Myocardial architecture and ventricular arrhythmogenesis.

Abstract

Conduction of the cardiac impulse is dependent on both the active membrane properties of cardiac cells (generating the action potential) and the passive properties determined by architectural features of the myocardium. The influence of tissue architecture on conduction is determined principally by the size, shape, and packing of individual myocytes and by the quantity, three-dimensional distribution, and physiological behavior of the specialized intercellular junctions responsible for impulse propagation from cell to cell, the gap junctions.1 It has long been recognized that abnormalities in conduction of the cardiac impulse are an important cause of arrhythmias2 in that they alter the relationships between conduction velocity, path length, and recovery of excitability (the determinants of reentrant excitation). However, it was not until more recently that myocardial architecture was considered important in determining patterns of activation and conduction velocity3 4 and was therefore thought to have a central role in arrhythmogenesis,5 and some of the evidence for this forms the basis of this review. ### Myocardial Syncytium: Anisotropy of Structure and Function The early observation that conduction properties in myocardium were different in different directions,6 with more rapid conduction in the direction parallel to the myocardial fiber axis than in the transverse direction (the definition of the anisotropic conduction characteristic of heart muscle [Figure 1⇓]), is attributable principally to the lower resistivity of myocardium in the longitudinal than the transverse direction. Gap-junctional channels create continuity between the cytoplasmic compartments of abutting myocytes but act as resistive discontinuities to the cytoplasmic current flow between the intracellular compartments of the cells (the “intracellular” conduction pathway). Longitudinal resistivity is lower than transverse because this intracellular pathway encounters fewer cell boundaries per unit distance in the longitudinal than the transverse direction. In normal ventricular myocardium, conduction in the direction parallel to the long axis of the myocardial fiber bundles …