Vectorcardiographic Quantification of Early Repolarization

Introduction: Although a recent consensus report has recommended a unified approach to electrocardiographic (ECG) quantification of the early repolarization (ER) syndrome there is a lack of understanding of the hallmark ECG features of ER: end-QRS notches and slurs. Using vectorcardiographic (VCG) analysis of end-QRS morphology we investigate the relationship between concave "bite" segments in the VCG loop and the presence or absence of notches and slurs in the ECG. Methods: We obtained digital 12-lead ECG recordings from 1561 persons in whom out-of-hospital cardiac arrest was later witnessed. The presence of ER notches and slurs in the 12-lead ECG was determined in accordance with the 2015 consensus criteria. The inverse Dower transform was used to derive the VCG from recorded leads and end-QRS bites in the 3D VCG were identified and quantified by an automatic algorithm developed by the authors. An end-QRS bite was defined as a bite with a starting point in the second half of the QRS loop and an end point within the last quarter of the QRS loop. Bite amplitude was measured as the largest 3D distance to the loop from a line connecting the start and end point of the bite. Results: We identified 85 persons (5.4%) out of 1561 who met the ECG consensus criteria for ER configurations (ER+), see table. Projection of a VCG bite onto the 12-leads plays a significant role in the ECG manifestation as either notches or slurs. We observed that larger bites were more likely than smaller bites to manifest as notches, whereas even relatively small bites due to a combination of the orientation and projection of the VCG loop can manifest as slurs. Conclusion: Our observations of VCG bite morphology indicate the existence of a continuous transition from ECG slurring to notching which is determined by the magnitude and orientation of a VCG bite in 3D. We suspect therefore, that “true ER” is associated with the presence of a VCG bite and plan to study its relationship to cardiac risk.