This strip shows a second-degree AV block. During most of the strip, 2:1 conduction is present. At the beginning, however, two consecutive p waves are conducted, revealing progressive prolongation of the PR interval. This usually represents a Type I , or nodal, block: progressive refractoriness of the AV node. However, the wide QRS ( possibly left bundle branch block), and the fact that the non-conducted p waves are "out in the open" where they should have conducted, points to Type II - an intermittant tri-fascicular block. Wenckebach periods in patients with LBBB can be caused by progressive conduction delay in the right bundle branch.