When the atrial rate is rapid and there is no evidence of distal conduction system disease, such as fascicular or bundle branch block, the site of 2:1 AV block is more likely to be within the AV node than distal to it.
In both tracings displayed here, the atrial rate is 140. In the upper tracing, there is 2:1 AV conduction but the blocked P waves (arrows) are difficult to see because they fall within the T wave. The lower strip was recorded the next day and shows type I 2nd degree AV block with 3:2 conduction. This suggests that the site of block associated with the 2:1 conduction was also within the AV node.