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Objective: To study the sequence of atrial activation and the interatrial electromechanical delay (IEmD) noninvasively in healthy subjects during sinus rhythm (SR).Methods and Results: In 66 SR healthy subjects P-wave activation was analyzed by means of vectorcardiography. The timing of atrial contractions was measured as the intervals between the P-wave and the A-wave of the Doppler right and left ventricular inflows (P-A(t) and P-A(m)), and IEmD was calculated as the algebraic difference P-A(m)- P-A(t). In the horizontal plane the vectorcardiographic P-loop was anteroposterior ("typical", 41 subjects), anterior (18), or posterior (7). IEmD (mean +/- SE 17 +/- 8 ms) was directly related to R-R and P-R intervals. IEmD was significantly shorter in anterior and posterior P loops than in the typical (6.5 +/- 5.3 and 8.1 +/- 10.1, respectively, vs 24.2 +/- 3.1 ms). In the posterior P-loop group, who exhibited longer P-A(t), mitral E-wave velocity and E/A ratio were reduced, and left atrial booster function was increased.Conclusion: IEmD was widely variable in SR, reflecting the origin site of sinus impulse, which independently influenced ventricular filling dynamics. [ABSTRACT FROM AUTHOR]