2009, pp 283-330

Virtual Electrode Theory of Pacing

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One of the most important contributions of biomedical engineering to medicine is the devel¬opment of pacemakers, external defibrillators, and implantable cardioverters/defibrillators.1 Engineers have been quite successful in designing these devices empirically, without a funda¬mental understanding of the underlying biophysical mechanisms. Over the past 15 years, two areas of research — optical mapping of electrical activity in the heart2 and mathematical modeling of the heart using the bidomain model3 — have provided insight into the basic mechanisms by which cardiac electric fields are produced and how externally applied electric fields interact with cardiac tissue. The goal of this chapter is to describe this research and to summarize what has been learned from it. We survey the contributions of many researchers, but the emphasis is on our own work, which, of course, we know best. We focus on basic mechanisms; clinical applications are better described by other authors.4 The fundamental knowledge gained from basic research in cardiac shock response is enabling the development of detailed mathematical models5,6 that can guide the further optimization of implantable cardiac stimulators.

The electrical properties of the heart have been reviewed elsewhere. In 1993 Henriquez3 summarized the bidomain model in a seminal paper that serves as an excellent foundation for the discussions in our chapter. Neu and Krassowska7 examined the limitations of the bidomain as a continuum model. Roth8 described mechanisms of electrical stimulation of excitable tissue, including cardiac tissue. In the past 10 years much work has been published in this field, particularly on comparing numerical simulations to experimental data. The agreement between theory and experiment is an important topic9 and is the focus of this review.