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Figure 3 The figure shows the four pulmonary venous flow pulses, the A-wave during atrial contraction, the S1 and S2 waves during ventricular systole and the D-wave during early diastole. The numbers indicate four major waves in the dPdU trace. During atrial contraction the rise in pressure and decrease in velocity cause a negative dPdU (1), indicating a net backward-going compression wave. During atrial relaxation and during early ventricular systole corresponding to S1, PV pressure is falling and velocity is rising. Therefore dPdU is negative (2), indicating a backward-going expansion wave. Later in systole, corresponding to S2, PV pressure is rising and velocity is rising, and therefore dPdU is positive (3). This indicates a net forward-going compression wave. During early diastole, PV pressure is falling and velocity is rising and dPdU is negative (4), indicating a net backward-going expansion wave. dPdU = product of increments in pressure (dP) and velocity (dU) over 5 ms intervals; PV = pulmonary vein. Other abbreviations as in Figure 2.