His Electrograms in the Interpretation of Cardiac Arrhythmias

A 63-year-old man with chronic hypertensive atherosclerotic heart disease and stable anginal syndrome had complained of intermittent palpitations for the past 2 weeks. The palpitations were unrelated to activity or food intake. He was compensated and not on any routine cardiac drug therapy. Sublingual nitroglycerin promptly relieved his anginal episodes. The tracings in Figure 1 were recorded when he was seen initially.A tracing recorded a few minutes later is shown in Figure 2.His bundle recordings were obtained to confirm the clinical impression (Figure 3).1. c. upper tracing: sinus rhythm with complete LBBB and His extrasystoles with slight aberrancy; blocked sinus P wavelower tracing: pseudo Mobitz II 2:1 AV block due to concealed His extrasystoles d. same as c, but extrasystoles are from a focus in the RBB d. same as c, but extrasystoles are from a focus in the RBBWhen the upper and the lower tracings are compared, similarities suggest the probability of concealed His bundle extrasystoles. The extrasystoles are similar in configuration to the sinus conducted beats. Note that the lower tracing has a similar rhythmic pattern, but without the extrasystoles. All P waves are sinus and appear at regular intervals. The extrasystoles seen in the upper tracing occur immediately before a blocked sinus P wave. In the upper tracing, the P waves following the His extrasystole are blocked in the AV node, which was rendered refractory by retrograde activation from a proceeding premature His extrasystole.In the lower tracing, the His extrasystole is blocked in its orthograde progression in ventricles not yet recovered from their activation by the preceding sinus conducted beat. The retrograde activation of the AV node by the His extrasystole made the AV node refractory to the subsequent sinus P wave. This gives the appearance of 2:1 AV block. Thus, the junctional or His extrasystoles seen in the upper tracing are not present in the lower tracing because, although the impulse initiating the extrasystole was discharged, it did not depolarize the ventricles because the ventricles at that point in time were still refractory (the ventricles had not recovered from the previous activation). A His bundle recording was obtained for confirmation. Without this definitive diagnostic aid, the interpretation would erroneously be 2:1 AV block and when added to the presence of abnormally widened QRS complexes, an artificial pacemaker could be inappropriately considered.2. b. sinus rhythm with complete LBBB, His extrasystoles, some concealed, creating pseudo first degree AV block c. same as b, but extrasystoles are from a focus in the RBBVarying PR intervals that do not satisfy the criteria of specific forms of AV block should be considered as being due to concealed extrasystoles with concealed conduction, as explained above. At first glance (Figure 2), the varying PR intervals are obvious and would lead one to assess for a Wenckebach (Mobitz type I) pattern. However, a Wenckebach period (ie, the progressive widening of the PR interval before a dropped QRS) is not present. The other option would be a third degree AV block in which the P waves march through at regular intervals and the QRS interval, while no longer associated with the P waves, also marches through in a regular pattern. In this tracing the QRS intervals are irregular and do not march through as seen in third degree AV block. However, the P waves appear to march through even though the third and fifth P waves are concealed by the third and fifth ventricular complexes. Although varying degrees of AV block are mimicked, none of the criteria for AV block are clearly satisfied. A pseudo first degree AV block with an LBBB is present. His bundle recordings were obtained to confirm the clinical impression.3. a. A, H, and V (P, PR, and QRS) are normal in the first 2 beats c. pseudo AV block above and below the His bundle d. H- (retrograde His) represents concealed extrasystoles below the His bundle.The first His bundle electrograms in the United States were recorded in 1969 by Scherlag et al.1 Atrial and His bundle electrograms are essential and integral parts of electrophysiological testing. Temporary electrode catheters are inserted through peripheral veins into the heart, where they are positioned along strategic positions of the conduction system. The origins of each electrical cardiac impulse then can be recorded in order to identify areas of concealed conduction versus heart block.2 This atrial and His bundle electrogram is correlated with the surface electrocardiogram (ECG).A His bundle ECG is shown in the lower tracing (Figure 3). The A wave depicts the sinus P wave; the PR interval is recorded when the A wave is followed by an H. An H- indicates a focus lower in the His bundle or in the bundle branch that is conducted retrograde. In Figure 3, the His bundle electrogram records a retrograde His deflection, H-, induced by a premature beat distal to the H deflection; that is, a focus in the lower His or bundle branch. When the H- is closer in time to the A wave (fifth A), then it is evident that the concealed His extrasystole activated the AV node, rendering it refractory to the sinus A wave. This gives an appearance of 2:1 AV block or Mobitz II, but, as explained above, it is actually due to physiological refractoriness of the AV node, hence the term pseudo AV block or pseudo Mobitz II. The absence of a V wave following the third A-H is indicative of block below the H. The absence of an H following the fifth A is indicative of block above the H.The first recordings of His bundle electrograms in humans in the United States were made by Scherlag et al.1 Historically, however, the earliest report on the electrical activity in the bundle of His was in the French literature in 1960.3 This procedure, which is often unappreciated, is nevertheless potentially critical to the diagnosis and management of complex arrhythmias.Supported in part by a grant from the Applebaum Foundation, in loving memory of Joseph Applebaum.