Figure 206-8

His bundle (HB) electrograms in atrioventricular (AV) block. The tracings are from three different patients with second-degree AV block. In A and B, the conducted QRS complexes are wide and associated with bundle branch block.
In A, the block is within the AV node (i.e., the A wave on the HB is not followed by an HB deflection).
In B, it can be appreciated that the block is distal to the HB even though the surface electrocardiogram (ECG) demonstrates a Wenckebach phenomenon. The latter can obviously occur in the His-Purkinje system as well, as depicted in this figure.
C. The Site of the block is within the HB. This is suggested by split HB potentials (labeled H and H'), and the block is distal to the H but proximal to the H'. Intra-His block is difficult to diagnose from the surface ECG but can be suspected when a Mobitz type II occurs in association with a normal PR interval and a narrow QRS complex.

(From Akhtar.Reproduced with permission from the publisher and author.)