The RR index test for the differentiation of atrioventricular nodal block from His--Purkinje block during incremental atrial pacing in patients with bifascicular block

AIMS: His-Purkinje block induced by incremental atrial pacing is highly predictive of an impending high degree atrioventricular block in patients with bifascicular block. The His potential is, however, sometimes not measurable or is lost in the ventricular depolarization. The aim of this study was to evaluate whether the comparison of RR intervals before and after atrioventricular block, induced by incremental atrial pacing, could differentiate between atrioventricular nodal and His-Purkinje block in patients with bifascicular block. METHODS AND RESULTS: In 98 patients, with bifascicular block, incremental atrial pacing was performed as part of an invasive electrophysiological study. An "RR index' was constructed by calculating the numerical difference between the RR interval immediately before and after the atrioventricular block divided by the RR interval immediately before the pacing induced block. Endocavitary recording of the His bundle potential was used for defining the level of atrioventricular block. The median RR index was 0.98 (range 0.88-1.02) in recordings with His-Purkinje block and 0.49 (range 0.11-0.89) in recordings with atrioventricular nodal block (P < 0.001). An RR index of > or = 0.85 had a sensitivity of 100% and a specificity of 99% for the identification of atrioventricular block localized to the His Purkinje system. CONCLUSION: The use of an RR index is a helpful tool in the differentiation of His-Purkinje from atrioventricular nodal block in patients with bifascicular block undergoing incremental atrial pacing as part of an invasive electrophysical study.