Peter N Burns PhD
University of Toronto, Canada

In clinical ultrasound studies, the Doppler signal is a consequence of the movement of the blood itself, with the Doppler shift frequency dependent on the velocity of this movement. Doppler information is, then, information concerning blood flow velocity. It is natural to ask how velocity might relate to other physiological variables, to pressure, flow rate, and peripheral resistance, for example, what velocity patterns should be considered normal for a given vessel and how might they change in the presence of a lesion that encroaches on the vessel lumen? What is the significance of the way in which velocity changes with time in arteries? In this article some of the concepts and relationships used to describe blood flow are introduced so as to form a background for the interpretation of Doppler signals obtained in clinical practice.

SUMMARY

1. Difference in total fluid energy is the impetus for flow. Fluid energy can take the form of pressure, weight or inertia of moving blood.
2. The principal source of resistance to flow in blood vessels is the blood itself. Blood viscosity remains relatively constant in large vessels.
3. Poiseuille's law holds that for steady flow in a long rigid tube, flow rate is proportional to pressure, with the constant of proportionality related to the inverse fourth power of vessel radius. In idealized conditions, the flow velocity profile is parabolic.
4. Energy dissipated in a stenosis through the production of turbulence results in a loss of pressure. Whereas this loss begins to become significant around a critical degree of stenosis, raised velocity and disturbed flow occur at milder degrees of stenosis.
5. In a pulsatile system, the relationship between pressure and flow is more complex. The total opposition to flow is represented by the fluid impedance; losses are greater than for steady flow. The velocity profile changes with time and can contain multidirectional flow.
6. Discontinuities in fluid impedance five rise to wave reflections. These reflections combine with the travelling velocity pulse to influence its shape.

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