Key Points
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Atrial flutter (AFL) is the second most common arrhythmia seen in clinical practice.
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The direct medical costs because of pharmacologic therapy, cardioversion, and hospitalization may be lessened by curative treatment options such as cryocatheter ablation (CCA).
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CCA of AFL is associated with minimal pain, low complication rates, high effectiveness rates, and durable (and often permanent) therapeutic end points.
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Safety concerns typically relate to problems with vascular access and catheter manipulation, unwanted tissue damage on cryoenergy delivery, and radiation exposure secondary to fluoroscopy.
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Acute success rates between 56% and 100% and symptom recurrence rates from 0% to 25% have been reported from clinical studies.
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Effective follow-up monitoring for new-onset atrial fibrillation (AF) postablation treatment for AFL may be required given reports of occurrence of the former arrhythmia during follow-up.
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Focal ablation catheters with better handling, more maneuverability, plus the ability to create deeper and wider lesions are needed to maximize the potential for CCA.
Given the prevailing intense pressure to use cost-effective treatments and drive down healthcare costs, it is timely to bring treatment modalities such as cryocatheter ablation (CCA) to the fore because they may help reduce the clinical and economic burden associated with common cardiac arrhythmias such as atrial flutter (AFL). This chapter first briefly presents the epidemiology, pharmacoeconomics, symptoms and clinical features, and pathophysiology for AFL. The discussion then continues with a more in-depth account of CCA for AFL, specifically giving a bench-to-bedside insight into the preclinical and clinical record of this treatment modality.
Epidemiology and Pharmacoeconomics
After atrial fibrillation (AF), AFL is the most common atrial tachyarrhythmia seen in clinical practice. Men, the elderly, and individuals with preexisting heart failure or chronic obstructive lung disease are at greatest risk for development of this heart condition. The incidence of AFL increases markedly with age, from 5 per 100,000 of those older than 50 years to 587 per 100,000 for those older than 80 years. With a greater segment of aged persons expected to make up the general population in the decades ahead, the prevalence of AFL is anticipated to trend further upward. A recent retrospective analysis of a national claims database estimated prevalence of 3.03 million persons in the United States with AF only, 0.07 million with AFL only, and 0.19 million with both AF and AFL in 2005. Moreover, prevalence by the year 2050 is projected to increase to 7.56 million for AF only, 0.15 million for AFL only, and 0.44 million for both AF and AFL. These estimates of disease prevalence may also be applicable to a similarly sized population within the European Union.
There is a significant burden of illness associated with AFL both to society and affected individuals. This is due to lost productivity, direct and indirect medical costs, and the poorer quality of life associated with this condition. Assuming similar annual medical management costs as reported for AF, the cost per patient with AFL may be estimated at approximately $10,000 per year merely for therapeutic management. Therefore, a need exists to bend the cost curve associated with AFL given this annual per patient expense combined with the increasing prevalence of AFL projected in the decades ahead. The wider use of potentially curative treatments such as catheter ablation, although costly in the short term, may alleviate the burden of illness associated with AFL. This may be achieved by minimizing the need for lengthy hospitalization, cardioversion, medication, and follow-up visits. Indeed, a recent multivariate analysis suggests that same-day discharge of patients who had a routine catheter ablation procedure for AFL may be feasible, safe, and yield the added benefits of patient satisfaction and cost savings.
Symptoms and Clinical Features
On examination, the upper chambers of the fluttering heart beat up to five times faster than normal and typically with a 2 : 1 or 4 : 1 ratio between atrial and ventricular contractions. The heart is not an effective pump under these conditions of accelerated cardiac activity plus discordant atrial and ventricular contractions, which manifest as the clinical symptoms experienced by those with AFL. Although some patients with AFL are asymptomatic, commonly reported symptoms include palpitations, dizziness, chest tightness, shortness of breath, and fatigue. Despite not being a life-threatening arrhythmia, AFL can cause hypotension, impair cardiac output, exacerbate pulmonary congestion, and initiate myocardial ischemia. Moreover, tachycardia-mediated cardiomyopathy may be a consequence of permanent AFL with a rapid ventricular rate, plus individuals with AFL are at increased risk for stroke versus the general population.
Pathophysiology
The precise cause of AFL has not been fully elucidated, although a number of possible causes have been proposed. AFL can arise after scarring in the heart resulting from prior cardiac disease or heart surgery, but it can also occur in some patients with no other identifiable heart problems. There are two types of AFL, namely, type I (also known as typical, common, or counterclockwise isthmus dependent) and type II (also referred to as atypical or nonisthmus dependent). Type I is separated from type II by flutter rate (240 to 340 beats/min compared with 340 to 440 beats/min in type II), the existence of an excitable gap, the ability to transiently entrain type I but not type II, and the observation that type II can change in a “stepwise” manner to type I. Moreover, typical AFL has an electrocardiogram showing the classic “sawtooth” pattern of flutter waves ( Figure 13–1 ) with negative polarity in leads II, III, and aVF. Type I AFL rotates around the tricuspid annulus with the crista terminalis or sinus venosa (an area between superior and inferior cava) thought to be the functional posterior barrier and the tricuspid annulus forming the anterior barrier. The tricuspid isthmus is a slow conduction zone and the standard target for ablation in type I AFL ( Figure 13–2 ). Ablation is sometimes difficult across this zone. This may be because of interpatient anatomic variation that exists, which includes sub-Eustachian pouches or large pectinate muscles that encroach onto the isthmus.
