In normal atria and ventricles, electrical propagation occurs at velocities of 20-50 cm /sec. In the AV-node propagation velocity may be as low as 1 cm/sec. Computer simulations and experiments have shown that inhibition of membrane ion currents in presence of normal cell-to-cell coupling cannot produce conduction block at velocities>10cm/sec. Two mechanisms can lead to very slow conduction: High degree of cell-to-cell uncoupling or a highly discontinuous tissue structure. Both share closely common conduction characteristics albeit occurring at a entirely different scale. Especially, the safety factor for propagation is increased in both states. The implications of these findings for arrhythmogenesis are discussed.