Electrophysiological Anatomy of Typical Atrial Flutter: The Posterior Boundary and Causes for Difficulty with Ablation

Clinical anatomy of the coronary venous system and relevance to retrograde cardioplegia and cardiac electrophysiological interventions

Anomalies of coronary venous system, the valve of the coronary sinus (Thebesian valve) and other cardiac malformations may make interventions through the coronary sinus difficult. These variants may pose a challenge in cannulating the coronary sinus for retrograde cardioplegia and for interventions performed through the coronary sinus by cardiac electrophysiologist/interventional cardiologist. Retrograde cardioplegia is an established method of myocardial protection with advantages, indications, and complications. A good knowledge of the anatomy of the coronary sinus and its variants is important in understanding the difficulties encountered while cannulating the coronary sinus for the delivery of retrograde cardioplegia, cardiac resynchronization therapy, treatment of arrhythmias, and percutaneous mitral valve annuloplasty.

Echocardiographic imaging of a bifurcated double barrel coronary sinus

BACKGROUND

The coronary sinus (CS) is the terminal collecting vessel of the myocardial venous network, which returns deoxygenated blood used by the heart to the right atrium. The advent of high-fidelity imaging via CT and transesophageal echocardiography (TEE) has further defined the anatomy of the CS and its multiple tributaries. Understanding this anatomy is crucial for cardiac surgical cases that require the cannulation of the coronary sinus to deliver retrograde cardioplegia. However, anatomical variants of the CS may frustrate surgical retrograde catheter placement, in turn increasing the risk of CS injury or leading to inadequate cardioplegia delivery. Here, we present an especially unique CS presentation, a bifurcated, double-barrel CS, which was discovered via intraoperative TEE imaging that revealed a CS with two smaller lumens instead of the singular large os.

CASE PRESENTATION

A 67-year-old male presented for ascending aortic dissection repair, aortic valve replacement, and single vessel coronary artery bypass graft. On the pre-bypass TEE exam, the anesthesiologist noted a bifurcated CS with two small lumens. The surgeon utilized this information to select a smaller diameter retrograde catheter to avoid damage or perforation of the vessel. With TEE guidance, the surgeon successfully cannulated one of the CS lumens. However, it was noted upon dosing of retrograde cardioplegia that all tributary vessels attached to the non-cannulated lumen remained devoid of cardioplegia. The surgeon was forced to repeatedly administer anterograde cardioplegia via a handheld catheter through the coronary ostium throughout the case. The operative field was also flooded with topical ice saline slush to ensure cardiac protection. Ultimately, the operation was completed without incident despite the non-ideal conditions resulting from this anatomic variant.

CONCLUSIONS

Discovery of this patient's double-barrel CS during the pre-bypass TEE was incidental, showing that such anatomical variants may be completely asymptomatic and benign in the non-operative setting. However, the delivery of cardioplegia proved challenging for this patient, highlighting some degree of risk with certain cardiac interventions. This case demonstrates the utility of intraoperative TEE to quickly ascertain unforeseen anatomical variants of the CS which could compromise the safety of cardiac surgery cases.

Anatomical knowledge for the ablation of left and right atrial flutter

The different forms of atrial flutter (AFL) and atrial macroreentrant tachycardias are strongly related to the atrial anatomy in structurally normal atria, and even more so in patients with dilated chambers or with previous interventions. Atrial anatomy, macro- and microscopic tissue disposition including myocardial fibers, conduction system and connective tissue is complex. This review summarizes knowledge of atrial anatomy for the interventional electrophysiologist to better understand the pathophysiology of and ablation options for these complex arrhythmias, as well as to perform catheter ablation procedures safely and effectively.

A case report of balloon-assisted tracking to overcome coronary sinus competent valve: a novel technique in left ventricular lead implantation

Background

Competent lateral and posterolateral valves showed proximal tortuosity that might hinder left ventricular (LV) lead implantation in cardiac resynchronization therapy (CRT).

