USMLE Does Step 1 distinguish between shunting in upper lobes vs lower?

[Bryan Manzana]

Hi! Long time lurker, about to sit for step 1 in about a couple of weeks. IMG grad so a bit far off from my basic sciences; highly dependent on high yield review materials.

My question as stated: since upper lobes vs lower lobes have different v/q ratios, does the USMLE expect you to know changes in physiology (paO2, pCO2, correctible with 100% O2 or not) should say a fairly massive shunt occur in the upper lobes vs the lower lobes? Is there even a difference at all or am I just imagining that there might be?

Asking because I know that what's written in FA is an oversimplification. Going through previous threads kinda helped but not tackling my particular concern.

Thanks!

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[walakin25]

Hi! Long time lurker, about to sit for step 1 in about a couple of weeks. IMG grad so a bit far off from my basic sciences; highly dependent on high yield review materials.

My question as stated: since upper lobes vs lower lobes have different v/q ratios, does the USMLE expect you to know changes in physiology (paO2, pCO2, correctible with 100% O2 or not) should say a fairly massive shunt occur in the upper lobes vs the lower lobes? Is there even a difference at all or am I just imagining that there might be?

Asking because I know that what's written in FA is an oversimplification. Going through previous threads kinda helped but not tackling my particular concern.

Thanks!

I can help you. Inbox me.

 

[worldbeater]

I haven't seen any questions regarding shunting, its more that the lungs are the only area in the body where vasoconstriction occurs during hypoxia, in order to perfuse as many lobes as possible to keep the lungs working. They will put up a chart on World demonstrating this.

For ventilation, the big picture is that the top of the lobes are always ventilated while the bottom lobes are ventilated at specific rate. So if your breathing at 16 breaths per minute, the bottom lobes are ventilated 16 times in one minute, while the top is ventilated 60 times in one minute.