pulmonary shunting

This is more a terminology question than anything else, as I actually get the underlying processes here.

Basically, when we have a situation like a blocked airway, the V/Q ratio approaches 0. There’s ample perfusion (Q), but the ventilation is blocked (V). This is called a shunt.

My question - why the heck is it called a shunt? It seems backwards to me!

A shunt is “is a hole or a small passage which moves, or allows movement of fluid from one part of the body to another.” In this case, the problem is that fluid (air) is NOT moving to where it needs.

And yet, a pulmonary shunt exists “when there is normal perfusion to an alveolus, but ventilation fails to supply the perfused region.”

I can kind of fathom it if I think that the blood is shunted away from the unventilated region of the lung (vasoconstriction in response to hypoxemia; the lung is different from the rest of the body in this regard). That seems like a stetch, though.

Is there something obvious I’m missing about why the term “shunt” is used to describe this?

Thanks for any help!


It sounds like what you are specifically describing is an absolute intrapulmonary shunt. This is different than a right-to-left anatomic shunt. Most right-to-left anatomic shunts seem to me to occur outside the lungs, such as in the heart or great vessels. The causes of a right-to-left anatomic shunt outside of the lungs can be either a hole (like a patent foramen ovale in the heart) or a small passage (like a patent ductus arteriosus between the left pulmonary artery & aortic arch).

An absolute intrapulmonary shunt is caused by blockage in an airway supplying alveoli. However, there can be right-to-left anatomic shunts in the lungs as well, but these are usually caused by blockage in a blood vessel supplying alveoli rather than by a hole or small passage as occurs outside the lungs.

I understand your confusion. I’m not exactly sure why intrapulmonary shunts have a definition that involves a blockage and extrapulmonary shunts have a definition that involves a hole or a small passage. Either way the net effect is that deoxygenated blood gets mixed with oxygenated blood. I’m not really sure if I answered your question or made you more confused.

DISCLAIMER: I’m not a doctor, or even a medical student. But I am a technical writer, so dealing with words is my stock in trade.

A shunt is a short-circuit of some sort. In electronics, a shunt is literally an alternate current path, or short-circuit. In medicine, the term “shunt” typically refers to communication between normally separate anatomical passages or regions.

The usage of “shunt” in this case seems to refer to the fact that blood is being “shunted” past the lungs – that is, blood does not get oxygenated in the alveoli. This is typically caused when the alveoli are filled with fluid, thus blocking the oxygen from reaching the blood. But it appears that any mechanism that allows the blood to bypass the oxygenation process as if shunted past the alveoli is called a “pulmonary shunt”.

Hope that helps.

Thanks, Tic and spox. As best I can tell, the only “shunting” that goes on (as the word is used in the rest of medicine ,and in engineering, as spox mentions) is that the blood is shunted away from the hypoperfused region. I guess that’ll have to do