Bidomain model

The bidomain model is a mathematical model to define the electrical activity of the heart. It consists in a continuum (volume-avarage) approach in which the cardiac mictrostructure is defined in terms of muscle fibers grouped in sheets, creating a complex three-dimensional structure with anisotropical properties. Then, to define the electrical activity, two interpenetrating domains are considered, which are the intracellular and extracellular domains, representing respectively the space inside the cells and the region between them.
The bidomain model was first proposed by Schmitt in 1969 before being formulated mathematically in the late 1970s.
Since it is a continuum model, rather than describing each cell individually, it represents the avarage properties and behaviour of group of cells organized in complex structure. Thus, the model results to be a complex one and can be seen as a generalization of the cable theory to higher dimensions and, going to define the so called bidomain equations.
Many of the interesting properties of the bidomain model arise from the condition of unequal anisotropy ratios. The electrical conductivity in anisotropic tissues is not unique in all directions, but it is different in parallel and perpedicular direction with respect to the fiber one.
Moreover, in tissues with unequal anisotropy ratios, the ratio of conductivities parallel and perpendicular to the fibers are different in the intracellular and extracellular spaces. For instance, in cardiac tissue, the anisotropy ratio in the intracellular space is about 10:1, while in the extracellular space it is about 5:2.
Mathematically, unequal anisotropy ratios means that the effect of anisotropy cannot be removed by a change in the distance scale in one direction.
Instead, the anisotropy has a more profound influence on the electrical behavior.
Three examples of the impact of unequal anisotropy ratios are
Formulation.
Bidomain domain.
The bidomain domain is principally represented by two main regions: the cardiac cells, called intracellular domain, and the space surrounding them, called extracellular domain. Moreover, usually another region is considered, called extramiocardial region. 
The intracellular and extracellular domains, which are separate by the cellular membrane, are considered to be a unique physical space representing the heart (formula_1), while the extramyocardial domain is a unique physical space adjacent of them (formula_2). The extramyocardial region can be considered as a fluid bath, especially when one wants to simulate experimental conditions, or as a human torso to simulate physiological conditions.
The boundary of the two principal physical domains defined are important to solve the bidomain model. Here the heart boundary is denoted as formula_3 while the torso domain boundary is formula_4
Unknowns and parameters.
The unknowns in the bidomain model are three, the intracellular potential formula_5, the extracellular potential formula_6 and the transmembrane potential formula_7, which is defined as the difference of the potential across the cell membrane formula_8.
Moreover, some important parameters need to be taken in account, especially the intracellular conductivity tensor matrix formula_9, the extracellular conductivity tensor matrix formula_10. The transmembrane current flows between the intracellular and extracellular regions and it is in part described by the corresponding ionic current over the membrane per unit area formula_11. 
Moreover, the membrane capacitance per unit area formula_12 and the surface to volume ratio of the cell membrane formula_13 need to be considered to derive the bidomain model formulation, which is done in the following section.
Standard formulation.
The bidomain model is defined through two partial differential equations (PDE) the first of which is a reaction diffusion equation in terms of the transmembrane potential, while the second one computes the extracellular potential starting from a given transmembran potential distribution.
Thus, the bidomain model can be formulated as follows:
where formula_15 and formula_16 can be defined as applied external stimulus currents.
Ionic current equation.
The ionic current is usually represented by an ionic model through a system of ordinary differential equations (ODEs). Mathematically, one can write formula_17 where formula_18 is called ionic variable. Then, in general, for all formula_19, the system reads
Different ionic models have been proposed:
Model of an extramyocardial region.
In some cases, an extramyocardial region is considered. This implies the addition to the bidomain model of an equation describing the potential propagation inside the extramyocardial domain.
Usually, this equation is a simple generalized Laplace equation of type
where formula_22 is the potential in the extramyocardial region and formula_23 is the corresponding conductivity tensor.
Moreover, an isolated domain assuption is considered, which means that the following boundary conditions are added
formula_25 being the unit normal directed outside of the extramyocardial domain.
If the extramyocardial region is the human torso, this model gives rise to the forward problem of electrocardiology.
Derivation.
The bidomain equations are derived from the Maxwell's equations of the electromagnetism, considering some simplifications.
The first assumption is that the intracellular current can flow only between the intracellular and extracellular regions, while the intracellular and extramyocardial regions can comunicate between them, so that the current can flow into and from the extramyocardial regions but only in the extracellular space.
Using Ohm's law and a quasi-static assumption, the gradient of a scalar potential field formula_26 can describe an electrical field formula_27, which means that
Then, if formula_29 represent the current density of the electric field formula_27, two equations can be obtained
where the subscript formula_32 and formula_33 represent the intracellular and extracellular quantities respectively.
The second assumption is that the heart is isolated, so that the current that leaves one regione need to flow into the other. Then, the current density in each of the intracellular and extracellular domain must be equal in magnitude but opposite in signe, and can be defined as the product of the surface to volume ratio of the cell membrane and the transmembrane ionic current density formula_34 per unit area, which means that
By combining the previous assumptions, the conservation of current densities is obtained, namely
from which, summing the two equations
This equation states exactly that all currents exiting one domain must enter the other.
From here, it is easy to find the second equation of the bidomain model subtracting formula_37 from both sides. In fact,
and knowing that the transmembral potential is defined as formula_39
Then, knowing the transmembral potential, one can recover the extracellular potential.
Then, the current that flows across the cell membrane can be modelled with the cable equation,
Combining equations () and () gives
Finally, adding and subtracting formula_37 on the left and rearranging formula_39, one can get the first equation of the bidomain model
which describes the evolution of the transmembral potential in time.
The final formulation described in the standard formulation section is obtained through a generalization, considering possible external stimulus which can be given through the external applied currents formula_15 and formula_16.
Boundary conditions.
In order to solve the model, boundary conditions are needed. The more classical boundary conditions are the following ones, formulated by Tung.
First of all, as state before in the derive section, there ca not been any flow of current between the intracellular and extramyocardial domains. This can be mathematically described as
where formula_48 is the vector that represents the outwardly unit normal to the myocardial surface of the heart.
Since the intracellular potential is not explicitily presented in the bidomain formulation, this condition is usually described in terms of the transmembrane and extracellular potential, knowing that formula_49, namely
For the extracellular potential, if the myocardial region is presented, a balance in the flow between the extracellular and the extramyocardial regions is considered
Here the normal vectors from the perspective of both domains are considered, thus the negative sign are necessary. Moreover, a perfect transmission of the potential on the cardiac boundary is necessary, which gives
Instead, if the heart is considered as isoleted, which means that no myocardial region is presented, a possible boundary condition for the extracellular problem is
Reduction to monodomain model.
By assuming equal anisotropy ratios for the intra- and extracellular domains, i.e. formula_54 for some scalar formula_55, the model can be reduced to one single equation, called monodomain equation
where the only variable is now the transmembrane potential, and the conductivity tensor formula_57 is a combination of formula_9 and formula_59
Formulation with boundary conditions in an isolated domain.
If the heart is considered as an isolated tissue, which means that no current can flow outside of it, the final formulation with boundary conditions reads 
Numerical solution.
There are various possible techniques to solve the bidomain equations. Between them, one can find finite difference schemes, finite element schemes and also finite volume schemes. Special considerations can be made for the numerical solution of these equations, due to the high time and space resolution needed for numerical convergence.