## Transfer function

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:

• If you want to display the transfer function on the heart surface (atrial or ventricular), you first have to select a node on the thorax. The function displayed (a row of the transfer matrix) is the transfer from all points on the heart surface to the selected node on the thorax. This function can be interpreted as the amount by which the (local) source elements of the heart surface contribute to the ECG at the node selected on the thorax.

• If you want to display the transfer function on the thorax, you first have to select a node on the heart surface. The function displayed (a column of the transfer matrix) is the transfer to all points on the thorax from the node selected on the heart surface. It can be interpreted as the amount by which the (single) EDL source element at the selected node on the heart surface contributes to the potential on the body surface.