Flow disturbances occur when a narrowing is present in a vessel. This appears as spectral broadening in the waveform. Flow patterns change from laminar, to disturbed, to turbulent as flow patterns break down. Turbulent flow is more likely to occur as velocities increase. When a critical narrowing is present, flow velocities increase and the flow patterns become disturbed. The velocity at which flow becomes turbulent can be determined by calculating the Reynold's number. The following formula defines the Reynold's number.
R = dnrm
R = Reynold's number
r = Density of the fluid
d = Diameter of the vessel
m = Viscosity
n = Velocity of fluid
Once the Reynold's number exceeds the critical value of approximately 2000, turbulent flow occurs. The presence of a critical stenosis can cause an increase in velocity. The clinical image below shows spectral broadening and increased velocities indicating turbulent flow.
