THE STOCHASTIC NATURE OF CARDIAC PROPAGATION AT A MICROSCOPIC LEVEL - ELECTRICAL DESCRIPTION OF MYOCARDIAL ARCHITECTURE AND ITS APPLICATION TO CONDUCTION

The object of this study is to present evidence that the myocardial architecture creates inhomogeneities of electrical load at the cellular level that cause cardiac propagation to be stochastic in nature; ie, the excitatory events during propagation are constantly changing and disorderly in the sense of varying intracellular events and delays between cells. At a macroscopic level, however, these stochastic events become averaged and appear consistent with a continuous medium. We examined this concept in a two-dimensional (2D) model of myocardial architecture by exploring whether experimentally observed V-max variability reflected different patterns of intracellular excitation events and junctional delays. The patterns of V-max variability at randomly chosen intracellular sites were similar experimentally and in the 2D model. The 2D cellular model produced marked variability in gap junction delays; however, on the average, different gap junctions were used for cell-to-cell charge flow during conduction in different directions. During longitudinal propagation (LP), the velocity increased from the proximal to the distal end of each myocyte, and V-max was lowest proximally, increased to a maximum at the distal fourth of the cell, and decreased distally. Transverse propagation (TP) produced rapid intracellular conduction with variable intracellular excitation sequences. TP V-max was greater than LP V-max in most subcellular regions, but near the ends of some myocytes, a reversed ''TP>LP V-max'' relation occurred. Total charge carried by the sodium current varied inversely with V-max, demonstrating feedback effects of cellular loading on the subcellular sodium current and the kinetics of the sodium channels. The results suggest that the stochastic nature of normal propagation at a microscopic level provides a considerable protective effect against arrhythmias by reestablishing the general trend of wave-front movement after small variations in excitation events occur.