Ventilation/Perfusion Ratio & Gas Exchange

[wAyRadikull]

I was flipping through the BRS earlier this week and came across the V/Q ratio section which kinda puzzled me a bit. BRS/Costanzo claims that there is more gas exchange in the apex due to a higher V/Q ratio.. where as my professor says that gas exchange is higher in the base due to a "closer to 1 ratio".... i checked a few other sources but didnt get a clear cut answer..

I'm tempted to stick with costanzo/brs just because.. but my class mates are "all about the lecture notes"

Can anyone shed some light on this and clarify it for me a bit???

Thanks.

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[FutureInternist]

Disclaimer - It's been 3 years since my physio class 🙂

I always thought that alveoli were most compliant when they were closed (hence making them very easy to open up). Given that the alveoli at the bottom of the lung are collapsed d/t gravity & given the fact that there is more blood flow to the bottom of the lung, I would think that the greatest gas exchange would be at the bottom.

 

[braluk]

I always understood the base of the lung as having the greatest gas exchange given that the ratio is nearly 1 to 1. The only reason why V/Q is high at the apex is because Q is pretty low, with some locations reaching nearly infinity given that Q is nearly zero. In that aspect the base would have the highest rate of gas exchange. I think the apex of the lung, however, will see the most amount of ventilation though.

 

[EMCC]

This basically comes down to the West zones of the lung. If you remember, the 3 zones are highly ventilated (zone 1), equal ventilation/perfusion (zone 2), and highly perfused (zone 3). Hence, the most "air" goes to zone 1, and the most blood goes to zone 3.

The V/Q ratio is closest to one in zone 2. In zone 3, the ratio approaches zero. In zone 1, the ratio approaches infiniti.

Does that make sense? Let me know...

 

[wAyRadikull]

Yeah thats what i was looking for: gas exchanges in terms of the three zones and V/Q ratio. So i guess it makes sense that the zone 1 has highest ventilation, zone 2 has equal V and Q and zone 3 has highest perfusion... so logically 1 is 1 would be met right in the middle.

So.. equal V/Q= more gas exchange= zone 2

Good deal. Let me know if there are any other physio geniuses out there who would like to get in on this...👍

 

[JohanK]

The correct answer is that the most gas exchange occurs at the bottom of the lung as some in this thread has correctly pointed out. I don't know what the author of that book has been smoking.

The apex of the lung is the least ventilated part of the lung due to the fact that the compliance of the lung is not constant but instead decreases as the pressure gradient over the alveolar wall increases. Due to the weight of the lung, the interstitial pressure is higher in the base of the lung. As the air pressure in the alveoli is constant, this means that the pressure gradient is lower at the base and hence this part of the lung is better ventilated.

The high V/Q at the apex is the equivalent of a physiological dead space which obviously impairs the gas exchange in this part of the lung as part of the small amount of ventilation that goes to this part of the lung is wasted. However I fail to see how the low V/Q at the base of the lung impairs the efficiency in the utilization of the ventilated air. Rather the lack of ventilation makes the perfusion inefficient resulting in a physiologic shunt. Hence the low V/Q at the base, even if it impairs the efficiency of the lung as a whole due to wasted perfusion, actually shows that the air that is ventilated is used more efficiently at the base than in the other parts of the lung. This is illustrated by the fact that the PCO2 and PO2 here is closer to the values in the venous blood.*

As the base of the lung, which is the best ventilated part of the lung, also extracts more oxygen out of each unit of ventilated air, we can conclude that most of the gas exchange occurs at the base and not the apex of the lung. It is not, however, due to a V/Q that is closer to 1, as it is in fact less than 1 at the bottom of the lung.

*As the PO2 in the inspired air is constant, a lower PO2 in the alveoli shows that more of the oxygen in the exchanged air has been absorbed by the blood, as the amount of oxygen in the air is proportional to the partial pressure of oxygen.

 

[lacrossefiend73]

In the upper region of the lung, there is less ventilation (as compared to the lower region) since your intrapleural pressure is more negative and your alveoli are more distended causing less compliance. There is also less perfusion (as compared to the lower region) since your intravascular pressure is lower, you have less distention and recruitment, and a higher resistance (causing less flow). So, the lower regions of the lung have higher ventilation and higher perfusion.

What is affecting the V/Q ratio are the gradients of perfusion and ventilation. The blood flow gradient from the bottom of the lungs to the apex is steeper than the ventilation gradient, therefore Q is what mostly changes the ratio.

