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How to Ablate Typical and Reverse Atrial Flutter

Javed M. Nasir, MD; John C. Evans, MD; Donald D. Hoang, BA; Mintu P. Turakhia, MD, MAS

Introduction

Typical atrial flutter (AFL) is a right atrial macroreentrant arrhythmia dependent upon conduction across the cavotricuspid isthmus (CTI). AFL is common and relatively refractory to medical therapies for rate and rhythm control. AFL may be an isolated arrhythmia or occur in conjunction with atrial fibrillation (AF). Even when AFL presents in the absence of AF or structural heart disease, it may have serious adverse effects, including stroke and systemic embolism, extreme tachycardia, myocardial ischemia, pulmonary venous congestion, tachycardia-induced cardiomyopathy, and heart failure. Catheter ablation of typical flutter is generally straightforward and success rates are high, while complication rates are quite low. Ablation can also be performed without advanced EP laboratory equipment such as electroanatomic mapping, intracardiac ultrasound, or non-contact mapping catheters, although these tools can be useful in complex cases or when the diagnosis is difficult to establish. For these reasons, catheter ablation of typical AFL is one of the most common procedures performed in invasive cardiac EP.

AFL Terminology

Since its discovery in 1920 by Sir Thomas Lewis,¹ numerous terms have been used to describe AFL. Specifically, these terms have been used to characterize the flutter circuits based on direction of conduction around the tricuspid annulus (counterclockwise or clockwise), whether it is right- or left-sided, whether its conduction is dependent upon the CTI (isthmus-dependent and non-isthmus dependent), or a combination of these factors (typical, reverse typical, atypical, type 1, type 2).² In 2001, a consensus document recommended standardized terminology for AFL.³ CTI-dependent, right atrial (RA) macroreentrant tachycardia in the counterclockwise direction of propagation around the tricuspid annulus is defined as “typical” AFL. “Reverse typical flutter” is defined as using the same anatomic circuit but propagating in the clockwise direction. For this chapter, we refer to both typical and reverse forms collectively as AFL.

Anatomy and Physiology

An understanding of CTI anatomy and physiology is fundamental for ablation of AFL. The CTI is bounded anteriorly by the tricuspid annulus and posteriorly by the inferior vena cava Eustachian ridge (Figure 2.1). These structures provide anatomical or functional barriers of conduction, which allow for depolarization through a protected zone of slow conduction and propagation of the arrhythmia. Slow conduction is predominantly in the CTI, where conduction may be 30%–50% of the AFL tachycardia cycle length (CL).⁴ In typical AFL, depolarization of atrial myocardium proceeds from the lateral portions of the isthmus to the medial portions, with continuation up the septal atrium and around the roof. With both the crista terminalis and Eustachian ridge serving as anatomic or functional barriers to conduction, the depolarization wavefront coming around the roof is then funneled anterior to the crista terminalis to reenter the isthmus. Therefore, disruption of conduction across the CTI eliminates the slow conduction that provides an adequate wavelength for reentry relative to atrial conduction velocities,⁵ and represents the preferred ablation strategy. Sustained bidirectional block across the CTI is the goal and endpoint of the procedure.

ECG Diagnosis

As seen in Figure 2.2, typical AFL is classically recognized on the surface 12-lead ECG by regular negative flutter waves in the inferior limb leads (II, III, aVF) and positive flutter waves in V₁.^3,6–8 The atrial rate is typically greater than 240 beats per minute (bpm) in the absence of antiarrhythmic drugs. The rate can be slowed in the presence of antiarrhythmic drugs, including both class I and class III antiarrhythmic drugs. QRS widening due to rate-related fascicular block, bundle branch block (BBB), or class Ic drug effect may also be observed. Clockwise, or reverse typical flutter (Figure 2.3) (using the same anatomic circuit but traveling clockwise around the CTI), is typically described as having positive flutter waves in the inferior leads with a similar CL, but the pattern is much less specific than that of counterclockwise typical flutter.

The classical appearance of 12-lead ECG of typical flutter is specific, but not sensitive, for a counterclockwise CTI-dependent circuit. The morphology of the flutter waves may be markedly different in the setting of a structurally abnormal atrium, as in the case of congenital heart disease or previous cardiac surgery. In these situations, flutter waves may have markedly reduced amplitude in the limb leads, reflecting slow isthmus or interatrial conduction, usually from severe left atrial scar or recurrence after prior isthmus ablation.

