Figure 2.

Flow volume measurement in the graft.

The flow volume calculation is based on the following formula: Qa = πr ² × TAMEAN, where r is the radius of the examined graft and TAMEAN is the time-averaged mean velocity inside. The latter is an integral value during the heart cycle of the average velocity layer (flow velocity is the fastest in the middle and the slowest along the vascular wall). TAMEAN must not be replaced by TAMAX, which is the time velocity integral of the fastest velocity only—using TAMAX would lead to significant overestimation of Qa. Pulse wave Doppler sample size should be wide enough to cover most of the vessel lumen because of the different speeds of blood inside the vessel from the center to the wall.