QRS complex configurations in 12-lead electrocardiograms of dogs with monomorphic ventricular tachycardia or complete bundle branch block

Introduction: The differentiation between ventricular tachycardias (VT) and supraventricular tachycardias (SVT) with bundle branch block (BBB) is clinically challenging. The aim of the study was to define by the 12-lead-electrocardiogram the QRS complex morphology in monomorphic VT (MVT) and in BBB. Methods: Twelve-lead-electrocardiograms were blindly retrospectively analyzed and categorized in four groups: sinus rhythm with left bundle branch block (SR-LBBB), sinus rhythm with right bundle branch block (SR-RBBB), MVT with RBBB configuration (MVT-RBBB), MVT with LBBB configuration (MVTLBBB). Measurements were not normally distributed, and they were normalized by Box–Cox transformation. Repeated-measures linear mixed-effects models were constructed according to the 3 measurements performed. Results: A total of 103 12-lead-electrocardiograms were retrospectively analyzed: 18 SR-RBBB, 18 SR-LBBB, 33 MVT-RBBB, 34 MVT-LBBB. Limb leads concordance was found in 100% of SR-RBBB, 100% of SR-LBBB, 54.5% of MVT-RBBB, 70.6% of MVT-LBBB. Precordial leads discordance was present in 100% of SR-RBBB, 100% of SR-LBBB, 78.8% of MVT-RBBB, 88.2% of MVT-LBBB. The transition point was located at V1-V2 in 100% of SR-RBBB, 100% of SRLBBB, 50.5% of MVT-RBBB and 71.5% of MVT-LBBB. Positive V1 with M shape morphology was detected in 100% of SR-RBBB and 12% of MVT-RBBB. The mean electrical axis on the frontal plane was -108.15° (−118.29 / −101.52) in RSSR-RBBB, 75.42° (71.78–80.46) in RSSR -LBBB, -93.46° (−102.75/−78.49) in MVT-RBBB and 82.27° (76.85–88.95) in MVT-LBBB. Discussion: In case of inability to identify signs of atrioventricular dissociation, the presence of standard limb leads discordance, precordial leads concordance or discordance with transition point other than V1-V2, left limb leads and left precordial leads discordance, aVR and V1 discordance and absence of M shape configuration of the QRS complex in lead V1 is likely to be MVT.