Figure 206-7

Atrioventricutar (AV) block in the His-Purkinje system (HPS).
A. ControL A 1:1 AV conduction is depicted in a patient with unexplained syncope. Following 150 mg of intravenous procainamide (B), a second-degree AV block in the HPS is noted( i.e., His bundle potential Is not followed by a complex, an abnormal response to a small dose of procainamide suggesting AV block in the HPS as a potential cause of syncope.