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V(y,t) = int A(x,y) S(x,t),
where 'int' denotes the integral of the surface of the ventricular 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 relates to the electric volume conductor properties of the thorax only. Its values follows from the laws of 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, you first have to select a node on the thorax. Then you can choose Transfer function from the surface function dialog from the Heart menu. 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) 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. Then you may choose Transfer function from the surface function dialog from the thorax menu. 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 selected node on the heart surface contributes to the ECG on the thorax.