Background: Electric pacing in the right atrium affects both the atrial depolarization and contraction and may alter the mechanical interaction between atrium and ventricle. Tissue Doppler and strain echocardiography has been validated as accurately depicting regional myocardial motion and deformation, respectively. The purpose of this study was to determine (1) the effect of VDD versus DDD pacing mode on left ventricular (LV) performance and (2) the mechanisms underlying the difference. Methods: We studied 20 patients with permanent dual-chamber pacemakers. The dual chamber pacemakers were programmed into the VDD and DDD modes, and all Doppler echocardiographic measurement and tissue Doppler imaging were repeatedly obtained in both pacing modes. Atrial synchrony was defined as the time difference between atrial mechanical activation time at the right atrium (RA), left atrial (LA) free walls and the interatrial septum. Atrial strain was measured to represent active atrialcontraction. Results: The mitral inflow time-velocity integral, and diastolic filling period were greater in VDD mode (p<0.001). Atrial strain wassignificantly higher in VDD mode in the RA (p=0.0017), interatrial septum (p=0.0056) and the LA (p=0.015). Mechanical delay between interatrial septum and LA (p=0.04) and between RA to LA (p=0.0014) were significantly prolonged in DDD mode. Left ventricular outflow tract time-velocity integral was greater in VDD mode (p<0.0001). Conclusion: In intrinsic atrial rhythm during VDD pacing, atrial contractility and atrial synchrony are preserved, resulting in better LV diastolic filling and LV performance.