Atrial Flutter

Atrial flutter is a macroreentrant atrial tachycardia characterized by an organized atrial rhythm, typically at rates of 250–350 bpm, with a defined circuit traversing large portions of the atrial myocardium. Unlike focal atrial tachycardias that arise from a single point source, flutter circuits revolve around anatomic or functional barriers, using corridors of slow conduction as the critical isthmus that sustains the arrhythmia. The classification of atrial flutter is fundamentally based on whether the circuit is dependent on the cavotricuspid isthmus (typical flutter) or follows an alternative path (atypical flutter).

Typical (CTI-Dependent) Flutter

Typical atrial flutter is the most common form, accounting for approximately 80% of all atrial flutter cases. The reentrant circuit resides entirely within the right atrium and is critically dependent on conduction through the cavotricuspid isthmus (CTI) — the corridor of atrial tissue bounded anteriorly by the tricuspid annulus and posteriorly by the inferior vena cava (IVC) and Eustachian ridge. The CTI represents the narrowest and slowest-conducting segment of the circuit, making it the ideal target for ablation.

The wavefront propagates around the tricuspid annulus, with the crista terminalis and Eustachian ridge serving as posterior barriers to conduction, and the tricuspid annulus itself as the anterior boundary. The superior vena cava (SVC) forms the superior limit, while the IVC and coronary sinus ostium define the inferior boundary. The crista terminalis, a thick muscular ridge running from the SVC to the IVC along the posterolateral right atrium, acts as a line of functional block during flutter — conduction across it is either very slow or absent, forcing the wavefront to travel around it.

There are two variants of typical flutter, defined by the direction of wavefront rotation around the tricuspid annulus:

• Counterclockwise (CCW) flutter (approximately 90% of typical flutter): The wavefront travels down the right atrial free wall (lateral wall), across the CTI from lateral to septal, up the interatrial septum, and over the roof of the right atrium. This is the classic and most common form. In the left anterior oblique (LAO) fluoroscopic view, the circuit rotates in a counterclockwise direction.
• Clockwise (CW) flutter (approximately 10% of typical flutter): The wavefront travels in the reverse direction — descending the septum, crossing the CTI from septal to lateral, ascending the right atrial free wall, and traversing the roof. Also called “reverse typical flutter.” Despite the reversed direction, the circuit remains CTI-dependent and is equally amenable to CTI ablation.

Atypical (Non-CTI-Dependent) Flutter

Atypical atrial flutter refers to any macroreentrant atrial tachycardia whose circuit does not depend on the CTI. These arrhythmias are more heterogeneous, often more challenging to map and ablate, and are increasingly encountered in the era of catheter ablation for atrial fibrillation. Common subtypes include:

• Perimitral flutter: The circuit revolves around the mitral valve annulus within the left atrium. The wavefront uses the corridor between the mitral annulus and the left pulmonary veins. This is the most common form of left atrial flutter and frequently occurs after incomplete left atrial ablation procedures. Ablation requires a mitral isthmus line connecting the mitral annulus to the left inferior pulmonary vein.
• Roof-dependent flutter: The circuit traverses the left atrial roof, often using gaps in prior ablation lines (e.g., incomplete roof lines from AF ablation). Activation proceeds across the roof in one direction and returns through the posterior wall or septum. A roof line connecting the superior pulmonary veins is the ablation target.
• Scar-related flutter: Macroreentrant circuits form around areas of atrial scar, whether from prior surgical (e.g., atriotomy scars from congenital heart disease repair) or catheter ablation lesion sets. These circuits are often complex, with multiple channels of slow conduction through scar tissue that can sustain one or more reentrant loops. Detailed activation and entrainment mapping is essential.
• Right atrial non-CTI-dependent flutter: Less common circuits within the right atrium that revolve around the SVC, around areas of right atrial scar (e.g., after atriotomy), or along the lateral right atrial wall using gaps in the crista terminalis.

Atypical flutters are particularly common in patients with structural heart disease, prior cardiac surgery (especially for congenital heart disease), and prior catheter ablation for atrial fibrillation. The incidence of left atrial macroreentrant tachycardia after pulmonary vein isolation ranges from 5–25% depending on the extent of the initial ablation lesion set.

