Transition from a continuous to discontinuous understanding of cardiac conduction.

Abstract

This personal reflection that celebrates the 50th anniversary of Circulation Research focuses on an area of integrative electrophysiology—the extension of continuous medium theory to the discontinuous nature of cardiac propagation produced by the cellular interconnections (gap junctions). It is appropriate to note that as a clinician who shifted to the laboratory, my collaboration with Drs Roger Barr, J. Francis Heidlage, and Paul Dolber has provided me long-term training and checks on ideas that involve conduction theory, computer modeling, and cardiac architecture. That process has been particularly influenced by advances in this field that involve the measurement and mathematical modeling of ionic channel currents, as well as the immunohistological targeting of cardiac gap junction connexins. Although journal references are not used here, I have felt compelled to mention a few scientists who advanced selected ideas about the spread of currents in cardiac muscle. It is noteworthy that the widespread use of optical mapping to measure spiral waves that represent the behavior of cardiac wavefronts at a macroscopic size scale illustrates the contemporary importance of continuous medium theory. Interestingly, van Cappele first described complex spiral wave activity for cardiac muscle in 1980 by generating a computer model of a continuous isotropic sheet. Nine years passed before this phenomenon was experimentally confirmed in cardiac muscle. With spiral waves, the nonuniformities necessary to initiate the conduction disturbances that lead to reentry are induced experimentally as repolarization inhomogeneities (primarily by introducing stimuli at two different sites). Spiral waves are mentioned at the outset because the concept of spiral waves at a macroscopic level and the concept of discontinuous conduction at a microscopic level have both gained acceptance during the past two decades as paradigms for mechanisms of cardiac arrhythmias. Thus, it may be of interest to the reader to consider how experiments done in the …