THE ARCHITECTURE OF THE ATRIAL MUSCULATURE BETWEEN THE ORIFICE OF THEINFERIOR CAVAL VEIN AND THE TRICUSPID-VALVE - THE ANATOMY OF THE ISTHMUS

Introduction: Electrophysiologists recognize a so-called isthmus'' in the right atrium through which passes the reentrant circuit of common atrial nutter. Ablative lesions placed in this narrow channel have pro ved effective in breaking the circuit, To the best of our knowledge, h owever, no study has been performed to establish the arrangement and o rientation of the atrial myocardial fibers in this crucial area. Metho ds and Results: We examined 28 normal heart specimens, identifying a q uadrilateral area composed of three morphologic sectors between the in ferior caval vein and the tricuspid valve confluent superiorly with th e triangle of Koch, Within this quadrilateral, there are constant rece sses, or sinuses, inferior and lateral to the orifice of the coronary sinus. The inferior isthmus measured an average of 31 +/- 4 mm (range 19 to 40), Gross examination identified marked differences in the atri al wall forming the quadrilateral, A smooth anterior component forming the vestibule of the tricuspid valve was found in all the hearts, but variations in the remaining sectors were seen in ten specimens, The u sually membranous posterior sector was noticeably muscular in three sp ecimens, while the middle, trabecular sector was more membranous in fi ve specimens. We demonstrated the orientation of the subendocardial at rial fibers by dissection in 14 specimens, revealing a relatively cons tant overall pattern in eight specimens and variations in fiber orient ation in the remaining specimens. Conclusion: There are considerable a natomic variations in the atrial wall that comprises the so-called ist hmus, The presence of recesses and membranous areas in some hearts and the variations in arrangement of the subendocardial fibers are releva nt in improving understanding of conduction in this area.