Differentiating the QRS morphology of posterior fascicular ventricular tachycardia from right bundle branch block and left anterior hemiblock aberrancy

Background Left posterior fascicular ventricular tachycardia (LPF-VT) is frequently misdiagnosed as supraventricular tachycardia with aberrant right bundle branch block (RBBB) and left anterior hemiblock (LAHB). The purpose of the present study was to define the morphological ECG characteristics of LPF-VT and attempt to differentiate it from RBBB and LAHB aberrancy. Methods and Results A systematic Medline search was used to identify or locate ECG tracings from patients with LPF-VTs. ECGs with LPF-VT were also collected from patients who underwent ablation of this arrhythmia at the Tel Aviv and Sheba Medical Centers. These ECGs were compared with ECGs of consecutive patients with RBBB and LAHB and no obvious cardiac pathology by echocardiography. Overall, 183 ECGs of LPF-VT were compared with 61 ECGs showing RBBB and LAHB. Univariate analysis demonstrated differences in QRS axis, limb (I, aVr), and precordial (V₁, V₂, V₆) ECG leads. On multivariate logistic regression analysis, LPF-VT was more often associated with atypical RBBB-like V₁ morphology (odds ratio, 5.1; P=0.004), positive QRS in aVr (odds ratio, 19.2; P<0.001), V₆ R/S ratio ≤1 (odds ratio, 6.7; P=0.01), and QRS ≤140 ms (odds ratio, 7.7; P<0.001). Using these 4 variables, a prediction model was developed that predicted LPF-VT with sensitivity and specificity of 82.1% and 78.3%, respectively. Patients with 3 of 4 positive variables had high probability of having LPF-VT, whereas patients with ≤1 positive variable always had RBBB plus LAHB. Conclusions The morphological ECG characteristics of LPF-VT were defined, and a high accurate tool for correctly differentiating LPF-VT from RBBB and LAHB aberrancy was developed.