How anatomy can guide ablation in isthmic atrial flutter

Creation of a complete bidirectional conduction block across the inferior right atrial cavo-tricuspid isthmus is the accepted marker for long-term success in patients with isthmus-dependent atrial flutter. 1 , 2 However, it may be extremely difficult to achieve or its effect is temporary in some patients. 3–5 Histopathologic findings have demonstrated that transmural ablation of the atrial wall is a prerequisite for success. 6–8 Factors making it difficult to obtain a complete, transmural, and permanent ablation line across the inferior isthmus may be haemodynamic but also anatomical owing to the variable and complex endocardial geometry of the isthmic region, and the unpredictable content of atrial myocardium and fibro-fatty tissues at different locations of this atrial territory. The cavo-tricuspid isthmus is limited posteriorly by the Eustachian valve/ridge and anteriorly by the annular insertion of the septal leaflet of the triscuspid valve. Post-mortem and in vivo imaging examination in normal hearts and in patients with atrial flutter have pointed to the anatomic variability of the dimensions, endocardial geometry, and muscular architecture across the anatomic landmarks of the inferior right atrium and its impact as anatomic determinant for catheter ablation. 9–17 As displayed in attitudinally orientation, the isthmus shows on its endocardial surface an irregular quadrilateral shape. The superior border of the quadrilateral is the paraseptal isthmus (so-called septal isthmus) extending between the septal insertion of the Eustachian valve just above or posterior to the orifice of the coronary sinus, and the most inferior paraseptal insertion of the tricuspid valve. The inferolateral border of the quadrilateral area contained pectinate muscles as a continuation of the final ramifications of the terminal crest. The middle zone of the isthmus, between the paraseptal and the inferolateral levels, is the central isthmus. The central isthmus is the shortest distance between the orifices of the inferior caval vein and the tricuspid valve and traverses through a pouch-like recess, the subeustachian sinus. The inferior pouch-like recess is composed mainly by membranous tissue and muscular trabeculations of different thickness and depths. A posterior pouch-like recess deeper than 5 mm can be seen in 20% of patients and it usually causes the most difficulty in achieving a complete line of block during isthmus ablation. In addition, a prominent and muscular Eustachian ridge (such as found in 26% of our specimens) may require tricky angling of the catheter for good contact. 11 Interpretation of isthmus anatomy derived from simple fluoroscopic examination during atrial flutter ablation is limited to the catheter's position and cardiac shadow; therefore, electrophysiologist had to imagine the anatomic landmarks from such weak references. Right atrial angiography was the first imaging technique used to accurately define isthmus profile. 12 It is a widely available and inexpensive technique not only to pre-visualize the anatomic characteristic of the isthmus but also to show the precise position of the ablation catheter in relation with the isthmus surface during radiofrequency (RF) application. 12–15 In the right anterior oblique projection, atrial angiogram identified the contours of the endocardial surface with a constant smooth myocardial vestibule of the tricuspid valve anteriorly and a variable pouch-like area anterior to the Eustachian valve and inferior to the orifice of the coronary sinus. Angiography demonstrated larger dimensions of the right atrium and the inferior isthmus in patients with isthmus-dependent atrial flutter than in normal controls. 12 In addition, studies have reported that both longer isthmus >35 mm and morphological characteristics such as concave deformation of the isthmus and deep pouch-like recesses were associated with a significantly increased procedure duration and number of RF applications during atrial flutter ablation. 13–15 Interestingly, isthmus angiographic evaluation may predict the effectiveness of an RF catheter because externally irrigated-tip catheter tends to be more effective for ablation of concave isthmus morphology and 8 mm-tip catheter in cases of straight morphology. 15 Recent studies have shown that 3D electroanatomic mapping systems provide a good reconstruction of the inferior right atrial isthmus with a good correlation with angiography for the isthmus anatomy and dimensions. 18–20 Imaging evaluation of the isthmus characteristics has also been reported using multi-slice computed tomography (CT) and cardiac magnetic resonance. 21–23 These studies demonstrated its feasibility to visualize the morphological variants of isthmus morphology and its impact on catheter ablation. Kirchhof et al.24 performed a magnetic resonance imaging (MRI) study of isthmus anatomy to characterize anatomic details in ‘difficult’ ablation procedures providing further insight of non-invasive isthmus evaluation prior to flutter ablation. Consistent with previous angiographic studies, difficult procedure was associated with a longer diastolic isthmus length demonstrated by cardiac MRI. 24 The variable isthmus angulation was previously observed by right atrial angiography but the study of Kirchof et al.24 demonstrated for the first time that an angulation between inferior caval vein and right atrial floor close to 90º renders ablation difficult. In contrast to angiography and cardiac CT, MRI does not require exposure to ionizing radiation or potentially nephrotoxic contrast media. Histological examination of the isthmus musculature demonstrated that when comparing the three isthmic levels, the central isthmus had the thinnest muscular wall and the paraseptal isthmus the thickest wall. 11 At all three levels, the anterior part was consistently muscular, whereas the posterior part was composed of mainly fibro-fatty tissue. Therefore, there is a non-uniform muscular content and thicknesses in the isthmus of evident relevance to create transmural lesion of the atrial wall during atrial flutter ablation. In contrast to angiography, 3D mapping system and multi-slice CT, phased-array intracardiac echocardiography can be used to characterize the variable wall thickness and to monitor the wall lesions during RF applications. 25 The study of Kirchhof et al.24 also demonstrated the feasibility of MRI to visualize the myocardial tissue. This is in our opinion the more important contribution of this study. As shown in this study and in previous anatomic studies in some patients the atrial myocardial layer ended before the Eustachian ridge, resulting in a shorter length of the isthmus when measured as the length of atrial myocardium. 9–11 These observations suggest that in some patients the length of the isthmus that needs to be ablated, i.e. the functional isthmus, can be considerably narrower than the area bounded by the anatomic barriers of the tricuspid valve anteriorly and the inferior caval vein posteriorly. Clinical corroborative evidence has shown that in some patients, ablation does not require a contiguous line of RF lesions across the entire isthmus and CTI conduction can even be eliminated by the application of RF energy at a single site. 13 , 26 Of the three levels of isthmus, the thinner wall and shorter length of the central isthmus should make it the preferred isthmus site (6 O'clock position in the LAO projection) for constructing a complete linear line with RF ablation. In some cases with a large and central pouch-like recess, a more inferolateral line should be the alternative to avoid difficulties in tissue contact and chest pain since ganglia and fibres of autonomic nervous system can be seen at the epicardial aspect of this region. 11 In contrast, the paraseptal isthmus (4–5 O'clock position in the LAO projection) has the thickest wall, is close to the artery to the atrioventricular node, and, in some cases, can contain the inferior extensions of the node. 11 The cavo-tricuspid isthmus is much more than a simple muscular band in the inferior right atrium. An ideal imaging evaluation should provide anatomic information of both morphological and architectural determinants for atrial flutter ablation at the time of the ablation procedure itself. 27 , 28 The study of Kirchhof et al.24 open the perspective to use real preprocedural anatomic information to guide isthmus ablation and evaluate atrial myocardium in patients with atrial flutter. Conflict of interest: none declared.