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Purpose: There is still controversy on whether interventricular delay (IVD) optimization of cardiac resynchronization therapy (CRT) should be routinely performed and which is the programming that would benefit most patients. Our aim is to test different IVD in a left bundle branch block (LBBB) swine model.Methods: 5 healthy Landrace x Large white pigs of 30 kg were studied. In each animal a LBBB was induced by radiofrequency ablation. A CRT device was implanted. Echocardiography was performed in each animal prior to and immediately after LBBB induction, and after the onset of CRT at three different IVD programmings: simultaneous biventricular pacing (VV0), right ventricle preactivation at 30 ms (VV+30) and left ventricle preactivation at 30 ms (VV-30). The presence of intraventricular dyssynchrony was studied analyzing the presence of a LBBB-induced septal flash (SF)). SF was defined as the presence of a fast septal inward/outward motion occurring during the isovolumic contraction period and its excursion was quantified in M-mode.Results: After LBB ablation the QRS width increased (48±5ms baseline vs 69±6ms post-ablation, p<0.05), and a SF acutely appeared(mean SF excursion 3.1±1 mm). Both VV0 and VV-30 reduced SF excursion in comparison to CRT off (1.1±0.2 vs 3.1±1 mm and 0.6±0.5 vs 3.1±1 mm, respectively, both p<0.05). No significant difference was noted between VV+30 and CRT off (2.5±1.2 vs 3.1±1 mm, p=n.s.). A non significant trend was observed towards better resynchronization with VV-30 in comparison to VV0 (See Figure). Conclusion: VV0 and VV-30 programings result in a better acute resynchronization in a LBBB swine model without structural heart disease. VV+30 did not significantly improve synchrony, suggesting that right preactivation may not be of benefit in a subset of CRT patients.