Case summary

A 57-year-old woman was diagnosed as non-ischaemic cardiomyopathy, no other comorbidities, complaining of dyspnoea [New York Heart Association (NYHA) class III], and on optimal medical therapy. Electrocardiogram showed left bundle branch block with QRS duration 150 ms. The patient was candidate for CRT. However, during LV lead implantation, a competent posterolateral vein valve and proximal tortuosity hindered LV lead implantation that was overcome by balloon-assisted tracking technique. At 9 months of follow-up, the patient had NYHA class II, ejection fraction improved to 38%, and all implanted leads were still in place.

Discussion

Balloon-assisted tracking technique can be used to cross coronary sinus and smaller veins with competent valves and coronary veins tortuosity.

Peculiarities in coronary sinus anatomy: implications for successful cannulation from an autoptic study

Aims

The number of cardiovascular procedures using the coronary sinus (CS) as a gateway is constantly increasing. The present study aimed to define specific structures within CS, which could potentially complicate CS cannulation and to develop a new Thebesian valve (TV) classification system.

Methods and results

The study was performed on 560 consecutive unfixed cadaveric hearts during routine autopsy examination (1–3 days post-mortem). Basic CS dimensions were measured and the presence and dimensions of the TV and the Vieussens valve (VV) were assessed. Thebesian valves were classified according to their morphology into six main types: remnant fold, semilunar, fenestrated, chord, fused strands, and mixed shaped. The median age of hearts was 48 years (range 16–95 years), and 38.9% were female. Thebesian valve was present in 79.5%. The most common TV type was semilunar (54%) followed by fenestrated (8.2%), remnant fold (5.5%), fused strands (4.8%), chord (4.0%), and mixed shaped (3.0%). In 1.1% of hearts, TV totally covered the coronary sinus ostium (CSO). The VV was detected in 67.9%. Potentially occlusive VV was found in 1.1% hearts and in all of which it coexisted with obstructive TV. The median CSO area was 87.9 mm² [interquartile range (IQR): 56.5–127.1 mm²] and median CS length was 38 mm (IQR: 29.5–45 mm). The CSO area and CS length correlated with each other and with the right atrium’s dimensions.

Conclusion

We identified six types of TVs, among which only 1.1% TVs caused total occlusion of CSO. The obstructive TV co-existed with potentially occlusive VV what might hinder CS cannulation.

A cavotricuspid isthmus pouch revealed to be a breakout site for gap conduction of recurrent common atrial flutter

A cavotricuspid isthmus pouch can be a breakout site for gap conduction of cavotricuspid isthmus block line. If the previous block line is electrically silent, high-density 3-D mapping and pouchgraphy are useful to find the pouch and ablate within it.

Biophysical properties, efficacy, and lesion characteristics of a new linear cryoablation catheter in a canine model

BACKGROUND

The cryoballoon (CB) catheter is an established tool for pulmonary vein isolation (PVI), but its use is limited for that purpose.

OBJECTIVE

The purpose of this study was to investigate the biophysical properties of a newly developed linear cryoablation catheter for creation of linear ablation lesions in an in vivo model.

METHODS

Twenty-nine dogs (14 acutely ablated, 15 chronically followed) underwent cryoablation using the linear cryoablation catheter. Regions of interest included the cavotricuspid isthmus (CTI), mitral isthmus (MI), left atrial (LA) roof, and LA posterior wall in an acute study. Cryoablations for CTI and MI were performed in 14 atrial fibrillation animals after PVI and followed over 1 month in the chronic study. Tissue temperature during cryoablation was monitored using implanted thermocouples in the regions of interest. Gross and microscopic pathologic characteristics of the lesions were assessed.

RESULTS

In acute animals, lesion length (transmurality) was CTI 34 ± 4 mm (89% ± 11%); MI 29 ± 4 mm (90% ± 13%); LA roof 19 ± 3 mm (90% ± 8%); and LA posterior wall 19 ± 2 mm (81% ± 13%), with 1 or 2 freezes. Chronic bidirectional block was achieved in 13 of 14 CTI (93%) and 10 of 14 MI (71%) ablations after 1-month follow-up and was consistent with lesion continuity and transmurality upon pathology. The lowest tissue temperature correlated well with the closest distance to the linear cryocatheter (r = 0.688; P <.001).

