We are occasionally asked why we designed our probe heads with a non-compressive fit.

“Wouldn’t you want the probe to grasp the vessel to hold it firmly in place?” they will ask.

No, as a clinician or an engineer, you would not. To understand this answer, we need only to understand the basics of flow itself.

There are two types of flow: laminar and turbulent. In laminar flow, everything (fluids, erythrocytes, solutes) is moving parallel in neat, even rows, like six lanes of interstate traffic, with every car moving in sync at the same speed. But you’ve driven on enough interstates to know that’s rarely the case. Anything can disturb the harmony: an exit, loss of a lane, a piece of cardboard in the road… cars begin jockeying for position, braking, accelerating.

Blood flow works the same way. Most disturbances transform laminar flow into turbulent. Eddies form, blood begins to swirl and pool, slowing in one area and accelerating in another. Instead of a continuous current, multiple currents of differing velocities form—and this creates what is known as shearing force. If you think back to highschool physics, you may recall the four fundamental internal forces: flexion, tension, compression, and shear.

Though we usually think of them in relation to solid members, shear also occurs in fluids, and it is not an occurrence to be overlooked. Shearing can do anything from inducing distention (aneurysms, we’re looking at you) to causing vessel proliferation (the body assumes the shear is occurring because pressure needs to be relieved.) Turbulence, and the shearing it causes, wreak havoc on speeds, volumes, and thereby the overall flows that those two factors combine to create.

So here’s the question: how well could you predict interstate traffic flow if the cars were running in laminar fashion: all together, same speed and direction? You could probably gauge it with precision. Conversely, how well would you do if the cars were moving in a turbulent fashion? Swerving around, changing lanes, accelerating then braking, suddenly impeding the traffic behind? Your assessment could be no less haphazard than the traffic itself.

Selling non-constrictive probes isn’t a marketing gimmick. Fluids are incompressible, so when the vessel is compressed, flow patterns are necessarily distorted. That is turbulence. Consider a simple kitchen analogy. How well would a measuring cup work if it automatically compacted the ingredients to an unknown degree every time you put them in it? Or an example from your workshop: do you think you could build a proper bookshelf if your tape measure compressed each plank to a random length every time you measured?

The primary tenant of a good measurement device is that it does not alter the object it is measuring.

We build non-compressive probes because that is what Mother Nature intended!

Thanks for reading,

             Transonic Systems, Inc.

                                 The Measure of Better Results