Pulmonary question

[LuckiestOne]

Patients X and Y both have respirations of 12/min
and tidal volumes of 500 mL. However, patient X
has an arterial PCO 2 of 35 mm Hg, while patient Y
has an arterial PCO 2 of 45 mm Hg. The difference in
PCO 2 between these patients is best explained by a
difference in which of the following?


(A) Cardiac output
(B) Dead space ventilation
(C) Forced expiratory volume in 1 sec (FEV 1 )
(D) Functional residual capacity
(E) Lung compliance

What do you guys think the answer is and can you please provide explanation justifying your answer? Thanks!

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[Phloston]

Patients X and Y both have respirations of 12/min
and tidal volumes of 500 mL. However, patient X
has an arterial PCO 2 of 35 mm Hg, while patient Y
has an arterial PCO 2 of 45 mm Hg. The difference in
PCO 2 between these patients is best explained by a
difference in which of the following?


(A) Cardiac output
(B) Dead space ventilation
(C) Forced expiratory volume in 1 sec (FEV 1 )
(D) Functional residual capacity
(E) Lung compliance

What do you guys think the answer is and can you please provide explanation justifying your answer? Thanks!

Dead space volume = Tidal volume x [(PaCO2 - PeCO2) / PaCO2]

Therefore:

[(Dead space volume x PaCO2) / Tidal volume] + PeCO2 = PaCO2

Therefore:

For PaCO2 to differ between patients, you know it's because of either tidal volume, dead space or expired CO2.

Because tidal volume and respiratory rate are the same, you know it's dead space that must be different.

Therefore, I'd go with choice B.

 

[LuckiestOne]

Thanks, besides cranking the equations, what is a conceptual way to look at this? What is actually causing the CO2 level to be different? It probably isnt' due to lower diffusion rate b/c CO2 is very permeable... I would also think that the arteriolar to alveolar CO2 gradient is similar in both cases since atmospheric CO2 level is pretty low...

 

[Myxedema]

What is actually causing the CO2 level to be different?

Without more information, it's not possible to answer this. This may be an appropriate response to metabolic alkalosis. Or this may be due to hypoventilation due to some pathology like CNS depression (morphine, barbiturates) or diaphragm paralysis.

 

[Phloston]

The same tidal volume and respiratory rate, but different PaCO2, means that the same level of gas exchange is not occurring.

There can be quite a few reasons for reduced gas exchange, such as diminished surface area, increased diffusion distance, etc. We don't know his specific pathology, but we do know that his dead space is increased.

Kaplan QBook had a good question on dead space in one of the physiology sections IIRC.