Monodomain model

The monodomain model is a reduction of the bidomain model of the electrical propagation in myocardial tissue. The reduction comes from assuming that the intra- and extracellular domains have equal anisotropy ratios. Although not as physiologically accurate as the bidomain model, it is still adequate in some cases, and has reduced complexity.

Formulation

The monodomain model can be formulated as follows^[1]

${\displaystyle {\frac {\lambda }{1+\lambda }}\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v\right)=\chi \left(C_{m}{\frac {\partial v}{\partial t}}+I_{\text{ion}}\right),}$

where ${\displaystyle \mathbf {\Sigma } _{i}}$ is the intracellular conductivity tensor, $v$ is the transmembrane potential, ${\displaystyle I_{\text{ion}}}$ is the transmembrane ionic current per unit area, $C_m$ is the membrane conductivity per unit area, $\lambda$ is the intra- to extracellular conductivity ratio, and $\chi$ is the membrane surface area per unit volume (of tissue).

Derivation

The bidomain model can be written as

${\displaystyle {\begin{aligned}\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v\right)+\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v_{e}\right)&=\chi \left(C_{m}{\frac {\partial v}{\partial t}}+I_{\text{ion}}\right)\\\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v\right)+\nabla \cdot \left(\left(\mathbf {\Sigma } _{i}+\mathbf {\Sigma } _{e}\right)\nabla v_{e}\right)&=0\end{aligned}}}$

Assuming equal anisotropy ratios, i.e. ${\displaystyle \mathbf {\Sigma } _{e}=\lambda \mathbf {\Sigma } _{i}}$, the second equation can be written

${\displaystyle \nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v_{e}\right)=-{\frac {1}{1+\lambda }}\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v\right).}$

Inserting this into the first bidomain equation gives

${\displaystyle {\frac {\lambda }{1+\lambda }}\nabla \cdot \left(\mathbf {\Sigma } _{i}\nabla v\right)=\chi \left(C_{m}{\frac {\partial v}{\partial t}}+I_{\text{ion}}\right).}$

References

1. Keener J, Sneyd J (2009). Mathematical Physiology II: Systems Physiology (2nd ed.). Springer. ISBN 978-0-387-79387-0.