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Purpose: Although the utility of two-dimensional speckle tracking echocardiography (2D STI) to quantify left ventricular (LV) function has been demonstrated, this methodology is limited by geometric modeling and the assumption that speckles can be tracked from frame to frame, despite their out of plane motion. To circumvent these limitations, a three-dimensional speckle tracking (3D STI) algorithm was recently developed. Our goal was to evaluate left ventricular (LV) function and dyssynchrony in patients with dilated cardiomyopathy (DCM) and complete left bundle block (CLBBB) by 3D STI.Methods: 37 DCM patients with CLBBB (65.2±12.3 old, mean QRS width 145.3±24.6ms) and 25 healthy volunteers (61.9±14.0 old) were imaged with Philips IE33 system by means of a 3D transthoracic probe (X3-1). Five apical full-volume data sets were recorded within one breath-hold, combining four real-time sub-volume during four cardiac cycles. We measured global 3D, longitudinal, circumferential, radial strain and standard deviation of time to peak from 3D strain of 16 segments by Tom-Tec 4-D LV analysis 3.0 software.Results: 1) In control group, uniformity in the average value of 3D strain was observed between individual segments (basal anterior: -34.7±4.8%, basal anteriospetal: -39.8±3.7%, basal inferoseptal: -39.4±7.2%, basal inferior: -35.6±3.8%, basal inferolateral: -38.3±4.6%, basal anterolatal: -32.4±3.9%, mid anterior: -36.4±4.8%, mid anteriospetal: -36.4±5.1%, mid inferoseptal: -37.2±3.9%, mid inferior: -34.9±3.6%, mid inferolateral: -36.1±4.8%, mid anterolatal: -35.8±5.2%, apical anterior: -38.6±5.4%, apical septal: -39.2±4.8%, apical inferior: -37.3±3.6%, apical lateral: -33.5±7.4%, P>0.05 for each segment); 2) Compared with control group, global 3D, longitudinal, radial and circumferential strains were significantly lower in DCM patients (-16.18±6.95% versus -37.18±4.62%, -8.57±3.05% versus -21.50±2.79%, 23.28±13.38% versus 94.37±16.05%, -8.90±3.86% versus -26.37±4.73%, respectively, P<0.01 for all); 3) Global 3D, longitudinal, radial and circumferential strains had excellent correlations with LVEF (r=0.92, 0.84, 0.78 and 0.81 respectively, P<0.01); 4) Standard deviation of time to peak from 3D strain of 16 segments in DCM patients with CLBBB was significantly longer (94.66±31.82ms) than that of volunteers (30.63±5.26ms, P<0.01).Conclusion:When image quality is optimal, 3D STI represents a promising novel technique for assessment of global LV function and dyssynchrony.