Why does 100% oxygen help patients with dead space but not pulmonary shunting?

[TheEugenius]

Everything seems to make sense until you take into account hypoxic pulmonary vasoconstriction.

For the sake of simplicity, imagine one entire lung has either dead space or pulmonary shunting and the other one is normal.

In the case of dead space, the diseased lung will not have any perfusion. All that blood that is supposed to flow through that lung will flow through the normal lung. The normal lung will thus have 2x as much blood flowing through it. As a result, giving a patient in this situation 100% oxygen will benefit them.

In the case of pulmonary shunting, the diseased lung will not have any ventilation. One may argue that both lungs will receive the same amount of blood as usual and thus, giving a patient 100% oxygen in this situation will not benefit them since the normal lung will have 1x amount of blood and will not be able to "extract" any more oxygen from its alveoli than it already does.

However, if you consider the pulmonary vasoconstriction that occurs under hypoxic conditions, all of this seems to stop making sense.

Imagine the patient with pulmonary shunting in one entire lung. One lung is not receiving any ventilation. As a result, would that not cause the vessels in that lung to vasoconstrict and send more blood to the normal lung? And if it does, would 100% oxygen not help this patient if the normal lung will be receiving 2x as much blood (for example) - which is the same scenario in the dead space situation?

Thank you

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[Professional Student]

This may not be correct but this is how I currently understand it so that it makes sense. I think you're missing the point that pulmonary shunting is a pathologic condition where an area of the lung that is not receiving any ventilation (like if it's filled with fluid) but is still receiving perfusion. Since it is by definition a pathologic condition, hypoxic vasoconstriction is either not occurring or not adequate (I don't know why not). This means your V/Q=0, which is why 100% O2 is not helpful. In dead space, hypoxic vasoconstriction is a physiologic response to a stressor. In functioning lung portions, O2 acquisition switches from normal perfusion-limited to abnormal diffusion-limited due to extra blood flow. Extra O2 helps in times of diffusion-limited O2 acquisition.

Basically, since pulmonary shunting is by definition a pathologic condition where there is perfusion of lung tissue that is not ventilating, I don't think you can consider hypoxic vasoconstriction since that is literally the physiologic antithesis of pulmonary shunting. I think a better question would be "Why does hypoxic vasoconstriction not occur with pulmonary shunting?", and I don't know the answer to that. It might be because the system is just too overwhelmed to respond appropriately (as in basically every pathologic condition). I'm sure the answer is multivariable and dependent on the specific insult.

 

[Deecee2DO]

In shunting you are perfusing areas that are not being ventilated so you get those deoxygenated vessels causing admixture with well oxygenated vessels bringing the O2 levels down in the blood. These well ventilated areas are already extracting the max amount of O2 they can so giving more O2 will not improve it since its maxed out and also the areas that are not being ventilated can't exchange gas anyways. However with dead space you have wasted air ventilating poorly perfused alveoli so this blood gets diverted to well perfused areas and causes a huge increase in Q in relation to V/Q leading to high Q in the denominator (V/Q mismatch). In order to normalize this V/Q mismatch you need to increase V so that V will catch up to the increased Q and you can do this by giving O2