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Background: A 30% rate of non-responders to cardiac resynchronization therapy (CRT) poses a challenge to better define the potential candidates before device implantation. A mechanism based approach to patient selection has been recently proposed, part of which suggests the septal flash as a sign of intraventricular dyssynchrony, predictive of CRT response. It has been demonstrated that immediate response to CRT implantation observed from the point of LV intraventricular dyssynchrony can be detected by resolution of the septal flash immediately following device activation, thus demonstrating that the presence of a septal flash prior to CRT pacing is a direct consequence of early septal activation in LBBB, correctable by CRT. We hypothesize that the amplitude of the septal flash can be predicitve of the magnitude of immediate volume response to CRT.Methods: Data from 15 consecutive patents (6F/9M, 56±14 years) referred for CRT implantation via a mini-thoracotomy were analyzed. Intraoperative transoesophageal echocardiography data were acquired pre- and post-CRT device activation. The septal flash was defined as an early ventricular inward and outward septal motion within the isovolumic contraction period, imaged using gray-scale M-mode. The amplitude of the septal flash was measured as the maximal excursion of the inward septal motion. Reverse remodeling was defined as a reduction of end-systolic volume (LVESV) >10%. The right atrial and ventricular leads were placed transvenously, while the LV screw-in lead was positioned epicardially on the lateral wall.Results: The septal flash was detected preoperatively in all patients and resolved immediately after onset of biventricular pacing. A significant reduction of the septal flash amplitude was noted post-CRT activation (pre: 7.6±4.4 mm vs. post: 1.7±0.6 mm, p<0.05). Immediately following pacemaker activation, a significant reduction of LVESV (171±88 ml vs.131±72 ml, p<0.05) and increase in EF (23±9% vs. 32±8%, p<0.05) were measured in all patients. A positive correlation was demonstrated between the pre-activation values of the septal flash amplitude and the percent change in LVESV (delta LVESV=23±12%, r=0.55, p<0.05).Conclusion: Preoperative presence of the septal flash is a valid predictor of response to CRT. Immediately after CRT device activation, the septal flash is resolved and LV reverse remodeling is observed while the preoperative amplitude of the septal flash can be predictive of the magnitude of immediate volume response.