V/Q mismatch or shunt

[Newyorkgiants]

I know this is a step 1 topic but UWorld and Step up to medicine are differing on which pathology result in a V/Q mismatch and shunt. UWorld is saying that Atelectasia, pulmonary edema, and pneumonia result in a mismatch while step up is saying that it causes a shunt. Which one is correct and can you explain the reasoning behind it.

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[DrBowtie]

I know this is a step 1 topic but UWorld and Step up to medicine are differing on which pathology result in a V/Q mismatch and shunt. UWorld is saying that Atelectasia, pulmonary edema, and pneumonia result in a mismatch while step up is saying that it causes a shunt. Which one is correct and can you explain the reasoning behind it.

Could you articulate your question more specific? Are you asking which things cause ventilation defects? Are you asking what causes perfusion defects? There are two sides to the VQ mismatch.

 

[Ismet]

I know this is a step 1 topic but UWorld and Step up to medicine are differing on which pathology result in a V/Q mismatch and shunt. UWorld is saying that Atelectasia, pulmonary edema, and pneumonia result in a mismatch while step up is saying that it causes a shunt. Which one is correct and can you explain the reasoning behind it.

V/Q mismatch is a broader term that can be referring to a defect in ventilation or a defect in perfusion. A shunt is a defect in ventilation (atelectasis because the alveoli are collapsed and thus not filling with air, and pneumonia/edema because there's consolidation instead of air). A shunt is a V/Q mismatch, but not all V/Q mismatches are shunts.

 

[Newyorkgiants]

the question was referring to whether pulmonary edema would respond to O2 therapy. According to step up to medicine it would not but the correct answer on Uworld was that it would respond to O2 therapy.

 

[MrGrrrMonster]

Pulmonary edema would be a V/Q mismatch, and not necessarily a shunt in my opinion. You could still have some oxygenation-albeit- slight from the involved alveoli, where as a atelectasis from a mucous plug prohibits any air exchange, thus the capillaries are not re-oxygenating at all..and are shunting. Not sure if this is absolute though.

 

[Phloston]

I know this is a step 1 topic but UWorld and Step up to medicine are differing on which pathology result in a V/Q mismatch and shunt. UWorld is saying that Atelectasia, pulmonary edema, and pneumonia result in a mismatch while step up is saying that it causes a shunt. Which one is correct and can you explain the reasoning behind it.

The clinical importance (and what would show up on the Step) is that you have a shunt if a hypoxic patient is given oxygen and his or her pO2 doesn't increase.

This refers to any ventilatory V/Q mismatch (it doesn't matter what it is - atelectasis, pulmonary edema, bronchitis) that becomes severe enough to prevent additional ventilation (i.e., oxygen) from making a difference. We generally think of deoxygenated blood as R-sided and oxygenated as L-sided. If oxygen doesn't help, whatever the pathology is becomes classified as a shunt because the R-->L effects are beyond a threshold where oxygen helps.