The source model implied in ECGSIM is the Equivalent Double Layer (EDL) type of surface source model (see references section). Accordingly, the potential V(y, t) at position y on the body surface at time t is given by
V(y,t) = int A(x,y) S(x,t)
where 'int' denotes the integral over the surface of the ventricular (or atrial) myocardium, S(x,t) the transmembrane potential at position x at the surface of the ventricular myocardium at time t, and A(x,y) the transfer function.
The transfer function quantifies the (passive) electric volume conductor properties of the thorax. Its values follows from the laws of electric current flow. The transfer function used in ECGSIM has been computed for a heart-thorax model, including the inhomogeneities caused by the higher conductivity of blood in the cavities and the lower conductivity of the lungs. The relevant geometry is derived from the MRI data of the subject whose reference ECG is involved.
The transfer function A(x,y) can be displayed on both the heart (x) and the thorax (y), in µV/cm²: