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The Inferior Right Atrial Isthmus: Further Architectural Insights for Current and Coming Ablation Technologies


  • Funding received from Grant SAF2004-06864 (J.A.C. and D.S.Q.) from Ministerio de Educación y Ciencia, Spain, Fondo de Investigaciones dela Seguridad Social (Redes Temáticas de Cooperación; Red Cardiovascular C01/03) (J.A.F. and J.F.), and Royal Brompton and Harefield Hospital Charitable Fund (S.Y.H.).

  • Manuscript received 13 October 2004; Accepted for publication 27 October 2004.

Siew Yen Ho, Ph.D., F.R.C.Path, F.E.S.C., Paediatrics, National Heart and Lung Institute, Imperial College, London SW3 6LY, UK. Fax: ++44 20 7351 8230; E-mail:


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.