Conduction blocks in the cardio electrical system are perhaps the most common cause of disrhythmias.  Blocks in the normal electrical conduction pathway of the heart cause reentry type disrhythmias when blocks in the normal path force retrograde entry of action potentials through previously closed pathways.  In the image below, the first graph is of a normal conduction pathway while the second image portrays pathological reentry.  Normal APs cancel each other out in the third phase shown in image 1 below.  However, when a block is present, retrograde reentrys occur, disrupting the normal cardiorhythm and leading to tachycardia (increased HR) or bradycardia (decreased HR).