Anisotropic conduction in the triangle of Koch of mammalian hearts: electrophysiologic and anatomic correlations.

OBJECTIVES The purpose of this study was to characterize anisotropy in the triangle of Koch by relating electrophysiology with anatomy. BACKGROUND Atrioventricular (AV) node fast and slow pathway characteristics have been suggested to be due to nonuniform anisotropy in the triangle of Koch. METHODS During atrial pacing, we determined the electrical activity within the triangle of Koch by multichannel mapping in 11 isolated hearts from pigs and dogs. Orientation of fibers was determined in nine hearts. RESULTS Fibers were parallel to the tricuspid valve annulus (TVA) in the posterior part of the triangle of Koch. In the midjunctional area, the direction of the fibers changed to an orientation perpendicular to the TVA. During stimulation from posterior and anterior sites, activation proceeded parallel to the TVA at a high conduction velocity (0.5 to 0.6 m/s). During stimulation from sites near the coronary sinus, a narrow zone of slow conduction occurred in the posterior part of the triangle of Koch where activation proceeded perpendicular to the fiber orientation. Above and below this zone, conduction was fast and parallel to the annulus. After premature stimulation, conduction delay in the triangle of Koch increased by 4 to 21 ms; in contrast, the AH interval increased by 80 to 210 ms. CONCLUSIONS Data support the concept of anisotropic conduction in the triangle of Koch. Activation maps correlated well with the arrangement of superficial atrial fibers. Comparison of conduction delay in the triangle of Koch and AH delay after premature stimulation disproves that anisotropy in the superficial layers plays an important role in slow AV conduction.