Fig. (2) A: Two-dimensional theoretical model. The active electrode tip is inserted into cardiac tissue to depth P=1.25 mm. Dimensions R, Z and L are obtained by means of convergence tests (see text). T₄ shows the temperature estimation point (4 mm deep). B: biological tissue is modeled as a dynamic system with an input (applied voltage) U=V²and output tissue temperature (at depth of 4 mm), which corresponds with temperature at T₄ (see Fig. 1A), thus obtaining a first order transfer function G(s).