The surface ECG is often the first clue to the diagnosis and can provide significant information about the type and direction of the flutter circuit. The atrial rate in flutter is typically 250–350 bpm (cycle length 170–240 ms), with ventricular response determined by the degree of AV conduction (most commonly 2:1, yielding a ventricular rate of ~150 bpm).

Counterclockwise Typical Flutter

The classic ECG pattern of CCW typical flutter is one of the most recognizable in clinical electrophysiology. The hallmark is the sawtooth pattern in the inferior leads (II, III, aVF), characterized by continuous undulating flutter waves (F waves) without a true isoelectric baseline. The F waves are inverted (negative) in leads II, III, and aVF, reflecting the caudal-to-cranial activation of the interatrial septum, and upright (positive) in lead V1, reflecting right-to-left septal activation. The flutter waves in lead I are typically low-amplitude or isoelectric.

Clockwise Typical Flutter

In clockwise (reverse) typical flutter, the wavefront travels in the opposite direction around the tricuspid annulus. The F waves are upright (positive) in leads II, III, and aVF and negative in lead V1 — essentially the mirror image of CCW flutter. This pattern can mimic other atrial tachycardias and is sometimes misdiagnosed as sinus tachycardia or ectopic atrial tachycardia. The sawtooth morphology may be less pronounced than in CCW flutter.

The 2:1 Block Pattern

The most common AV conduction ratio in untreated atrial flutter is 2:1, producing a regular ventricular rate of approximately 150 bpm. This is a critically important clinical scenario because every regular narrow-complex tachycardia at ~150 bpm should raise suspicion for atrial flutter with 2:1 block. The flutter waves may be partially hidden within the QRS complex and T wave, making the diagnosis less obvious. Carotid sinus massage or administration of IV adenosine transiently increases the degree of AV block, revealing the underlying flutter waves and confirming the diagnosis.

Atypical Flutter ECG Patterns

Atypical flutter produces more variable ECG patterns that depend on the location and direction of the circuit. Perimitral flutter may produce predominantly positive F waves in lead V1 with variable morphology in the inferior leads. Roof-dependent flutter can mimic typical flutter or produce atypical patterns. Scar-related circuits produce the most heterogeneous patterns, and the ECG alone is often insufficient to localize the circuit — detailed intracardiac mapping is required.

The electrophysiology study is both diagnostic and therapeutic for atrial flutter. The primary goals are to confirm the macroreentrant mechanism, determine whether the circuit is CTI-dependent, and guide ablation. A systematic approach using activation mapping and entrainment mapping is essential.

Activation Mapping

Activation mapping during flutter demonstrates a macroreentrant circuit with continuous electrical activity spanning the entire tachycardia cycle length. Using a multipolar catheter (e.g., a 20-pole “Halo” catheter placed around the tricuspid annulus) or electroanatomic mapping systems (CARTO, EnSite), the activation sequence reveals the wavefront propagating around the tricuspid annulus in either a counterclockwise or clockwise direction. In typical CCW flutter, activation proceeds: lateral RA (cranial to caudal) → CTI (lateral to medial) → septum (caudal to cranial) → RA roof. The entire cycle length is accounted for by continuous sequential activation.

With electroanatomic 3D mapping systems, a color-coded activation map displays the “head meets tail” pattern characteristic of macroreentry — the earliest and latest activation times converge, with a continuous spectrum of colors rotating around the tricuspid annulus. This visual representation confirms the macroreentrant mechanism and helps identify the critical isthmus.

Entrainment Mapping

Entrainment mapping is the gold standard for confirming a reentrant mechanism and identifying whether a given site lies within the flutter circuit. Overdrive pacing at a cycle length 10–30 ms shorter than the tachycardia cycle length (TCL) from various sites around the circuit is performed, and the post-pacing interval (PPI) is measured after the last pacing stimulus.

PPI-TCL Analysis

The post-pacing interval (PPI) measured at the pacing site after cessation of entrainment pacing is a critical measurement. The PPI reflects the time for the wavefront to travel from the pacing site to the circuit, traverse the circuit, and return. When PPI − TCL ≤20 ms, the pacing site is within or very close to the circuit. A PPI − TCL >30 ms indicates the pacing site is outside the circuit (bystander). Entrainment from the CTI with a short PPI − TCL and concealed entrainment confirms CTI dependence. Conversely, if entrainment from the CTI yields a long PPI − TCL, the flutter is not CTI-dependent (atypical), and alternative circuit locations must be sought.