CONCLUSION

This linear cryocatheter created continuous and transmural linear lesions with "single-shot" cryoenergy application and has the potential for clinical use in the setting of various arrhythmias.

Multi-electrode mapping of complex macroreentry atrial tachycardia

BACKGROUND

Multi-electrode mapping (MEM) is increasingly applied in ablation of complex atrial arrhythmias. This study aimed to evaluate MEM for analysis and treatment of complex macroreentry atrial tachycardia (MAT).

METHODS

Patients with MAT related to scarring, history of heart surgery or atrial linear ablation were studied. Patients were mapped with conventional activation mapping (CAM) or MEM. After characterizing the mechanism of atrial tachycardia (AT), the ablation was performed.

RESULTS

The study consisted of 114 eligible patients, 74 in the CAM and 40 in MEM. Compared with CAM, MEM had a shorter procedure duration (156.7 ± 59.1 ms vs. 127.3 ± 59.3 ms, P = 0.003) and mapping duration (62.6 ± 35.7 ms vs. 30.5 ± 15.3 ms, P < 0.001) and more mapping points (1364.9 ± 828.7 points vs. 148.3 ± 79.6 points, P < 0.001). There were no significant differences between CAM and MEM in acute ablation success rate, complication, postoperative AADs, and ablation duration. The mean disease-free survival time in CAM versus MEM was 20.8 (95% CI: 17.6-24.1) months versus 26.6 (95% CI: 22.7-30.4) months. The median disease-free survival time in the CAM versus MEM was 20.0 (95% CI: 13.9-26.1) months versus 30.0 (95% CI: 26.7-36.3) months. The AT recurrence risk of MEM was 0.522 times that of CAM (HR 95% CI: 0.282-0.968; P = 0.039).

CONCLUSION

MEM is strongly recommended in ablation of complex MAT.

Anatomic Considerations Relevant to Atrial and Ventricular Arrhythmias

Knowledge of relevant cardiac anatomy is crucial in understanding the pathophysiology and treatment of arrhythmias, and helps avoid potential complications in mapping and ablation. This article explores the anatomy, relevant to electrophysiologists, relating to atrial flutter and atrial fibrillation, ventricular tachycardia relating to the outflow tracts as well as endocardial structure, and also epicardial considerations for mapping and ablation.

Use of Intracardiac Echocardiography in Interventional Cardiology: Working With the Anatomy Rather Than Fighting It

The indications for catheter-based structural and electrophysiological procedures have recently expanded to more complex scenarios, in which an accurate definition of the variable individual cardiac anatomy is key to obtain optimal results. Intracardiac echocardiography (ICE) is a unique imaging modality able to provide high-resolution real-time visualization of cardiac structures, continuous monitoring of catheter location within the heart, and early recognition of procedural complications, such as pericardial effusion or thrombus formation. Additional benefits are excellent patient tolerance, reduction of fluoroscopy time, and lack of need for general anesthesia or a second operator. For these reasons, ICE has largely replaced transesophageal echocardiography as ideal imaging modality for guiding certain procedures, such as atrial septal defect closure and catheter ablation of cardiac arrhythmias, and has an emerging role in others, including mitral valvuloplasty, transcatheter aortic valve replacement, and left atrial appendage closure. In electrophysiology procedures, ICE allows integration of real-time images with electroanatomic maps; it has a role in assessment of arrhythmogenic substrate, and it is particularly useful for mapping structures that are not visualized by fluoroscopy, such as the interatrial or interventricular septum, papillary muscles, and intracavitary muscular ridges. Most recently, a three-dimensional (3D) volumetric ICE system has also been developed, with potential for greater anatomic information and a promising role in structural interventions. In this state-of-the-art review, we provide guidance on how to conduct a comprehensive ICE survey and summarize the main applications of ICE in a variety of structural and electrophysiology procedures.