The target in type II AFL is more difficult to identify and ablate compared with type I AFL. The former arrhythmia is nonisthmus dependent and may arise from the right or left atrium. Right-sided arrhythmia includes upper loop re-entry, free wall re-entry, and figure-of-eight re-entry, whereas left-sided arrhythmia pertains to mitral annular, pulmonary vein related, and left septal. Macroreentrant circuits, where activation rotates around large obstacles, are the most common arrhythmias found in patients with type II AFL. Successful ablation in any of these settings depends on the exact identification of the reentrant circuit responsible for the arrhythmia, as well as its critical zone. The need to match activation maps with anatomy precisely makes computer-assisted, anatomically precise mapping an especially useful tool for cryoablation of type II AFL. Wider use of specialized systems that allow creation of three-dimensional reconstructions of the atria, which plot atrial activation sequence in tachycardia and location of scar tissue or conduction block, may facilitate mapping and ablation.
Cryoablation from Bench to Bedside
Use of cryoenergy as a therapeutic agent has a long history. It has been used in many noncardiac disciplines of medicine. But practical use of the unique property that cryoenergy engenders, namely, its ability to reversibly alter the electrical activity of cardiac tissue, dates back to the 1960s. The advantage that this property endows to CCA is apparent in the present-day practice of cryomapping . It was not until the late 1990s that cryoenergy delivery at temperatures close to −80°C was successfully combined with a highly selective and precise means of ablating myocardial tissue via a focal ablation catheter. A preclinical study by Dubuc and colleagues showed the feasibility of using an intravenous steerable catheter for percutaneous CCA of cardiac tissue in dogs. Follow-up investigation demonstrated the safety of CCA within the high-risk region of the atrioventricular (AV) node, the low risk for endocardial disruption, and minimal thrombogenicity. No acute or chronic complications were identified. Moreover, gross and histopathologic examinations showed that focal catheters of various tip sizes created continuous and transmural lesions.
Preclinical Studies Demonstrate the Feasibility of Cryoablation
A major advantage of cryoenergy delivery is the ability to reversibly suppress the electrophysiologic properties of cardiac tissue before the creation of permanent irreversible conduction block. This property was demonstrated with a steerable ablation catheter that was able to produce effective cardiac lesions to focal targets like the AV node in dogs. A few years later, Timmermans and colleagues showed that a similar catheter could successfully ablate the cavotricuspid isthmus (CTI) and create permanent bidirectional conduction block at this target zone in dogs. With the feasibility of CCA for AFL demonstrated in these animal studies, there was impetus to investigate whether a steerable cryoablation catheter could safely and efficaciously treat AFL in humans.
Basic Features of the Cryoablation Procedure
CCA is a minimally invasive procedure involving a few key steps. Multipolar catheters are advanced into the right atrium through insertion in the right femoral vein. The catheters are appropriately positioned under fluoroscopic guidance (a typical setup for flutter ablation in our institution is depicted in Figures 13–3 and 13–4 ). Successful ablation is dependent on identifying the critical portion of the re-entry circuit where it can be interrupted. Once targeted, the macroreentrant circuit is interrupted by creating focal lesions within the critical zone of slow conduction that extends to anatomic borders. Cryoenergy, at a mean temperature of approximately −80°C ± 5°C for 3 to 5 minutes per application, is typically delivered by a point-by-point method to create bidirectional conduction block across the critical zone. Some investigators have modified this last parameter in an attempt to maximize efficacy and procedure benefits. Isthmus block is verified by demonstrating bidirectional conduction block across the flutter isthmus after ablation through differential pacing from the coronary sinus and right atrium, lateral to the line.
Sometimes, the presence of non–CTI-dependent AFL may not be apparent until ablation in the CTI fails to abolish the arrhythmia, with the ablation of non–CTI-dependent AFL typically being quite difficult. Moreover, if multiple potential re-entry circuits are present, the electrical pathway may switch back and forth among different circuits. The presence of multiple reentrant circuits complicates attempts to identify an appropriate target zone for ablation.