V/Q will be >1 at the apex since there is more ventilation than perfusion while it will be <1 at the base since there is more perfusion than ventilation. Since there is lower flow (no flow in Zone 1) at the top, more gas exchange occurs at the bottom.

I probably just said what everyone else said... but what the heck.

 

[Dirt]

I was flipping through the BRS earlier this week and came across the V/Q ratio section which kinda puzzled me a bit. BRS/Costanzo claims that there is more gas exchange in the apex due to a higher V/Q ratio.. where as my professor says that gas exchange is higher in the base due to a "closer to 1 ratio".... i checked a few other sources but didnt get a clear cut answer..

I'm tempted to stick with costanzo/brs just because.. but my class mates are "all about the lecture notes"

Can anyone shed some light on this and clarify it for me a bit???

Thanks.

Depends on what you mean by gas exchange.

Overall there is more gas exchange happening at the base for reasons mentioned above. But, in zone 1, there is more gas exchange per unit of blood. In other words, the blood leaving zone 1 has the highest PO2.

V/Q ratio is a means of representing gas exchange/per unit of blood. In zone 1 you have the highest gas exchange/per unit blood. In zone 3 you have the lowest gas exchange/ per unit blood, but since there is such a larger amount of blood going through it, it ends up having the most total gas exchange. In zone 2, you have the optimal ratio.

So, the book is talking about gas exchange per unit of blood, which is highest in zone 1. Your professor is talking about overall gas exchange which is highest in zone 3, although I don't know what he is talking about with regard to zone 3 having a V/Q closest to 1, zone 2 is closest to 1.

 

[NRAI2001]

Top of the lung: High Ventilation/perfusion = mainly bc there is little perfusion at the top of the lung mainly bc of gravity. Interpleural pressure always stays negative so that alveoli never collapse (low compliance bc little volume changes for pressure changes).

Bottom of the lung: lower ventillation/pefusion "ratio" bc of greater perfusion. The lower Va (alveolar ventillation) doesnt effect oxygenation bc even at P-oxygen of 60mmhg there is over 90% HB saturation.

***compliance of alveoli is higher in the lower lung which leads to greater volume changes and maybe considered as "greater ventillation"; interpleural pressure actually reaches positive values at bottom of lung allowing it to actually collapse and then re-expand during inspiration (larger volume changes per given pressure).


As for the zones, zone 1 doesnt exist in normal patients. Zone 3 grows because of distention and recruitment of BVs during excercise so that perfusion of the overall lung increases.

Correct me if I m wrong bc I only read this stuff once last week👍

 

[TheRealMD]

I was flipping through the BRS earlier this week and came across the V/Q ratio section which kinda puzzled me a bit. BRS/Costanzo claims that there is more gas exchange in the apex due to a higher V/Q ratio.. where as my professor says that gas exchange is higher in the base due to a "closer to 1 ratio".... i checked a few other sources but didnt get a clear cut answer..

I'm tempted to stick with costanzo/brs just because.. but my class mates are "all about the lecture notes"

Can anyone shed some light on this and clarify it for me a bit???

Thanks.

Higher V/Q != better gas exchange. The best gas exchange occurs when V/Q is equal to 1 because one variable (i.e. air) isn't out of whack in comparison to the other (blood) and vice versa.

And like other people have said, V/Q is highest in the apex of the lungs, but not because there is more air there, but because it has the least amount of perfusion due to gravity. Most gas exchange still takes place at eh base of the lung.

 

[NRAI2001]

I would always go with the profs answer bc he is the one that determines if you move forward to the next term or repeat the course.. if you really have a big issue with his answer you could always go ask him about how his answer compares to the BRS answer, it could be and likely is the same answer as BRS but just stated in a different way.

 

[coldweatherblue]

both values of perfusion (Q) and ventilation (V) are highest at the base of the lung and lowest at the apex. However, there is much more difference in Q because of gravity so the V/Q ratio will be highest at the apex of the lung. This means that arterial blood flowing through the apex of the lung will have a greater partial pressure of oxygen and a lower partial pressure of carbon dioxide compared with the base. The ratio of gas exchange is highest at the apex; I think this is what BRS means by "more gas exchange".

Overall I think everyone would agree that most of the oxygen the body needs for daily metabolism comes from gas exchange at the base (Zones II, III) of the lung. This is what your professor is trying to say.