Supraventricular Arrhythmias: Clinical Framework and Common Scenarios for the Internist

Supraventricular arrhythmias can cause uncomfortable symptoms for patients. Often, the first point of contact is in the primary care setting, and thus, it is imperative for the general internist to have a clinical framework in place to recognize this cluster of cardiac arrhythmias, be familiar with immediate and long-term management of supraventricular tachycardias, and understand when cardiac electrophysiologic consultation is necessary. The electrocardiographic characteristics can have subtle but important clues to the diagnosis and initial management. An understanding of the mechanisms of these arrhythmias is essential to provide proper therapy to the patient. In addition, there are common practice strategies that should be emphasized to avoid common misperceptions that could pose risk to the patient. In this review, we provide a framework to more easily recognize and classify these arrhythmias. We also illustrate the mechanism for these arrhythmias to provide an understanding of the interventions generally used.

Clinical Anatomy of the Cavotricuspid Isthmus and Terminal Crest

The aim of this study was to provide useful information about the cavotricuspid isthmus (CTI) and surrounding areas morphology, which may help to plan CTI radio-frequency ablation. We examined 140 autopsied human hearts from Caucasian individuals of both sexes (29.3% females) with a mean age of 49.1±17.2 years. We macroscopically investigated the lower part of the right atrium, the CTI, the inferior vena cava ostium and the terminal crest. The paraseptal isthmus (18.5±4.0 mm) was significantly shorter than the central isthmus (p<0.0001), and the central isthmus (24.0±4.2 mm) was significantly shorter than the inferolateral isthmus (29.3±4.9 mm) (p<0.0001). Heart weight was positively correlated with all isthmus diameters. Three different sectors of CTI were distinguished: anterior, middle and posterior. The middle sector of the CTI presented a different morphology: trabeculae (N = 87; 62.1%), intertrabecular recesses (N = 35; 25.0%) and trabecular bridges (N = 18; 12.9%). A single sub-Eustachian recess was present in 48.6% of hearts (N = 68), and a double recess was present in 2.9% of hearts (N = 4) with mean depth = 5.6±1.8mm and diameter = 7.1±3.4mm. The morphology of the distal terminal crest was varied; 10 patterns of the distal terminal crest ramifications were noted. There were no statistically significant differences in any of the investigated CTI parameters between groups with different types of terminal crest ramifications. The presence of intertrabecular recesses (25.0%), trabecular bridges (12.9%) and sub-Eustachian recesses (48.6%) within the CTI can make ablation more difficult. We have presented the macroscopic patterns of final ramifications of the terminal crest within the quadrilateral CTI area.

Anatomical barriers in the right atrium to the coronary sinus cannulation

Background. The coronary venous system is an increasingly frequent target of minimally invasive cardiac procedures. The purpose of this paper is to assess the anatomical barriers in the right atrium to coronary sinus cannulation.

Methods. We examined the anatomy of the right atrium, coronary sinus ostium, inferior and superior vena cava ostia in 110 randomly selected autopsied human hearts of both sexes (27% females; mean age 49.2 ± 17.5 years).

Results. The Eustachian valve was present in 79 cases (71.8%) with mean height =4.9 ± 2.6 mm. The valve was perforated in 11 cases (13.9%). It is typically too small to hinder the coronary sinus catheterization, but in some cases (about 2%) a significantly protruding valve may be an obstacle. Chiari’s network (4.6%) is not a barrier to catheter entry into the right atrium but may significantly impede further catheter manipulations inside the heart venous system. A typical Thebesian valve leaves enough space for the passage of the standard catheter to the coronary sinus.

Discussion. Detailed anatomy of various anatomical structures within the right atrium that could play a potential role in coronary sinus cannulation is discussed.

