NBME 18 - Respiratory physiology question

[zeevee]

This is a simple question but recently I have started mixing up the terms .May be someone can help me here .

Following an operation ,a 65 year old patient has a lung region that is underventilated but well perfused.The condition will lead to an increase in which of the following ?

a) alveolar dead space
b) physiological dead space
c) anatomic dead space
d) Physiological shunt
e) PO2

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[jqueb29]

Physiological dead space = anatomic dead space + alveolar dead space. So if A increased, by definition B would have to increase as well. And if C increased, B would have to increase as well. Thus we can rule out A, B, and C.

E can be ruled out as well because obviously we're not going to get a greater PO2 when there is underventilation.

That leaves us with D. The best way to think of physiologic shunt, in my opinion, is that it is the amount of deoxygenated blood that enters the left ventricle. Normally, we have a small physiologic shunt that arises from bronchial artery blood being returned to the LV, as well as some coronary artery veins that drain into the LV. In the case described in the question, we will now have more physiologic shunt, as blood flows through this underventilated capillary and on in to the LV. The blood flow through this capillary will obviously be decreased due to hypoxic vasoconstriction, but this doesn't entirely eliminate flow through that capillary, so there is an increase in physiologic shunt as this blood cannot be oxygenated adequately.

 

[zeevee]

Physiological dead space = anatomic dead space + alveolar dead space. So if A increased, by definition B would have to increase as well. And if C increased, B would have to increase as well. Thus we can rule out A, B, and C.

E can be ruled out as well because obviously we're not going to get a greater PO2 when there is underventilation.

That leaves us with D. The best way to think of physiologic shunt, in my opinion, is that it is the amount of deoxygenated blood that enters the left ventricle. Normally, we have a small physiologic shunt that arises from bronchial artery blood being returned to the LV, as well as some coronary artery veins that drain into the LV. In the case described in the question, we will now have more physiologic shunt, as blood flows through this underventilated capillary and on in to the LV. The blood flow through this capillary will obviously be decreased due to hypoxic vasoconstriction, but this doesn't entirely eliminate flow through that capillary, so there is an increase in physiologic shunt as this blood cannot be oxygenated adequately.

Thanks bro
Makes sense .

I think my confusion arises from the fact that we also call a pulmonary embolism a shunt though it is an well ventilated underperfused area ; opposite to what the question has

 

[jqueb29]

Thanks bro
Makes sense .

I think my confusion arises from the fact that we also call a pulmonary embolism a shunt though it is an well ventilated underperfused area ; opposite to what the question has

Yeah I agree, this area can be kind of tricky. It's one of those things where the science behind everything makes perfect sense, but they decided they had to make things difficult somehow so they decided to use confusing terminology lol