Why do Doppler velocity waveforms from upper extremity vessels differ from lower extremity vessels?

Doppler velocity waveforms from upper extremity vessels differ from those in lower extremity vessels primarily due to the differences in resistance. Upper extremity resistance is typically lower than that of the lower extremities due to factors such as the vascular resistance encountered by blood flow, as well as the varying levels of demand for blood supply in the muscles and tissues of the arms compared to the legs.

This lower resistance in the upper extremities leads to a higher flow velocity and more distinct waveforms that reflect the characteristic, less pulsatile blood flow found in these vessels, such as those supplying the arms. In contrast, lower extremity vessels must accommodate greater vascular resistance owing to factors like gravitational effects and the need for greater blood supply to the large muscle groups involved in locomotion and weight-bearing activities.

While turbulence can indeed be a factor in lower extremity arteries and diameter differences can also exist, the fundamental reason for the difference in Doppler waveforms primarily stems from the resistance characteristics of the vascular systems in the upper versus lower extremities.