Anisotropic structural complexities in the genesis of reentrant arrhythmias.

T he paper by Brugada et a1l in this issue of Circulation adds importantly to mounting evidence that the anisotropic passive electrical properties of cardiac muscle play an important role in atrial2-4 and ventricular5-6 reentrant arrhythmias. The anisotropic properties involved are determined at a microscopic size scale (<200 jam) by the alignment of cardiac myocytes, their complex shapes, the distribution of their interconnections (gap junctions), and the geometry of interstitial (extracellular) space. These electrical properties are different in the atria and ventricles of normal adult hearts. For descriptive purposes, the different types of anisotropic properties have been characterized simply as "uniform" and "nonuniform" anisotropy.7 Normal ventricular muscle generally has uniform anisotropic properties due to the presence of frequent side-toside electrical connections between cells and groups of cells8,9; such bundles produce low amplitude smooth unipolar extracellular waveforms when propagation occurs across cells (transverse propagation).7'8 In many normal adult atrial bundles, however, the side-to-side connections are sparse, pro-