The Venturi effect and cerebrovascular ultrasound

One of the most frequent diagnostic pitfalls in cerebrovascular ultrasound is the evaluation of the vertebrobasilar system in the case of vertebral artery (VA) hypoplasia because the diagnostic findings are sometimes suggestive of significant pathologies, but may be completely normal as well. The Venturi effect is a phenomenon of hydrodynamics which describes a drop in hydrostatic pressure along areas of high flow velocities.

We review the physical background of the Venturi effect and describe how it can explain both physiological and pathological flow phenomena encountered during cerebrovascular ultrasound investigations. We focused on flow phenomena at the vertebrobasilar junction by theoretically calculating conditions with different influx angles and flow velocities on two-dimensional digital subtraction angiography images. In the discussion, we also address other areas of cerebrovascular ultrasound, where the Venturi effect is of particular interest.

The blood flow velocity within the VAs and the angle between a hypoplastic VA and the basilar artery (BA) are important determinants of the intraluminal pressures of these arteries and thus their resistances. In the case of angles below 90 degrees between the distal VA and the BA, abnormal extracranial flow profiles may suggest downstream VA stenosis or occlusion, whereas in the case of angles above 90 degrees, a diagnosis of an AV fistula downstream to the V4 segment may be assumed. Furthermore, various hemodynamic effects at the site of a stenosis of the brain supplying arteries can also be explained by a transstenotic pressure drop created by the Venturi effect, particularly the generation of musical murmurs. The systolic dip ('systolic deceleration') at the origin of the VA due to subclavian artery stenosis is also a consequence of the Venturi effect.