Diagnosis and mechanism of this bradyarrhythmia answer (July 2008)

The correct answer is paroxysmal AV block. Note the VPD with retrogade conduction (inverted p wave). This suggests that there is intact retrograde conduction to the atrium through the AV Node (ie there is no AV nodal block). The subsequent non conducted p waves occur due to spontaneous depolarization of the her purkinje tissues following a pause (ie phase 4 block). This phenomena occurs in abnormal or diseased tissues.