Free 132 Q44

[Tajima]

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Why does pulmonary vascular resistance increase during hypovolemic shock?

 

[walakin25]

Why does pulmonary vascular resistance increase during hypovolemic shock?

Vasoconstriction

 

[Tajima]

Why do the pulmonary arteries vasoconstrict during episodes of hypovolemic shock?

It seems counterintuitive to me.

 

[worldbeater]

Why do the pulmonary arteries vasoconstrict during episodes of hypovolemic shock?

Hypovolemia means your losing volume -> delivery of O2 to tissue in the lung is decreased -> vessels become hypoxic -> causes vasoconstriction to 100% perfuse as many lobes of the lung to survive

This is the only place in the body an artery would constrict (UWorld question, they will put a graph), an artery any where else like in the brain, would vasodilate due the lack of O2(hypoxia)/increase of CO2.

 

[Tajima]

I'm still not getting it. I was under the impression that hypoxic vasoconstriction is a phenomenon that reflects low oxygenation of airways and low capillary/alveolar oxygen exchange. I did not think it had anything to do with blood supply to the alveoli.

I am still quite confused about this, and my efforts to search the literature have come up with nothing.

 

[DrPicard]

Pulmonary venous constriction mobilizes blood stored in them. Pulmonary arteries constrict to maintain perfusion pressure across the lungs. Pulmonary vasculature is very compliant + low pressure, this constriction shouldn't really increase the workload of the right ventricle.

Systemic veins will also vasoconstrict to mobilize stored/unstressed blood.

Extrinsically controlled vasculature (eg gut, sk muscle etc) will constrict under the influence of catecholamines. Autoregulated vasculature will dilate (brain, heart). Vasoconstriction throughout the body will function to raise pressure to help push blood across the vasodilated coronary and cerebral circulations.

 

[walakin25]

Pulmonary venous constriction mobilizes blood stored in them. Pulmonary arteries constrict to maintain perfusion pressure across the lungs. Pulmonary vasculature is very compliant + low pressure, this constriction shouldn't really increase the workload of the right ventricle.

Systemic veins will also vasoconstrict to mobilize stored/unstressed blood.

Extrinsically controlled vasculature (eg gut, sk muscle etc) will constrict under the influence of catecholamines. Autoregulated vasculature will dilate (brain, heart). Vasoconstriction throughout the body will function to raise pressure to help push blood across the vasodilated coronary and cerebral circulations.

this man knows his physiology. great explanation!

 

[worldbeater]

Pulmonary venous constriction mobilizes blood stored in them. Pulmonary arteries constrict to maintain perfusion pressure across the lungs. Pulmonary vasculature is very compliant + low pressure, this constriction shouldn't really increase the workload of the right ventricle.

Yeah exactly, nice. I was just going to say if you want to ignore the oxygen component, any time your losing volume (hypovolemia) you will venoconstrict first (since 60% of blood is pooled in your veins) and then vasoconstrict (your arteries) to get blood back into your circulation. There is also an NBME question (spoiler alert) that asks what parameters decrease in hypovolemia, and the answer is renal arteriolar perfusion pressure.

I am still quite confused about this, and my efforts to search the literature have come up with nothing.

I looked up the exact page and the concept is explained on p.262 of The PASS Notes in 4 short lines if the original poster wants the source.