This strip of lead V1 is from the tracing recorded the next day. The atrial rate has slowed slightly (PP interval is now 1.00 to 1.08 seconds) and there is type I second degree AV block with progressive prolongation of the PR interval prior to the blocked P wave and a 4:3 conduction ratio. Note that the PR interval after the pause is prolonged (0.36 seconds) indicating 1st as well as 2nd degree block. This ECG suggests that the site of 2:1 block shown on the preceding page was within the AV node.