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#1
05-13-2015
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Q. o2 tension in upper lobe vs lower lower lobe

In first aid & U-world it is mentioned that both ventiltion and perfusion are greater at the base of the lung than at the apex of the lung due to the effects of gravity. But again, in the context of secondary (reactivation) tuberculosis, it is mentioned that bacteria establish infection in the apex due to increased oxygen tension . My understanding was oxygen has tendency to go up, which correlates well with the Mycobacterium TB lodging in the upper lobe . But, these contrasting explanations in U-world made me very confused.Is oxygen tension more in upper lobe or lower lobe? Can anybody pls clear my confusion with explanation addressing the above scenario mentioned.Thanks.

#2
05-13-2015
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I guess the answer to this question lies in the fact that both ventilation and perfusion individually are greater in the lower lobe than to what in the upper lobe ie say,(random values for explanation)

Upper Lobe : Ventilation = 100 , Perfusion = 20
Lower Lobe : ventilation = 120 , Perfusion = 60

Although both are individually high that what in the upper lobe but the ratio varies greatly ie,

Ratio(upper lobe)= 100/20 = 5
Ratio(lower lobe)= 120/60 = 2

Since the ventilationerfusion ratio is high at the upper lobe so less amount of oxygen coming to the upper lobe is actually being taking up the blood so excess amount of unused oxygen prevails in the upper lobe which is utilized by the tubeculous bacilli in secondary tuberculosis.

Taking about real numbers if V/Q ratio of lower lobe is 1 (4.8L gas exchange,5L blood coming via pulmonary artery) , V/Q of upper lobe is 3, so there is 3x more oxygen available to the bacilli for use, hope this helps

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#3
05-13-2015
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Wonderful explanation clever fox...thanks aumsai because I also had the same q in my mind...now it's clear.
#4
05-13-2015
 USMLE Forums Addict Steps History: --- Posts: 137 Threads: 62 Thanked 3 Times in 3 Posts Reputation: 13

Sorry, I am not still very clear. Just wanted to confirm what I understood is right or not. From your explanation what I understood is-
Even though the Oxygen tension is higher at the lower lobe, but that O2 is utilized during gaseous exchange. But, O2 at upper lobe is not utilized as compared to lower lobe because perfusion is less at the upper lobe than lower lobe due to gravity. Thus,this unused O2 at the upper lobe is utilized by Mycobacterium TB.

Pls let me know if my understanding is correct or not?

Thanks a lot.
#5
05-13-2015
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 237 Threads: 4 Thanked 160 Times in 114 Posts Reputation: 172

Yep that's what I said...u made the summary without beating around the bush =)
#6
05-13-2015
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If that is the case then why not bacteria deposit in upper lobe in primary tb also? Why in primary TB, bacteria prefers lower lobe and why in secondary TB, bacteria prefers upper lobe? Thanks for patiently helping to solve my doubt.
#7
05-13-2015
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 237 Threads: 4 Thanked 160 Times in 114 Posts Reputation: 172

Quote:
 Originally Posted by Aumsai If that is the case then why not bacteria deposit in upper lobe in primary tb also? Why in primary TB, bacteria prefers lower lobe and why in secondary TB, bacteria prefers upper lobe? Thanks for patiently helping to solve my doubt.
Look I am not an expert but I think I read the same thing what your asking in CMMRS(Gladwin) what I think is that the most airflow is that to the lower lobe so Bacteria comes and deposits there(what you said gravity effect) and does primary TB and if it gets chance of recurrence it goes to the upper lobe and colonizes there.This is my logic since nothing is clearly given in CMMRS also.
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#8
05-13-2015
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Yes, you are right. Sometimes, it is better to accept the fact because everything doesn't have explanation as well.Thanks cleverfox.

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