Does His bundle electrogram recording localize the site of block in patients with complete heart block?
His bundle electrograms demonstrate that the majority of complete heart block cases occur distal to the His bundle, likely due to bilateral bundle-branch block.
Twenty-one patients with complete heart block (CHB) were studied by recording His bundle (BH) electrograms. The site of block was localized proximal or distal to the site at which the BH electrogram was recorded. Eighteen patients had block distal to the recorded BH deflection. One of these 18 patients had a narrow QRS complex indicating block within the His bundle; the other 17 had wide QRS complexes probably due to bilateral bundle-branch block. The three patients with block proximal to the His bundle are examples of A-V nodal block or block in the uppermost portion of the BH. Two of these three had normal QRS complexes; the third with a left bundle-branch block pattern demonstrated that during CHB the ECG criteria for the localization of the site of block are not dependable. The conduction time from the atrium to the BH (A-H time) was measured in the 18 patients with CHB distal to the BH and was within the normal range. With atrial pacing there was progressive lengthening of the A-H time, and Wenckebach phenomena were demonstrated in some cases. The conduction time from the His bundle to the ventricles (H-V time) in patients with block proximal to the His bundle was also within the normal range. Bilateral bundle-branch block was probably the mechanism of block in the majority of the patients with CHB; this emphasizes the clinical significance of ECG patterns indicating bilateral bundle-branch block during normal sinus rhythm with 1:1 A-V conduction.
Narula et al. (Sun,) studied this question.