The basic mechanism underlying directional differences in excitability, con- duction velocity, andsafetyfactor thatleadtocircusmovementreentry incardiac muscle is generally attributed to a spatial difference in the refractory period as originally described by Mines [1] or to a depressed segment as described by Schmitt and Erlanger [2]. A departure from this depolarization in cardiac muscle involve quantities, such as Vmax' that are not directly descriptive of the underlying mechanisms of propagation.
