Fig. 1. Velocity components in Doppler recordings of pulmonary venous flow. During ventricular systole, there are two velocities (S1 and S2) that are often fused during normal atrioventricular conduction and form one wave, the S wave (S). Ventricular diastolic velocity, the D wave (D) is normally 20% smaller than S. Late ventricular diastole is marked by flow in the opposite direction caused by the atrial contraction, the A wave (A).