The Inferior Right Atrial Isthmus: Further Architectural Insight
The Inferior Right Atrial Isthmus: Further Architectural Insight
Background: Although linear ablation of the right atrial isthmus in patients with isthmus-dependent atrial flutter can be highly successful, recurrences and complications occur in some patients. Our study provides further morphological details for a better understanding of the structure of the isthmus.
Methods and Results: We examined the isthmic area in 30 heart specimens by dissection, histology, and scanning electron microscopy. This area was bordered anteriorly by the hinge of the tricuspid valve and posteriorly by the orifice of the inferior caval vein. With the heart in attitudinal orientation, we identified and measured the lengths of three levels of isthmus: paraseptal (24 ± 4 mm), central (19 ± 4 mm), and inferolateral (30 ± 3 mm). Comparing the three levels, the central isthmus had the thinnest muscular wall and the paraseptal isthmus the thickest wall. At all three levels, the anterior part was consistently muscular whereas the posterior part was composed of mainly fibro-fatty tissue in 63% of hearts. The right coronary artery was less than 4 mm from the endocardial surface of the inferolateral isthmus in 47% of hearts. Inferior extensions of the atrioventricular node were present in the paraseptal isthmus in 10% of hearts, at 1—3 mm from the endocardial surface.
Conclusions: The thinner wall and shorter length of the central isthmus together with its distance from the right coronary artery, and nonassociation with the atrioventricular node or its arterial supply, should make it the preferred site for linear radiofrequency ablation.
The inferior right atrial cavo-tricuspid isthmus, a critical link for the macro-reentrant circuit of isthmus-dependent atrial flutter, is the target of catheter ablation techniques that have become the treatment of choice for this arrhythmia. Creation of a complete bidirectional conduction block across the isthmus is the accepted electrophysiological ablation endpoint for long-term success but its achievement may be difficult or temporary in some patients. In addition, experimental animal studies have demonstrated that a transmural lesion of the atrial wall is required to achieve ablation success. The ease of obtaining a complete, transmural, and permanent isthmus ablation line depends on architectural factors of the inferior isthmus such as its size, endocardial geometry, and content of myocardial and fibro-fatty tissues at the ablation zone. Ablation catheters with irrigated or large-tip electrodes, producing deeper lesions than conventional 4-mm tip electrodes, are now standard for the ablation of isthmus-dependent atrial flutter. Reported rates of atrial flutter recurrences with these new ablation techniques are variable, but in some centers they may be as high as 10%. Cryoablation of atrial flutter, although less painful than radiofrequency catheter techniques, is followed by an 11% recurrence rate of the arrhythmia after an apparent acute bidirectional isthmic block.
Apart from success and recurrence rate, additional issues for isthmic catheter ablation in patients with atrial flutter are the production of pain, and the possibility of damaging elements of the normal AV nodal conduction pathway, or the neighboring vasculature, and the regional innervation. The gross morphological features of the inferior isthmus have been described in postmortem specimens recognizing the existence of sectors with a variable content of myocardium and fibro-fatty tissue. However, architectural details on the relations among the possible target areas for ablation with the underlying myocardial thickness, coronary vessels, nerve endings, and elements of the atrioventricular node, remain to be examined. The present morphological and histological study was designed to provide these details for a better understanding of the target ablation areas in isthmus-dependent atrial flutter particularly in connection with current and emerging ablation techniques aimed at producing larger and deeper lesions in this region.
Background: Although linear ablation of the right atrial isthmus in patients with isthmus-dependent atrial flutter can be highly successful, recurrences and complications occur in some patients. Our study provides further morphological details for a better understanding of the structure of the isthmus.
Methods and Results: We examined the isthmic area in 30 heart specimens by dissection, histology, and scanning electron microscopy. This area was bordered anteriorly by the hinge of the tricuspid valve and posteriorly by the orifice of the inferior caval vein. With the heart in attitudinal orientation, we identified and measured the lengths of three levels of isthmus: paraseptal (24 ± 4 mm), central (19 ± 4 mm), and inferolateral (30 ± 3 mm). Comparing the three levels, the central isthmus had the thinnest muscular wall and the paraseptal isthmus the thickest wall. At all three levels, the anterior part was consistently muscular whereas the posterior part was composed of mainly fibro-fatty tissue in 63% of hearts. The right coronary artery was less than 4 mm from the endocardial surface of the inferolateral isthmus in 47% of hearts. Inferior extensions of the atrioventricular node were present in the paraseptal isthmus in 10% of hearts, at 1—3 mm from the endocardial surface.
Conclusions: The thinner wall and shorter length of the central isthmus together with its distance from the right coronary artery, and nonassociation with the atrioventricular node or its arterial supply, should make it the preferred site for linear radiofrequency ablation.
The inferior right atrial cavo-tricuspid isthmus, a critical link for the macro-reentrant circuit of isthmus-dependent atrial flutter, is the target of catheter ablation techniques that have become the treatment of choice for this arrhythmia. Creation of a complete bidirectional conduction block across the isthmus is the accepted electrophysiological ablation endpoint for long-term success but its achievement may be difficult or temporary in some patients. In addition, experimental animal studies have demonstrated that a transmural lesion of the atrial wall is required to achieve ablation success. The ease of obtaining a complete, transmural, and permanent isthmus ablation line depends on architectural factors of the inferior isthmus such as its size, endocardial geometry, and content of myocardial and fibro-fatty tissues at the ablation zone. Ablation catheters with irrigated or large-tip electrodes, producing deeper lesions than conventional 4-mm tip electrodes, are now standard for the ablation of isthmus-dependent atrial flutter. Reported rates of atrial flutter recurrences with these new ablation techniques are variable, but in some centers they may be as high as 10%. Cryoablation of atrial flutter, although less painful than radiofrequency catheter techniques, is followed by an 11% recurrence rate of the arrhythmia after an apparent acute bidirectional isthmic block.
Apart from success and recurrence rate, additional issues for isthmic catheter ablation in patients with atrial flutter are the production of pain, and the possibility of damaging elements of the normal AV nodal conduction pathway, or the neighboring vasculature, and the regional innervation. The gross morphological features of the inferior isthmus have been described in postmortem specimens recognizing the existence of sectors with a variable content of myocardium and fibro-fatty tissue. However, architectural details on the relations among the possible target areas for ablation with the underlying myocardial thickness, coronary vessels, nerve endings, and elements of the atrioventricular node, remain to be examined. The present morphological and histological study was designed to provide these details for a better understanding of the target ablation areas in isthmus-dependent atrial flutter particularly in connection with current and emerging ablation techniques aimed at producing larger and deeper lesions in this region.
Source...