physiology q2 - katy2011

[ArchivalUser]

3. As a result of a stab wound to the lower left chest, an
otherwise normal 47-year-old man suddenly develops a
regional but significant atelectasis in the underlying
lung region. Which of the following combinations of
gas pressures (in mm Hg) is most consistent with that
condition?
alv alv sys sys
pco2 po2 artpco2 artpo2
(A) 32 110 32 109
(B) 39 101 39 98
© 32 110 39 60
(D) 35 98 33 52
(E) 50 88 50 87

i dont have the answer for this...so please explain

[ArchivalUser]

It is C. ,atelectasis ,produce shunt I.e blood pass without oxygenation, no division of o2 from alv to blood.

[ArchivalUser]

DD.

Alv pCO2 35 mmHg
Alv pO2 98 mmHg
Arterial pCO2 33 mmHg
Arterial pO2 52 mmHg

Atelectasis results in a V/Q mismatch and a left-to-right shunt is created. Thus, there will be hypoxemia with increase in (A-a) gradient. So choices A, B, and E are out.

Also, in patients with atelectasis, there will be a certain degree of dyspnea. More CO2 is eliminated. Therefore, systemic arterial pCO2 will be lower than the Alveolar pCO2.

So I would go with choice "D" as the answer.

Good Question. Thank you.

[ArchivalUser]

I agree with psychmledr, right asnwer is DD. I remember clearly reading that CO2 stays normal or goes down in pulmonary shunt bc of increased alveolar ventilation. I also remember and not understanding it, so please, some more explanation psychmledr:

If O2 cannot increase because of the shunt (that part I get , how come increased ventilation can decrease CO2? I would imagine that if there is no O2 exchange, than there is no CO2 exchange either. What I am missing?

[ArchivalUser]

@ psychmledr
the shunt is RIGHT to LEFT.
got me confused for a sec. lol. but great explanation

[ArchivalUser]

@interferon_advocate:

You are right. It should be RIGHT-TO-LEFT shunt. Thank you for correcting the error.

***In addition, I believe the answer should be "C" because in this case, and the Q stem refers to the atelectasis as "significant," there is a decreased exchange of CO2 and O2 between the Alveolar air and the blood.
Therefore, the PaCO2 can be expected to be higher than the PACO2, while the PaO2 would be lower than the PAO2.
A: Alveolar
a: arterial