Anatomical variations of the coronary sinus valve (Thebesian valve): implications for electrocardiological procedures

AIMS

The Thebesian valve (TV) can be a significant obstacle to coronary sinus (CS) cannulation. The aim of this study was to evaluate the characteristic features of the CS valve--TV anatomy. In particular, emphasis was placed on identifying specific structures of the TV that could potentially complicate CS cannulation.

METHODS AND RESULTS

We examined 273 autopsied human hearts. The height of the TV and the diameter of the CS were measured. The valves were classified according to their shape into five types: remnant, semilunar, fold, cord, and mesh and fenestrated. The mean transverse CS ostium (CSO) diameter was 12.2 ± 3.5 mm. The TV was present in 224 (82.1%) cases. The most common type of TV was semilunar: 32.6%; followed by remnant: 25.5%; fold: 17.4%; cord: 14.3%; and lastly mesh and fenestrated: 10.3%. The mean TV height for remnant-semilunar-fold types was 5.8 ± 3.0 mm. In seven cases, the present TV (2.6%) covered the entire orifice of the CS. Hearts with larger CSO diameter had lower TV height (P < 0.001).

CONCLUSIONS

We propose a new classification of the TV shapes based on the largest sample to date. We assessed that only in 2.6% of all 273 cases the presence of an obstructive TV can cause unsuccessful cannulation. The height of the TV was inversely correlated to the CSO diameter (r = -0.33; P < 0.001).

Does diffuse irrigation result in improved radiofrequency catheter ablation? A prospective randomized study of right atrial typical flutter ablation

AIMS

Recent developments of open irrigated catheters have sought to create uniform cooling of the entire ablating electrode. The aim of this randomized study was to assess whether the diffuse irrigation of the Coolflex(®) (CF) catheter results in improved short-term procedural benefits in patients undergoing ablation of right atrial typical flutter.

METHODS AND RESULTS

Sixty consecutive patients (age 62 ± 13) with typical atrial flutter were prospectively randomized to ablation of the cavotricuspid isthmus (CTI) using either a standard 3.5 mm tip ablation catheter with six distal irrigation channels (6C) (30 patients) or a 4 mm tip fully irrigated ablation catheter (CF) (30 patients). There were no significant differences seen between procedures performed with the diffusely irrigated CF catheter and the standard six-channel irrigated-tip catheter. This concerned the total procedural duration RF duration, fluoroscopic duration, the total amount of irrigation fluid, and the occurrence of steam pop.

CONCLUSIONS

The use of a diffuse irrigation at the ablation catheter tip does neither facilitate lesion formation nor reduce the amount of irrigation during RF ablation for typical right atrial flutter using recommended flow and power settings.

Standardized review of atrial anatomy for cardiac electrophysiologists

Catheter ablation of cardiac arrhythmias has rapidly evolved from a highly experimental procedure to a standard form of therapy for various tachyarrhythmias. The advances in this field have included, first, the development of techniques of catheter ablation that often requires the precise destruction of minute amounts of arrhythmogenic tissues and, second, techniques of resynchronization therapy that require pacing different parts of the ventricles. A detailed prepocedural knowledge of cardiac anatomy can improve the safety of the procedure and its rate success. It helps the electrophysiologist to choose the appropiate region for ablation, shortening the procedural time. The atrial anatomy structures are usually localized before ablation by different imaging techniques such as fluoroscopy, electroanatomic mapping, intracardiac echocardiography or multidetector computed tomography. In this review, we describe the normal anatomy of the atria, highlighting the landmarks of interest to intervencional cardiologist, stressing their relationship to other structures. This article is part of a JCTR special issue on Cardiac Anatomy.