Distinguishing Typical from Atypical Flutter

The critical determination during the EP study is whether the flutter is CTI-dependent. Entrainment from the CTI with concealed fusion and PPI − TCL ≤20 ms confirms CTI dependence. If CTI entrainment yields a long PPI or manifest fusion, the operator should perform entrainment from alternative sites — the mitral isthmus, left atrial roof, or around areas of scar — to identify the true circuit. In patients with prior atrial fibrillation ablation, multiple reentrant circuits may coexist, and one flutter may transition to another during the procedure, requiring repeated remapping.

Catheter ablation of typical atrial flutter targeting the CTI is one of the most well-established and successful procedures in clinical electrophysiology. The procedure is curative for the CTI-dependent circuit, with acute success rates of 95–99% and long-term recurrence rates of only 3–10%. Atypical flutter ablation is more complex but increasingly successful with modern electroanatomic mapping technologies.

CTI Ablation Technique

The goal is to create a continuous line of transmural lesions across the CTI, from the tricuspid annulus (anterior boundary) to the IVC/Eustachian ridge (posterior boundary). The ablation catheter is typically positioned at the 6 o’clock position on the tricuspid annulus in the LAO view. A systematic, point-by-point lesion approach is used, with the catheter dragged slowly from the ventricular aspect of the tricuspid annulus posteriorly toward the IVC while delivering radiofrequency energy.

Key technical considerations include:

• Power and duration: Radiofrequency energy is typically delivered at 30–50 W with a target temperature of 43–50°C. Contact force sensing catheters (target 10–30 g) improve lesion quality. Each application is maintained for 30–60 seconds or until local electrogram attenuation is achieved.
• Anatomic challenges: The CTI is not flat — it contains recesses, pouches, and prominent Eustachian ridges that can prevent transmural lesion formation. The sub-Eustachian recess is a common site of incomplete block. In some patients, the CTI is broad or heavily trabeculated, requiring more extensive ablation.
• Ablation during flutter vs sinus rhythm: Ablation can be performed during ongoing flutter (with the endpoint of flutter termination followed by bidirectional block confirmation) or during sinus rhythm/pacing (with the sole endpoint of bidirectional block). Flutter termination alone is insufficient as an endpoint — bidirectional block must always be confirmed.

Atypical Flutter Ablation

Ablation of atypical (non-CTI-dependent) flutter requires detailed electroanatomic mapping to identify the circuit and critical isthmus. The approach varies by circuit location:

• Perimitral flutter: A linear lesion is created across the mitral isthmus, connecting the mitral annulus to the left inferior pulmonary vein. This line traverses the posteroinferior left atrium and often requires ablation within the coronary sinus to achieve complete block (epicardial connections via the CS musculature can maintain conduction). Confirmation of bidirectional block is analogous to CTI ablation, using differential pacing from both sides of the mitral isthmus line.
• Roof-dependent flutter: A linear lesion along the left atrial roof connecting the two superior pulmonary veins is performed. Bidirectional block is confirmed by demonstrating descending activation of the posterior wall during anterior pacing (and vice versa) rather than direct conduction across the roof.
• Scar-related flutter: The critical isthmus is identified by entrainment mapping and voltage mapping of the scar. Ablation targets the narrowest conducting channel between scar borders or between scar and an anatomic boundary (e.g., valve annulus, vein ostium). These are often the most challenging cases and may require extensive mapping and multiple ablation applications.

Success Rates and Recurrence

Typical CTI-dependent flutter ablation has an acute success rate of 95–99% with a long-term recurrence rate of 3–10% (usually due to recovery of conduction across the CTI). Recurrence is more common when the acute endpoint was flutter termination alone without confirmed bidirectional block. Atypical flutter ablation success rates are lower (70–90%) with higher recurrence, reflecting the greater complexity of these circuits. Importantly, 25–50% of patients with typical atrial flutter will develop atrial fibrillation within 5 years of successful flutter ablation, underscoring the shared substrate and risk factors between these arrhythmias.