V/Q mismatch

[soon2Bmd2005]

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i can't seem to understand V/Q mismatch...first aid picture (2007 edition) shows that ventilation is relatively higher at apex of lungs and perfusion is relatively higher at the base. however...both ventilation and perfusion are highest at the base. so why does TB reactivation occur at the apex if there's more total oxygen at the base? and also what do the Pa > PA > Pv etc. mean at each of the levels? i've never been able to understand this concept...and i guess a week before the exam would be a good time to start. any help would be appreciated. thanks!

 

[medhacker]

i can't seem to understand V/Q mismatch...first aid picture (2007 edition) shows that ventilation is relatively higher at apex of lungs and perfusion is relatively higher at the base.

Both are higher at the base.

however...both ventilation and perfusion are highest at the base.

Yes, because of gravity

so why does TB reactivation occur at the apex if there's more total oxygen at the base?

There's more ventilation at the base but O2 tension is highest at the apex

and also what do the Pa > PA > Pv etc. mean at each of the levels? i've never been able to understand this concept...and i guess a week before the exam would be a good time to start. any help would be appreciated. thanks!

Pa = arterial partial pressure
PA = alveolar partial pressure
Pv = not sure

 

[Blesbok]

i can't seem to understand V/Q mismatch...first aid picture (2007 edition) shows that ventilation is relatively higher at apex of lungs and perfusion is relatively higher at the base. however...both ventilation and perfusion are highest at the base. so why does TB reactivation occur at the apex if there's more total oxygen at the base? and also what do the Pa > PA > Pv etc. mean at each of the levels? i've never been able to understand this concept...and i guess a week before the exam would be a good time to start. any help would be appreciated. thanks!

Although they are both higher at the base, the difference in ventilation in very minimal, where the difference in perfusion is significant. Therefore the V/Q is higher in the apex than in the base.

 

[78222]

i can't seem to understand V/Q mismatch...first aid picture (2007 edition) shows that ventilation is relatively higher at apex of lungs and perfusion is relatively higher at the base. however...both ventilation and perfusion are highest at the base. so why does TB reactivation occur at the apex if there's more total oxygen at the base? and also what do the Pa > PA > Pv etc. mean at each of the levels? i've never been able to understand this concept...and i guess a week before the exam would be a good time to start. any help would be appreciated. thanks!

I reviewed this last week so hopefully I get this right:

TB prefers areas with low blood flow/high ventilation which is why it chooses the apex preferably.

Pa/PA/Pv relate to the pressure at each level. In the Apex, PA is highest which means there is more alveolar pressure than arterial/veinular pressure. The significance of this is that because Alveolar pressure is higher than Pa, the alveoli can compress the artery in cases of positive pressure (i.e. PEEP) - this would cause a V/Q mismatch since the alveoli would continue to be ventilated without any perfusion.
At the base of the lungs, gravity increases the Pa and Pv.

 

[USArmyDoc]

Directly from BRS Physio:

Perfusion highest at base due to gravitational forces. (makes sense)

Ventilation--> highest at base (makes sense)

At apex: Po2 is highest and Pco2 is lower because there is more gas exchange

At base: Po2 is lowest and Pco2 higher b/c there is less gas exchange.

I guess what they are saying is the more gas exhange the lower the Pco2 b/c it is being exhaled.