This animation shows a computer model of a pacemaking SA node embedded in surrounding atrial tissue. Individual SA node cells are based on the model of Cai, Winslow, and Noble (IEEE Trans. BME. 41(3):217-231). Atrial cells are modeled as described in Winslow et al. (Proc. Roy. Soc. Lond. B, 254: 55-61). Cell-to-cell coupling in the SA node is 10 nS, and is 50

nS in the surrounding atrium. Coupling at the border between SA node and atrium is 50 nS. At this level of coupling, the pacemaking node is able to generate a propagating wave of excitation in the atrium.

Model of Mammalian SA Node and Atrium: Pacemaker Conduction Failure

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In this animation, cell-to-cell coupling within the atrium and at the SA node - atrial border is increased from 50 to 100 nS. This coupling level is so high that electrotonic loading of peripheral SA node cells by surrounding atrial cells prevents them from oscillating, and the pacemaker potential fails to conduct onto the atrium. Electrotonic loading occurs since atrial cells have a resting potential (roughly -90 mV) which is considerably more hyperpolarized than the maximum diastolic potential of peripheral SA node cells (-70 mV).

Role of Interdigitation

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In this animation, cell-to-cell coupling both within the atrium and at the SA

node - atrial border is held constant at the value used in the previous study.

However, the insertion of fine strands of atrial tissue into the SA node, as

described by ten Velde et al (Circ. Res. 76: In Press) is included in the

model. Atrial cells at the tips of these insertions are now activated by the

surrounding pacemaking SA node cells. The wave of excitation exits from

the SA nodes by propagating along the atrial insertions, to excite the

surrounding atrial tissue. Interdigitation of SA node and atrial tissue therefore

helps prevent atrial loading of the SA node.

Spiral Reentrant Waves: Oxsoft Heart V4.5 Atrial Network Model

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Spiral reentrant waves are believed to be a possible model of tachycardia. In this animation, a spiral reentrant wave is induced in a 256x256 lattice of atrial cells modeled as described for Oxsoft Heart V4.5 (D. Noble). The reentrant wave was initiated using the so-called S1-S2 protocol. In this protocol, a plane wave is initiated by current injection along one edge of the lattice. When this plane wave has propagated across half the lattice, a second pulse is applied to cells along the orthogonal edge. The spiral reentrant wave induced using this procedure has a rotational period of about 100 msec. This period is in reasonable agreement with that observed in animal models.

Ectopic Beating

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This animation illustrates a form of cardiac arrhythmia known as ectopic

beating. Ectopic beating can arise in cardiac cells when internal sodium (Na)

levels become elevated, as can occur during ischemia. In this simulation, a

region of atrial tissue in the upper right corner is made "ischemic" by

increasing internal Na concentration in cells within this region. These cells

begin to oscillate spontaneously, generating an excitatory wave which

propagates across the lattice. Frequency of the ectopic pacemaker site is

roughly 2 Hz. An elliptical SA node is shown in the lower left corner. The

node oscillates at a frequency of roughly 3 Hz. In this simulation, the more

rapidly oscillating node entrains the more slowly oscillating ectopic focus.