P. embolus vs pneumo thorax pathophys... please help.

[boardsbandit524]

I have the toughest time wrapping my head around the pathophys of these two subjects, please any help you can give me would be much appreciated:

P.Embolus
--------------------------------
PAO2 increases, PACO2 decreases (by tachypnea)
somehow PaO2 and PaCO2 simultaneously decrease (this according to a world question, which stated that often hyperventilation [ in other cases could also caused by edema or pneumonia] can present this way


Pneumothorax
-------------------------------------
PAO2 decreases, PACO2 increases (decreased ventilation due to collapse, tachypnea still present)
because of this PaO2 decreases, PaCO2 increases. '

The pnuemothorax makes sense to me, the P.E. part with the arterial blood gases does not, specifically why does PaCO2 decrease,? Do I have this right?

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[agranulocytosis]

PO2 decreases b/c of one of 4 causes of hypoxia is present here: a V/Q mismatch. A PE results in a dead space, thereby decreasing diffusing capacity and low PO2. But because of this, the patient will try to compensate by hyperventilating, decreasing PCO2. Remember that PCO2 is what determines PO2: i.e. if a person spontaneously hypoventilates, PCO2 increases which decreases PO2, and if a person spontaneously hyperventilates, PCO2 decreases, increasing PO2.

But in the case of PE, the capacity for diffusion is already impaired, so the PO2 can never reach "normal" levels, thus it remains permanently low until the PE is resolved.

 

[boardsbandit524]

any chance you could denote that with PA vs Pa, I'm having trouble distinguishing between arterial vs alveolar in your post... I'm trying though.

 

[Monica Lewinsky]

any chance you could denote that with PA vs Pa, I'm having trouble distinguishing between arterial vs alveolar in your post... I'm trying though.

I believe he is referring to arterial gas partial pressures only.

My question is what increases if PaC02 and Pa02 both decrease (pp of everything has to add up to 760)? Maybe I'm not thinking about this correctly.

 

[KCUMBscarCONTRT]

Ya, I think the 760 only applies to air in you are breathing in, not in your blood - I think when you have a large PE the reason that PaO2 and PaCO2 both decrease is related to the natural ability of the two gases to diffuse and equilibrate across the alveolar membrane - Oxygen diffuses more slowly and CO2 more quickly.

So if you suddenly block the blood flow to one lung then the blood flow to the other lung will be twice as large at a given cardiac output - and if the diameter of the vessels doesn't change much you can assume the velocity is twice as large. Thus, the blood has half as much time to equilibrate. CO2 is still able to diffuse across since it goes faster and since you are hyperventilating it is all blown off and thus the low PaCO2. O2 on the other hand doesn't have enough time to reach its max conc in blood so low PaO2.

In the pneumothorax i guess either you are just really not able to "hyperventilate" - at least not enough to blow off the CO2 - or you are still sending blood to the non/under-ventilated lung and this blood will be low in O2 and high in CO2 and will combine with the "normal" blood from the other lung and the product will still have a net decrease in O2 and increase in CO2. I guess in theory you'd be better off sending all that blood to the good lung - which I you try to do via hypoxic vasoconstriction - but I guess its not enough to snunt all of the blood to just one side . ???

 

[butaloh666]

In the pneumothorax i guess either you are just really not able to "hyperventilate" - at least not enough to blow off the CO2 - or you are still sending blood to the non/under-ventilated lung and this blood will be low in O2 and high in CO2 and will combine with the "normal" blood from the other lung and the product will still have a net decrease in O2 and increase in CO2. I guess in theory you'd be better off sending all that blood to the good lung - which I you try to do via hypoxic vasoconstriction - but I guess its not enough to snunt all of the blood to just one side . ???

I agree with your last sentence. Your lungs can try to divert as much blood flow away from the collapsed lung or lobes, but you just can't completely stop perfusing the region. So blood flow through those areas of the lung will continue, which results in shunting.

 

[boardsbandit524]

beautiful, thanks.