100% O2 and V/Q

[usmlesuccess]

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Why does 100% O2 NOT improve oxygenation when there is a shunt, v/q=0

 

[MrBeauregard]

Because even though the patient is inhaling 100% oxygen, deoxygenated (venous) blood is being shunted into the arterial circulation, thus depressing the oxygen content. No amount of oxygen inhalation will reverse the hypoxemia because the actual cause of the hypoxemia, the shunt, is still present.

 

[HelpPleaseMD]

Why does 100% O2 NOT improve oxygenation when there is a shunt, v/q=0

cause the V = O meaning no oxygen gets there. Let's say a ball got stuck there and no air can pass so it makes no difference how much oxygen you give the patient

 

[frankyazz]

If V/Q = 0, that means ventilation is zero. This means that inhaled oxygen is not becoming involved in gas exchange because it is not coming into contact with perfused areas. The lungs will respond by vasconstricting pulmonary vasculature in poorly ventilated areas, thus shunting blood to well ventiliated areas so they can participate in gas exchange. The purpose of shunting is to help improve V/Q mismatch by not wasting perfusion to areas that are not ventilated.

A way you can think of it....If you have an obstruction, areas of the lung will not be ventilated (V=0), but will be perfused. Stating the obvious, no air/oxygen is getting through to participate in gas exchange. So no matter what percentage of oxygen you give, its not going to be able to hit perfused areas and improve oxygenation.

 

[usmlesuccess]

So when there is a blockage, q=0, then the reason that 100% oxygen improves arterial o2 is because that blood is shunted to well ventilated areas?

 

[HelpPleaseMD]

Yes blood will be shunted to other areas, but at least from my understanding the aterial 02 wont improve significantly cause the hemoglobin are already saturated at normal O2 concentrations.