Is emphysema physiologic dead space or shunt?

Answer 1

Emphysema is a shunt my friend. Shunts are diffusion limited by definition. There is a limitation to diffusion because the endothelial alveolar interface is destroyed in emphysema. Several factors go into diffusion. The permeability of the membrane/solubility of the gas, the thickness of this interface, the surface area available for exchange, and the difference in partial pressure of the gas of interest on both sides (in the blood vessel and the alveolus). In emph., SA is diminished (there is little fibrosis in emph., it's not a thickness issue). We say that surface area is diminished because of less healthy lung tissue participating in gas exchange for the reasons I just mentioned. Also think of pulmonary fibrosis, where thickness is increased. Such people cannot be treated with 100% O2 very successfully. You are not doing anything to ameliorate the initial problem: bad lungs. Blasting them with O2 will do little.

Dead space or perfusion limitation on the other hand does respond to 100% O2. In the case of a pulmonary embolism, there is perfusion limitation. This is tricky, because perfusion limitation shows equilibration of gases by the end of the capillary, so what good would adding more O2 do? Again, increasing the partial pressure difference across HEALTHY lung (in this instance) will cause a higher oxygen saturation than typical in the remaining capillary beds not blocked by the embolus. That is why 100% O2 will treat a pulmonary embolus or anything perfusion limited, assuming that you have less than 100% dead space.

I literally know nothing more on the subject.

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