The 2015 American Heart Association, American College of Cardiology, and Heart Rhythm Society Guideline for the Management of Adult Patients with Supraventricular Arrhythmias provided updated recommendations for the management of AFL.6 The guidelines recommend catheter ablation for typical flutter that is symptomatic or refractory to pharmacologic rate control (class I indication; Level of Evidence: B). The guidelines also state that it may be reasonable to perform catheter ablation for the asymptomatic patients with recurrent atrial flutter (class IIb indication; Level of Evidence: C). Catheter ablation is also reasonable for AFL occurring after use of class Ic agents or amiodarone for the treatment of AF (class I; Level of Evidence: B) or in patients undergoing catheter ablation of AF who also have a history of documented clinical or induced CTI-dependent atrial flutter (class IIa; Level of Evidence: C).

In general, we consider ablation first-line therapy for symptomatic AFL. Cardioversion, with or without antiarrhythmic therapy, may still be attempted as the first line therapy, but this strategy is usually reserved for acute management. Pharmacologic agents (e.g., ibutilide) are effective for the chemical cardioversion of AFL.^9–11 However, recurrence after cardioversion is very high (70%–90%), even in the presence of antiarrhythmic drug therapy to attempt maintenance of sinus rhythm.^12,13 Prospective, randomized trials comparing catheter ablation to medical therapy^13,14 have shown markedly decreased recurrence rates, fewer hospitalizations, and improved quality of life. With asymptomatic AFL, we tend to offer medical therapy, but will consider ablation on a case-by case basis.

Preprocedure Planning

Anticoagulation

AFL is believed to confer a stroke risk similar to AF.^6–8 Therefore, risk stratification and therapy for stroke prevention should parallel management of AF. Termination of AFL from catheter ablation should be viewed no differently than cardioversion. Therefore, for patients presenting for the procedure in AFL, a preprocedure transesophageal echocardiogram (TEE) should be perform prior to ablation to exclude left atrial thrombus. Alternatively, for patients taking warfarin, documentation of therapeutic anticoagulation for 3 weeks prior and up to the procedure is reasonable to avoid TEE.^6–8,15 We prefer to document three weekly therapeutic prothrombin times or INR measurements as proof of therapeutic anticoagulation. In patients taking NOACs, there are data showing NOACs are safe alternatives to warfarin for cardioversion,^16–18 and we will therefore not perform TEEs in patients on new (or novel) oral anticoagulants (NOACs), for ≥ 3 weeks. If there is documentation of subtherapeutic INRs, concerns about NOAC adherence, or a history of prior stroke, then we will perform a TEE. We will also perform ablation on uninterrupted warfarin, based on data from several studies, which have demonstrated safety and efficacy of this approach for catheter ablation with normal INR targets.^19,20 For patients presenting in normal sinus rhythm in whom the ECG meets all criteria for typical counterclockwise CTI flutter, ablation of the CTI may be performed in sinus rhythm without induction or termination or flutter, potentially obviating the need for TEE or anticoagulation.

Procedure

Patient Preparation

We perform CTI ablation on patients in the postabsorptive state after a fast of at least 6 hours. Previously, we used conscious sedation with fentanyl and midazolam rather than general anesthesia. However, new-generation irrigated-tip catheters can cause substantially more pain than previous catheters and we now rely on general anesthesia for most cases. Ablation on the inferior aspect of the CTI ablation line can be exquisitely painful, particularly near the inferior vena cava (IVC).

