Nodal and infranodal atrioventricular conduction block: electrophysiological basis to correlate the ECG findings.

A 68-year-old woman with a history of dilated non-ischemic cardiomyopathy presented with syncope. The index ECG showed sinus rhythm with left bundle branch block. On telemetry episodes of sinus rhythm with narrower QRS complexes conduced in 2:1 pattern were noted. Invasive electrophysiological study was performed to determine cause of syncope. Normal conduction up to the AV node with an AH interval of 79 ms (normal = 55-125 ms) was observed. However, every alternate sinus beat was blocked after the inscription of His deflection (infra-Hisian block). The narrow beats conducted through the His bundle with HV intervals of 54 ms (normal = 35-55 ms). When 1:1 conduction resumed further abnormality of the His-Purkinje conduction system became evident with a QRS morphology that of an LBBB and prolongation of HV interval (HV = 96 ms). Criteria to differentiate nodal versus infranodal block based on electrophysiological properties of the nodal and infranodal system are discussed.