Imaging of the coronary sinus: normal anatomy and congenital abnormalities

Knowledge of the anatomy of the coronary sinus (CS) and cardiac venous drainage is important because of its relevance in electrophysiologic procedures and cardiac surgeries. Several procedures make use of the CS, such as left ventricular pacing, mapping and ablation of arrhythmias, retrograde cardioplegia, targeted drug delivery, and stem cell therapy. As a result, it is more important for physicians interpreting the results of computed tomographic (CT) examinations dedicated to the heart or including the heart to be able to identify normal variants and congenital anomalies and to understand their clinical importance. Abnormalities of the CS range from anatomic morphologic variations to hemodynamically significant anomalies such as an unroofed CS, anomalous pulmonary venous connection to the CS, and coronary artery-CS fistula. It can be important to identify some anatomic variations, even though they are clinically occult, to ensure appropriate preprocedural planning. Both CT and magnetic resonance imaging provide excellent noninvasive depiction of the anatomy and anomalies of the CS. Supplemental material available at http://radiographics.rsna.org/lookup/suppl/doi:10.1148/rg.324105220/-/DC1.

Impact of respiration on electroanatomical mapping of the right atrium: implication for cavotricuspid isthmus ablation

PURPOSE

Cavotricuspid isthmus (CTI) ablation for typical atrial flutter (AFL) has become the preferred treatment for this arrhythmia. The aims of this study were to assess the impact of respiratory gating (RG) on electroanatomical mapping of CTI and to assess the efficiency of CTI ablation guided by the Carto3® system equipped with the new respiration gating software.

METHODS

Forty-four consecutive patients (mean age, 60 ± 13 years; 25 male) undergoing cavotricuspid ablation for symptomatic common AFL were randomly assigned to CARTO™ mapping with or without enabling RG module (Group A, RG OFF, Group B, RG ON).

RESULTS

A significant reduction in mean RA volume, CTI central length and CS ostium maximum diameter has been observed in the RG maps. The mean total procedural, fluoroscopy and radiofrequency (RF) time were 102.9 ± 35.3, 10.6 ± 3.3, 22.9 ± 14.2 min in group A and 75.3 ± 21.7, 3.6 ± 4.5, 10.4 ± 5.7 min in group B, respectively (p < 0.05).

CONCLUSIONS

Electroanatomical mapping systems' accuracy may be strongly influenced by respiration movements. The current study showed that automatic respiratory gated acquisition resulted in a better visualization of CTI, and this determines a relevant reduction in fluoroscopy and RF times.

Distribution of Mean Cycle Length in Cavo-Tricuspid Isthmus Dependent Atrial Flutter

Although cycle length (CL) constitutes a fundamental descriptor of any arrhythmia, there is not larger study describing mean CL in electrophysiologically confirmed cavo-tricuspid isthmus (CTI)-dependent atrial flutter (AFL). We analyzed retrospectively digital recordings of 121 patients (98 men; age 64±11 years) referred for radiofrequency ablation of persistent CTI-dependent AFL. Median of mean AFL CL was 240 ms (interquartile range (IQR) of 222-258 ms, overall range of 178-399 ms). The distribution of CL was not normal (Shapiro Wilk test, p<0.001). Both counterclockwise and clockwise (14.9 % of all cases) AFLs were comparable in their CL; 240 (IQR 222-258) ms vs. 234 (217-253) ms, respectively. AFL CL<200 ms and AFL CL<190 ms was noticed in 5 (4.1 %) and 3 cases (2.5 %), respectively. In multivariate regression analysis, age (increase by 6±3 ms per decade of age, p=0.036), treatment with specific antiarrhythmic drugs (increase by 11±6 ms, p=0.052) and the history of cardiac surgery (increase by 26±9 ms, p=0.004) were independently associated with AFL CL. In conclusions, the distribution of AFL CL is not normal. The prevalence of AFL with short CL is low. Short CL<200 ms does not rule out the CTI-dependent AFL, especially in young and otherwise healthy patients.

