When there is 2:1 AV block, it is often impossible to determine from the ECG whether the site of block is within the AV node or the more distal conduction system since the characteristics of the PR intervals prior to the blocked P wave can not be established. This ECG shows sinus rhythm with an atrial rate of 68 (PP interval = 0.88 seconds) and 2:1 AV block. There is right bundle branch block with left axis deviation, and the PR interval of the conducted P waves is 0.20 seconds. Although the tracing suggests diffuse conducting system disease, it is impossible to determine the site of block of the non-conducted P waves from this ECG.
The tracing from the next day, shown the following page, helps to identify the site of block.