LAUSR

tially life-threatening condition because an abrupt increase in the atrial rate can result in paroxysmal AV block and asystole. Unfortunately, 2:1 heart block is frequently missed by interpretation softwares and by clinicians. In cases of apparent sinus bradycardia, it is important to actively search for nonconducted P waves in the electrocardiogram, which would indicate 2:1 block.1 Once 2:1 AV block has been diagnosed, one should establish whether the block occurred within the AV node or in the distal conduction system. Important factors suggesting a more malignant distal site include stable PR intervals before and after the block, wide QRS complexes, such as bundle-branch block and bifascicular block, and most importantly, the fact that block occurred during acceleration of the atrial rate.6 In these cases, the use of atropine can be hazardous.