The Thebesian valve: Gatekeeper to the coronary sinus

Clinical cardiac procedures such as electrophysiology studies, catheter ablation of arrhythmias, retrograde cardioplegia delivery, cardiac resynchronization therapy and, more recently, percutaneous mitral annuloplasty, involve cannulation of the coronary sinus (CS). The presence of a membrane closing the orifice of the CS may cause difficulties during these interventions. Thus, detailed knowledge of the variations and anomalies of the valve of the CS, or the Thebesian valve, now has practical significance. To improve our understanding of this structure, classic anatomical dissection of 50 hearts from dissection room cadavers was performed. A Thebesian valve was present in the overwhelming majority (88%) of cases. Its morphology varied widely, from a few small strands of tissue, to a membrane covering more than half the CS ostium. A significant number (20%) of valves occluded >65% of the ostium, making them "potential complicating factors" in cannulation of the CS. An understanding of these anatomical variations may help in identifying and overcoming potential difficulties during clinical cardiac interventions.

Usefulness of pre-procedure cavotricuspid isthmus imaging by modified transthoracic echocardiography for predicting outcome of isthmus-dependent atrial flutter ablation

BACKGROUND

Anatomic characteristics of the cavotricuspid isthmus (CTI) have been reported to be related to the outcome of atrial flutter ablation therapy. However, preprocedural evaluation of CTI anatomy using modified transthoracic echocardiography to guide atrial flutter ablation has not been well described.

METHODS

Transthoracic echocardiography was prospectively performed before atrial flutter ablation in 42 patients with typical CTI-dependent atrial flutter. A modified apical long-axis view was designed to visualize and evaluate anatomic characteristics of the CTI and Eustachian ridge (ER). A prominent ER, extending from the inferior vena cava to the interatrial septum, is defined as an extensive ER.

RESULTS

Twenty-eight patients had straightforward ablation procedures, and 14 patients had difficult ablation procedures. Two patients with difficult procedures had unsuccessful ablation. Multivariate analysis (using CTI length, the presence of a pouch or recess, ER morphology, and significant tricuspid regurgitation as variables) showed that the presence of extensive ER was the only independent predictor of a difficult ablation procedure. The ablation time in patients with extensive ER (n = 13) was significantly longer than in those patients with nonextensive ER (n = 29) (1,638.4 ± 1,548.3 vs 413.8 ± 195.5 sec, P = .015). The incidence of difficulty in achieving bidirectional isthmus block was also higher in patients with extensive ER (10 of 13 vs four of 29, P < .001).

CONCLUSION

Preprocedural transthoracic echocardiography using a modified apical long-axis view is useful to characterize the morphology of the CTI and the ER. An extensive ER is a strong predictor for difficult ablation of CTI-dependent atrial flutter.

Mechanisms of termination and prevention of atrial fibrillation by drug therapy

Atrial fibrillation (AF) is a disorder of the rhythm of electrical activation of the cardiac atria. It is the most common cardiac arrhythmia, has multiple aetiologies, and increases the risk of death from stroke. Pharmacological therapy is the mainstay of treatment for AF, but currently available anti-arrhythmic drugs have limited efficacy and safety. An improved understanding of how anti-arrhythmic drugs affect the electrophysiological mechanisms of AF initiation and maintenance, in the setting of the different cardiac diseases that predispose to AF, is therefore required. A variety of animal models of AF has been developed, to represent and control the pathophysiological causes and risk factors of AF, and to permit the measurement of detailed and invasive parameters relating to the associated electrophysiological mechanisms of AF. The purpose of this review is to examine, consolidate and compare available relevant data on in-vivo electrophysiological mechanisms of AF suppression by currently approved and investigational anti-arrhythmic drugs in such models. These include the Vaughan Williams class I-IV drugs, namely Na(+) channel blockers, β-adrenoceptor antagonists, action potential prolonging drugs, and Ca(2+) channel blockers; the "upstream therapies", e.g., angiotensin converting enzyme inhibitors, statins and fish oils; and a variety of investigational drugs such as "atrial-selective" multiple ion channel blockers, gap junction-enhancers, and intracellular Ca(2+)-handling modulators. It is hoped that this will help to clarify the main electrophysiological mechanisms of action of different and related drug types in different disease settings, and the likely clinical significance and potential future exploitation of such mechanisms.