[Drug-induced major anomalies of ventricular repolarization masked by a left branch block]

The presence of a left bundle branch block (LBBB) may hamper the electrocardiographic diagnosis of diseases that involve the QRS complexes. That it may conceal, or even completely erase, major abnormalities of ventricular repolarization induced by certain drugs is not so well known. In this paper, two highly demonstrative examples of such abnormalities observed with bepridil (case 1) or with the amiodarone-aprindine combination (case 2) are reported. In both cases, the intermittent character of the LBBB revealed the phenomenon which deserves to be known as it is not without practical consequences: in patients under treatments likely to modify repolarization and induce severe dysrhythmias (notably torsades de pointes) the presence of a LBBB indicates that the QT and/or QU intervals must be very carefully measured. In case of phase 3 LBBB compression of the carotid sinus or intravenous ATP injection helps the diagnosis since in the presence of even moderately prolonged ventricular cycles such manoeuvres create a refinement of QRS complexes which then demonstrate the major alteration of the underlying repolarization.