This case report is kindly provided by Michael Rosengarten from McGill and is part of the McGill Cases. These cases come from the McGill EKG World Encyclopedia.
These two rhythm strips show Mobitz I A/V block with a gradual increase in the PR interval before the dropped p wave. Note the 2:1 block in the lower strip, and that one can not use this to determine if the block is Mobitz I or II as more than one conducted P wave is required to do this.