The different ECG-based indices of mechanical dyssynchrony assessment used in our study; (A) shows a still 2D image of the assessment of ApRock and SF (see Supplementary data online, Video S1 1); (B) shows the assessment of systolic stretch index (SSI) in a patient with LBBB which is the sum of the systolic pre-stretch of the lateral wall (SPS_lat) and the systolic rebound stretch of the septum (SRS_sept); (C) shows pressure-strain loops of the septum (S), and lateral wall (LW) of a patient with LBBB. The loop area represents the myocardial work where the LW–S work difference is the absolute difference in work values, calculated with the strain of each wall averaged over its mid- and basal-segments in the 4CH view. AVO, aortic valve opening; AVC, aortic valve closure.