End Points to Assess the Safety and Efficacy of Cryoablation in Clinical Studies
Procedure and device-related complications, discomfort on cryoenergy delivery to cardiac tissue, bidirectional conduction block, noninducibility of AFL after ablation, symptom recurrence during follow-up, and conduction recurrence during follow-up are among the major clinical end points that have been used to assess the safety and efficacy of CCA. Acute success is typically measured in terms of the creation of bidirectional conduction block intraoperatively. Chronic success is generally appraised in terms of symptom recurrence and/or conduction recurrence on repeat electrophysiologic study (EPS) during follow-up. But most reported data on recurrence during follow-up are subjective. Consequently, there may be an underestimation of the true failure rate of catheter ablation by those studies that have not measured conduction recurrence by repeat EPS but have instead relied solely on patient reporting of symptoms during follow-up. Only an objective measure, such as verified persistence of bidirectional conduction block on repeat EPS during follow-up, confirms the lasting success of catheter ablation. A few clinical studies have measured conduction recurrence by repeat EPS to determine the persistency of bidirectional conduction block. But an obstacle to the broader use of repeat EPS as a follow-up assessment is the refusal of many asymptomatic patients to undergo this invasive procedure.
Clinical Studies Demonstrate the Safety and Efficacy of Cryoablation
With feasibility shown in the aforementioned preclinical studies, numerous clinical studies followed that demonstrated the safety and efficacy of CCA as treatment for AFL ( Table 13–1 ). Although the safety profile of CCA in clinical studies has largely been excellent, nonetheless, there are reports that should prompt due diligence by clinicians. Safety concerns typically pertain to tissue injuries associated with gaining vascular access and catheter manipulation, unwanted tissue damage on cryoenergy delivery, and radiation exposure secondary to fluoroscopy.
| AUTHOR | CATHETER SIZE | PATIENTS TREATED WITH CRYO ABLATION (N) | SAFETY OUTCOMES FOR CRYOABLATION | EFFICACY OUTCOMES FOR CRYOABLATION | MEAN FOLLOW-UP (MONTHS) |
|---|---|---|---|---|---|
| Rodriguez (2002) | 10-Fr 6 mm | 15 | No complications | 100% acute success | ~3 |
| Timmermans (2003) | 10-Fr 6 mm | 7 | No complications | 100% acute success; 0% symptom recurrence | 6 |
| Rodriguez (2004) | 10-Fr 6 mm | 73 | No complications | 99% acute success; 11% symptom recurrence | 15 |
| Manusama (2004) | 10-Fr 6 mm | 40 | No complications | 98% acute success; 5% symptom recurrence | 11.7 |
| Manusama (2004) | 10-Fr 6 mm | 35 | No major adverse events; no thromboembolic complications | 97% acute success; 11% symptom recurrence | 17.6 |
| Daubert (2005) | 10-Fr 6 mm | 48 | 1 serious procedure-related complication (femoral hematoma) | 94% acute success; 25% symptom recurrence | 6 |
| Montenero (2005) | 7-Fr 6 mm | 45 | No complications | 87% acute success; 31% conduction recurrence at 3-month repeat EPS; 0% symptom recurrence rate at 9-month follow-up | 9 |
| Montenero (2005) | 9-Fr 8 mm | 77 | No complications | 96% acute success; 30% conduction recurrence rate at 3-month repeat EPS; 0% symptom recurrence rate at 6-month follow-up | 6 |
| Montenero (2005) | 9-Fr 8 mm vs. 7 Fr 6 mm | 94 | No complications | Acute success rate 100% vs. 88%; conduction recurrence rate 35% vs. 32%; symptom recurrence rate 0% vs. 0% | 9 |
| Collins (2006) | 9-Fr 8 mm | 14 | No complications | 93% acute success rate; 14% symptom recurrence rate | 14.7 |
| Kuniss (2006) | 9-Fr 8 mm | 50 | No complications | 100% acute success rate; 19% conduction recurrence rate | 1 |
| Wang (2007) | 9-Fr 8 mm | 9 | No complications | 100% acute success rate; 0% symptom recurrence rate | 22 |
| Feld (2008) | 10-Fr 6.5 mm | 160 | Procedure-related complications (6.5%) included atrial fibrillation, groin hematoma, cardiac tamponade, atrial flutter, sick sinus syndrome, and complete atrioventricular block | 87.5% acute success rate; 10% symptom recurrence rate | 6 |
| Moreira (2008) | 10-Fr 6.5 mm | 180 | No complications | 95% acute success rate; 9% symptom recurrence rate | 27 |
| Thornton (2008) | 9-Fr 8 mm | 32 | 1 pericardial effusion | 69% acute success rate; 0% symptom recurrence rate | 4 |
| Malmborg (2009) | 9-Fr 8 mm | 20 | No complications | 56% acute success rate; 20% symptom recurrence rate | 15.1 |
| Kuniss (2009) | 9-Fr 8 mm | 90 | No complications | 89% acute success rate; 34% conduction recurrence rate | 3 |
| Manusama (2009) | 10-Fr 10 mm | 37 | No complications | 97% acute success rate; 50% conduction recurrence rate; 8.3% symptom recurrence rate | 37 |
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