Figure 4: Ladder diagram illustrates our proposed mechanism for intermittent RBBB and AV block seen on ECG. A His extrasystoles conducts retrogradely through the AV node to the atrium but blocks antegradely in the RBB will give rise to a QRS with RBBB. A shorter coupled His extrasystoles conducts retrograde into the slow AV node pathway to the atrium and into the fast AV node pathway causing the preceding sinus beat to block in the AV node will give rise to an inverted P wave without QRS mimicking AV block.