Question on mannitol/ADH

[ZagDoc]

This came up on my UW the other night.

Patient presents with head trauma and symptoms of increased ICP. Following CT, patient is given infusion of mannitol. What is most likely to happen shortly after infusion begins?

a. ADH released due to blood volume changes
b. ADH released due to Posm changes
c. ADH released due to blood volume and Posm changes
d. Assorted other B.S. answers

UW says its (c.) and that ADH is being released due to an acute DROP in blood volume after infusion of mannitol. It doesn't explain why. I get that mannitol, since it doesn't leave the intravascular compartment, jacks up oncotic pull into the vascular space and sucks in water and assorted goodies and that the rise in Posm will cause ADH release. But the blood volume drop seems counterintuitive to me. Wouldn't mannitol, since its sucking in osmotic reagents and water, be acutely EXPANDING the intravascular space? Or am I overthinking things cause of the word "shortly" and they're just trying to imply that ADH will be released because mannitol will cause a diuresis?

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[FutureChiDoc]

This came up on my UW the other night.

Patient presents with head trauma and symptoms of increased ICP. Following CT, patient is given infusion of mannitol. What is most likely to happen shortly after infusion begins?

a. ADH released due to blood volume changes
b. ADH released due to Posm changes
c. ADH released due to blood volume and Posm changes
d. Assorted other B.S. answers

UW says its (c.) and that ADH is being released due to an acute DROP in blood volume after infusion of mannitol. It doesn't explain why. I get that mannitol, since it doesn't leave the intravascular compartment, jacks up oncotic pull into the vascular space and sucks in water and assorted goodies and that the rise in Posm will cause ADH release. But the blood volume drop seems counterintuitive to me. Wouldn't mannitol, since its sucking in osmotic reagents and water, be acutely EXPANDING the intravascular space? Or am I overthinking things cause of the word "shortly" and they're just trying to imply that ADH will be released because mannitol will cause a diuresis?

Mannitol will cause a massive osmotic diuresis. It will get filtered in the kidney, and will osmotically pull a TON of water out with it. That will cause a massive decrease in water, and ADH will then respond.

 

[bola]

I wasn't exactly sold on their explanation too...I feel the question was poorly worded/need to be more precise as to what timeline they were referring to. Soon after infusion of mannitol I didn't think there could be such a massive drop in blood volume as to cause ADH release. The osmotic concentration increase in the blood I definitely agree with.

Maybe I'm just underestimating the time till onset of mannitol's diuretic effect...

 

[Captopril]

it's the pee