Vascular Access and Catheter Configuration

For diagnostic catheters, there must be electrodes at various points around the circuit. We favor a comprehensive approach where RA activation can be assessed to determine the full path of the AFL circuit (Figure 2.4). A duodecapolar catheter with electrodes extending from the HRA catheter to the inferior RA lateral to the CTI allows for mapping of activation anterior to the crista and may be helpful in identifying flutter variants that break through the crista. A His bundle catheter can be helpful for both the identification of the septum and as a marker of the His bundle electrogram (EGM) for added safety. A CS catheter provides useful information on both left atrial activation, which may change during RF ablation or mimic AFL in coarsely organized fibrillation, and direction of propagation across the atrial septum when compared to a more superiorly located His catheter. Extending electrodes to the left side via the CS also ensures that the left atrium is not an obligatory part of the circuit. Inserting the duodecapolar catheter across the isthmus and into the CS can obviate the need for separate RA and CS catheters, but it can interfere with the ablation catheter as it lies in the isthmus. If this approach is used, careful attention must be used to ensure that the ablation catheter is below any catheter straddling the isthmus for optimum contact and ablation. Finally, a right ventricular catheter is generally not necessary, but it may be useful to preempt pauses or asystole upon termination of AFL in patients with severe sinus node dysfunction. In cases where a preemptive right ventricular catheter has not been placed, pacing from the distal electrodes of the His bundle catheter can be used for backup ventricular pacing. There is large operator variation in the use of catheters and a minimalist approach with only an ablation catheter and a single reference or diagnostic catheter may be reasonable as long as there is high pretest probability of a CTI-dependent arrhythmia.

Vascular access, including the number of catheters, is tailored to each patient’s case and depends on catheter configuration and patient preferences. We limit the femoral veins to three catheters and the internal jugular to a single access. When a four-catheter setup is required, using ablator, RA, CS, and His, we defer to patient preference when deciding between placing the CS from the neck with three catheters in the groin or dividing up two catheters in each groin (Figure 2.5). However, the risk of groin complications is most likely reduced when only femoral access is obtained from only one side, especially when patients are anticoagulated after the procedure. Unilateral femoral access also better facilitates same-day discharges for elective procedures. To minimize the risk of catheter-associated venous thrombosis, we recommend all vascular access ports be flushed with heparinized saline and continuous, low-volume infusion through the side ports of any long sheaths, if used. Additionally, some operators will give a small intravenous heparin bolus of 30–50 U/kg after vascular access has been obtained.

Fluoroscopic Orientation of Catheters

Figure 2.5 demonstrates typical catheter positions for the ablation of the CTI. Fluoroscopy should demonstrate the CS proximal electrode at the CS ostium. More distally placed CS catheters may make it more difficult to ascertain bidirectional isthmus block with differential pacing or to recognize slow residual isthmus conduction. In the left anterior oblique (LAO) view, the proximal CS catheter should be positioned nearly as leftward as the His catheter. The importance of having the crista terminalis catheter placed anteriorly cannot be understated, as the activation patterns can be deceiving with variable block across the crista terminalis. If the catheter is placed too anteriorly, as can happen with some duodecapolar catheters with an anterior bias to facilitate placement, the distal electrodes may extend across the tricuspid annulus and into the right ventricle, which can be identified by EGMs or fluoroscopically. The right anterior oblique (RAO) projection best demonstrates the position of this catheter. In cases with significant RA enlargement or significant tricuspid regurgitation, extra-large curves of duodecapolar catheters may improve stability. In cases with a small RA, a standard- or large-curve duodecapolar can often be placed across the CTI and into the proximal CS.

Use of Electroanatomic Mapping

For CTI ablation, electroanatomic mapping is not necessary as long as catheters are positioned to ensure appropriate assessment of RA activation and isthmus conduction. Electroanatomic mapping is more useful and recommended in situations where ECG patterns are atypical. Electroanatomic mapping can be useful to minimize fluoroscopy¹⁷ exposure and “low fluoro” approach more commonly used for simple and even complex ablation.

For straightforward typical flutter, electroanatomic mapping can be used to create an activation map, but it is most useful for marking catheter potentials, His potentials, and ablation lesions. The position of the His bundle can be noted on an electroanatomic map (Figure 2.6), obviating the added safety provided by having a His catheter. The position of the ablation catheter can also be noted to allow the ablation catheter to return to the same location if the operator needs to maneuver around other catheters, such as a duodecapolar catheter, extending across the isthmus and into the CS. This is especially useful in labs that have only monoplace fluoroscopy, where the mapping system can be used to complement a single LAO or RAO view. Marking ablation lesions can also be useful in identify gaps in the ablation line or in ensuring that a line is continuous, which can be challenging in cases with tortuous or difficult CTI anatomy.

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