What is a normal Doppler waveform?
The “normal” Doppler waveform depends on the vessel being imaged. In general, veins have continuous low-velocity flow that varies with respiration. The PV normally has hepatopetal flow (flow toward the liver) that ranges from 15 to 18 cm/sec. The HVs have triphasic and pulsatile flow directed away from the liver into the IVC. Arterial flow varies dramatically with the cardiac cycle, with high-velocity flow during systole and relatively high flow (i.e., low resistance) during diastole.
The change in frequency of reflected sound waves from flowing blood (the Doppler frequency shift) and the angle at which the US beam interfaces with the flowing blood (the Doppler angle) are used to calculate the velocity and direction of blood flow. The Doppler angle should be less than 60 degrees to avoid erroneous velocity calculations. The operator determines whether flow directed toward the transducer is displayed above or below the baseline on grayscale imaging and whether blood flowing toward the transducer is blue or red on color imaging, with flow in arteries and veins normally assigned a different color.