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 pacingis highly predictive of an impending high degree atrioventricularblock in patients with bifascicular block. The His potentialis, however, sometimes not measurable or is lost in the ventriculardepolarization. The aim of this study was to evaluate whetherthe comparison of RR intervals before and after atrioventricularblock, induced by incremental atrial pacing, could differentiatebetween atrioventricular nodal and His—Purkinje blockin patients with bifascicular block. METHODS AND RESULTS: In 98 patients with bifascicular block, incremental atrial pacingwas performed as part of an invasive electrophysiological study.An ‘RR index’ was constructed by calculating thenumerical difference between the RR interval immediately beforeand after the atrioventricular block divided by the RR intervalimmediately before the pacing-induced block. Endocavitary recordingof the His bundle potential was used for defining the levelof atrioventricular block. The median RR index was 0·98(range 0·88–1·02) in recordings with His—Purkinjeblock and 0·49 (range 0·11–0·89)in recordings with atrioventricular nodal block (P<0·001).An RR index of 0·85 had a sensitivity of 100% and a specificityof 99% for the identification of atrioventricular block localizedto the His—Purkinje system. CONCLUSION: The use of an RR index is a helpful tool in the differentiationof His—Purkinje from atrioventricular nodal block in patientswith bifascicular block undergoing incremental atrial pacingas part of an invasive electrophysical study.