimitral flutter" reentry, LA roof dependent flutter and reentry around scars from previous surgery or ablation in atria. Obviously, these flutters are not amenable to ablation of the CTI, but common AFL often coexists with these atypical reentry circuits [4].

#### 3.1. Anatomy of the CTI

2. Epidemiology

24 Cardiac Arrhythmias

Overall, the incidence of AFL in the United States is 88 per 100,000 person-years. 15% of supraventricular arrhythmias are AFL and usually coexists with AF. More than 80% of patients who undergo RFA of typical AFL will have AF within the following 5 years. The incidence of AFL in men is more than twice that of women. Paroxysmal AFL can be seen in patients with no structural heart disease (SHD), whereas chronic AFL is frequently associated with underlying SHD, such as valvular disease or heart failure. Acute AFL may happen secondary to acute disease process, such as pericarditis, pulmonary embolism, exacerbation of lung disease,

AFL is defined as abnormal atrial activity inside a reentrant circuit with a diameter more than 2 cm<sup>2</sup> at a high rate of 240–320 bpm which makes a continuous oscillation without an isoelectric baseline [2]. In contrast, focal atrial tachycardia (AT) is a rapid abnormal atrial rhythm originating from a "point source" with a baseline between P waves on ECG. The most practical classification is based on isthmus versus non-isthmus dependency (Diagram 1). According to the new classification, typical AFL is a macroreentrant atrial tachycardia that usually proceeds up the atrial septum (counterclockwise or CCW), down the lateral atrial wall, and through the CTI between the tricuspid valve annulus and inferior vena cava (IVC). It is also known as "common AFL" or "CTI-dependent AFL." When the circuit rotates in the opposite direction, it is referred to as clockwise (CW) typical AFL or reverse typical AFL (Figure 1). Clockwise AFL is observed in only 10% of clinical cases. However, the flutter wave morphology might change in the presence of underlying atrial disease, prior surgery, or previous ablation which makes

Atypical flutter, or "non-CTI-dependent macroreentrant atrial tachycardia," is attributed to those flutters that do not use the CTI originating in the right (RA) or left atrium (LA) [3]. In this

following heart or lung surgery, or myocardial infarction. [1]

the flutter wave morphology not a reliable indicator of AFL type [2, 3].

Diagram 1. Classification of atrial flutter (see the text for discussion).

3. Definition and classification

The CTI is bounded anteriorly by the tricuspid annulus and posteriorly by the ostium of the IVC and the eustachian ridge. The width and muscle thickness of CTI are variable, from several millimeters to around 3 cm in width and depth of 1 cm roughly. The CTI is wider in the lateral portion and thinner in the central portion. The central isthmus is concave and pouch-like in 47 and 45% of patients, respectively. The subeustachian isthmus is the area between the tricuspid annulus and the eustachian ridge which ends in IVC junction. The pectinates, spare the myocardium just in atrial part of the tricuspid valve and makes the smooth portion of the CTI which is referred to as the vestibular portion. Of note, the septal part of the CTI is adjacent to the posterior extensions of the AV node as well as the middle cardiac vein [5, 6]. This anatomic proximity explains the higher risk of AV block if ablation is done in the septal aspect. Also, the smooth vestibular portion around the tricuspid valve lies very close to the right coronary artery.

#### 4. Clinical manifestation

The patients with flutter sometimes are asymptomatic or may present with a variety of symptoms including palpitations, dyspnea, fatigue, dizziness or reduced functional class. However, it might be the first presentation of more serious conditions like acute pulmonary embolism, acute coronary syndrome or acute pulmonary edema. The severity of symptoms closely depends on the baseline left ventricular ejection fraction (LVEF), ventricular rate during the flutter and underlying SHD. As a common scenario, the patients present with a stroke or with decompensated heart failure secondary to tachycardia-induced cardiomyopathy. AFL occurs in nearly 25% of patients with AF.

6. Typical flutter

6.1. Electrocardiographic characteristics

Figure 2. The typical CCW flutter ECG characteristic (see the text).

sequence in focal AT with silent periods causing flat isoelectric line.

In "typical" counterclockwise AFL, the wave of depolarization propagates through the lateral right atrium, then travels through the CTI in a lateral-to-medial direction. The wave of depolarization arrives at the inferior part of the interatrial septum, splits and propagates caudocephalically up the septum, finally traveling across the roof to arrive lateral RA to complete the circuit. At the same time, the depolarization wave propagates from inferior septum to the lateral wall of the left atrium. Flutter waves have constant morphology and polarity with same CL. In typical AFL, they are most visible in lead V1 and the inferior leads (II, III, aVF) with a sawtooth appearance. The propagation of depolarization wave is through interatrial septum which makes it positive or negative in inferior leads in clockwise (high to low) or counterclockwise (low to high) AFL, respectively. The wave includes a slow downsloping portion, with a sharp negative deflection, followed by a rapid positive deflection, merging to the next downsloping deflection (Figure 2). Because of constant activation inside the circuit, there is no electrically silent period and consequently no isoelectric period. This is as opposed to focal AT with silent periods between focal discharges (Figure 3). However, focal

Atrial Flutter: Diagnosis and Management Strategies http://dx.doi.org/10.5772/intechopen.74850 27

Figure 3. The top diagram demonstrates the atrial activation sequence correlated with typical CCW atrial flutter. The slow conduction through CTI causes the flatter portion of flutter wave. The bottom diagram shows atrial activation
