When should we diagnose incomplete right bundle branch block?

An rSr' pattern with QRS duration of less than 0.12 s in the right precordial leads can be due to incomplete right bundle branch block (which may progress to complete right bundle branch block) or can be a normal electrophysiological variant. To identify other ECG features that may help to distinguish between these two possibilities, ECGs of 15 patients who progressed from normal to complete right bundle branch block through an intermediate rSr' pattern of incomplete right bundle branch block were analysed. The following features in the right precordial leads (V1, V2) that preceded or accompanied the appearance of the rSr' were identified: diminution of the S wave depth (100%), inversion of ratio of the S wave depth to SV1 > SV2 (93%), slurring of the downstroke or upstroke of the S wave (27%) and prolongation of the QRS duration to > or = 0.10 s (73%). When a further 79 subjects with rSr' pattern in the right precordial leads and QRS duration of < 0.12 s were divided into those with SV1/SV2 ratio > 1.0 and those with SV1/SV2 < 1.0, compared with the latter the subjects with SV1/SV2 ratio > 1.0 were found to be significantly older (59.8 +/- 18.4 years vs 32.8 +/- 18.1 years, P < 0.001), to exclusively show S wave slurring (37% vs 0%), and to more likely have a QRS duration > or = 0.10 s (74% vs 7%).(ABSTRACT TRUNCATED AT 250 WORDS)