A MODEL STUDY OF EXTRACELLULAR STIMULATION OF CARDIAC-CELLS

Point source extracellular stimulation of a myocyte model was used to study the efficacy of excitation of cardiac cells, taking into account the shape of the pulse stimulus and its time of application in the ca rdiac cycle. The myocyte was modeled as a small cylinder of membrane ( 10 mum in diameter and 100 mum in length) capped at both ends and plac ed in an unbounded volume conductor. A Beeler-Reuter model modified fo r the Na+ dynamics served to simulate the membrane ionic current. The stimulus source was located on the cylinder axis, close to the myocyte (50 mum) in order to generate a nonlinear extracellular field (phi(e) ). The low membrane impedance associated with the high frequency compo nent of the make and break of the rectangular current pulse leads to a current flow across the membrane and an abrupt change in intracellula r potential (phi(i)). Because the intracellular space is very small, p hi(e) is nearly uniform over the length of the myocyte and the membran e potential (V = phi(i) - phi(e)) is governed by the applied field phi (e). There is then a longitudinal gradient of membrane polarization wh ich is the inverse of the gradient of extracellular potential. With an anodal (positive) pulse, for instance, the proximal portion of the my ocyte is hyperpolarized and the distal portion is depolarized. Based o n this principle and considering the voltage-dependent activation/inac tivation dynamics of the membrane, it is shown that a cathodal (negati ve) pulse is the most efficacious stimulus at diastolic potentials, an anodal current is preferable during the plateau phase of the action p otential, and a biphasic pulse is optimal during the relative refracto ry phase. Thus a biphasic pulse would constitute the best choice for m aximum efficacy at all phases of the action potential.