Sodium channel blockers act to decrease the current of Na ions into myocytes, thus reducing the amplitude, rate of rise and velocity of action potentials.  Na channel blockers can also affect K channels resulting in prolonging of the ARP. 

EKG effects: Longer PR interval, wider & later QRS.

Clinical: Na channel blockers are given to treat reentrant arrhythmias, most commonly tacharrhythmias.