. ___ .-_---_----Objective: Unidirectional conduction block in the heart can occur at a site where the impulse is transmitted from a small to a large tissue volume. The aim of this study was to evaluate the occurrence of conduction block in a 2-dimensional and 3-dimensional computer model of cardiac tissue consisting of a narrow strand abruptly emerging into a large area. In this structure, the strand diameter critical for the occurrence of block, h,, was evaluated as a function of changes in the active and passive electrical properties of both the strand and the large medium. M&hods: The effects of changes in the following parameters on h, were analysed: (I 1 maximum sodium conductance (gNa,,, 1, (21 longitudinal (R,) and transverse (R,) intracellular resistivities, and (3) inhomogeneities in gNa,,,, and R, and R," between the strand and the large area. Three ionic models for cardiac excitation described by Beeler-Reuter, Ebihara-Johnson, andLuo-Rudy ionic current kinetics were compared. Results: In the 2-dimensional simulations, h, was 175 pm in Ebihara-Johnson and Beeler-Reuter models and 200 pm in the Luo-Rudy model. At the critical strand diameter, the site of conduction block was located beyond the transition, i.e. a small circular area was activated in the large medium, whereas with narrower strands conduction block occurred within the strands. The decrease of gNa,,, resulted in a large increase of h,. This increase was mainly due to the change of d%7l,x in the large area, while h, was almost independent of giva,,, in the strand. Changing R, had no effect on h,, whereas the increase of R, decreased h, and reversed conduction block. Inhomogeneous changes of R, and R, in the strand versus the large medium had opposite effects on h,. When the resistivities of the strand alone were increased, h, also increased. In contrast, the increase of the resistivities in the large area reduced h,. In the 3-dimensional model, h, was 2.7 times larger than the corresponding Sdimensional values at the various levels of gNa,,, and resistivity. Conclusions: (1) At physiological values for active and passive electrical properties, h, in the 2D simulations is close to 200 pm in all three ionic models. In the 3-dimensional simulations, I?, is 2.7 larger than in the 2-dimensional models. (2) The excitable properties of the large area but not of the strand modify h,.. The decrease of intercellular coupling in the large medium facilitates impulse conduction and reduces h,., while the same change in the strand increases h,.. (3) Occurrence of conduction block at an abrupt geometrical transition can be explained by both the impedance mismatch at the transition site and the critical curvature beyond the transition.