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Q about Mannitol

Discussion in 'Anesthesiology' started by 318038, Jul 23, 2011.

  1. 318038

    318038 A Fan of Medicine

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    I have a random, possibly dumb question about Mannitol. I know that in anesthesia Mannitol is administered as an osmotic agent to control increased intracranial pressure. I know that it is not used to treat lower extremity edema and usually a loop diuretic is preferred in that instance.

    My question is why does mannitol target the cerebral vasculature yet it cannot be used for similar issues in the rest of of the body? My thought was that it can use this same osmolar effect throughout the vasculature? Sorry if this is a terrible question but thanks in advance :)
     
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  3. Oggg

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    bbb?
     
  4. mikey014

    mikey014 nervous...
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    I thought that osmotic diuretics like mannitol work both in the proximal tubule and the loop of Henle. And I was under the impression that osmotic diuretics are very strong diuretics and are only used during emergency situations like when there is increased cranial pressure or closed angle glaucoma. Maybe because in order to be effective it is usually given IV because very large concentrations of it must be attained in order to be effective.

    And although loop diuretics like furosemide also work in the same location, they work via a different mechanism by inhibiting reuptake of ions. Therefore, it those can (but don't have to be) taken orally. And the unlike osmotic diuretics the ions that in the lumen are charged and would reach a limited electrochemical potential and therefore will be less able to be as potent.

    And I think the previous poster touched upon it. That osmotic diuretics seem to be able to cross the blood brain barrier and decrease pressure in the brain & the eye.

    http://en.wikipedia.org/wiki/Osmotic_diuretic
     
  5. Idiopathic

    Idiopathic Newly Minted
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    Osmotic diuretics specifically do not cross the BBB. They pull water from extravascular tissue because they cant cross blood vessels. You shouldnt use these meds in most people with CHF (assuming thats the LE edema you are referring to) because they increase intravascular volume acutely.
     
  6. rsgillmd

    rsgillmd ASA Member
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    :thumbup:

    Nicely put.
     
  7. Idiopathic

    Idiopathic Newly Minted
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    and also worth remembering that swelling due to intraparenchymal hemorrhage (i.e. hemorrhagic stroke) may not be best managed with mannitol, because the BBB is disrupted and the mannitol may get deposited in the bleed area, worsening the situation.
     
  8. 318038

    318038 A Fan of Medicine

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    Perfect answer, makes a lot of sense. Thanks!
     
  9. gtb

    gtb Member
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    Mannitol raises plasma osmolarity, and in the case of a functional blood brain barrier, water will diffuse water from the parenchymal tissue, into the intravascular space. Remember the Starling equation for peripheral capillaries? That accurately explains why high osmolarity fluids work in capillaries outside the brain. Raise up the osmolarity with a colloid or high weight molecule, and Starling forces will pull fluid into the intravascular space. In the kidney, mannitol gets filtered, but not reabsorbed in the tubules. So water follows the mannitol into the foley. Because of the tight junctions, astrocytic feet, and unique transporters, porins, etc. within the brain capillaries, the Starling equation doesn't really adequately explain the action of mannitol, or any other hypertonic fluid (3% saline for example). Instead, you have to use this gnarly formula called the "van't Hoff equation." Useful only for people exploring the details of the BBB. In my case, I consider that only water molecules are freely diffusible across the BBB. High intravascular OSM, and the brain slackens, low intravascular OSM, and the brain swells. Keep the OSM somewhere near normal, and the brain, assuming an intact BBB, doesn't swell, no matter how much fluid you flog a patient with. Consider massive trauma (no TBI), when you flood a person with dozens of liters of crystalloid, and they wind up with pulmonary edema, abdominal compartment syndrome, and physically resemble the Marshmallow Man. But the brain doesn't swell, or at least it doesn't swell enough to cause herniation. Pretty cool system when it works. Mannitol has lots of other adverse effects, so use it cautiously.
     
  10. fakin' the funk

    fakin' the funk ASA Member
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    What other osmotic diuretics are there?

    I gave 50g (that's 250ml of 20%) mannitol to a dude with EF 20% having a crani the other day, carefully, and fully aware of the acutely-intravascular-volume-expanding properties, and ... nothing acutely happened. He peed an extra few hundred ml's after that.
     
  11. fakin' the funk

    fakin' the funk ASA Member
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    +1
     
  12. Idiopathic

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    not the norm, these guys usually put out liters over the course of a case in that situation. i would imagine he had a profound diruesis afterwards.

    i dont know of many other osmotic diuretics in clinical use, but some drugs have that property in addition to their other mechanism (mostly sugar derivatives?)
     
  13. Foodie

    Foodie ASA Member
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    Usually 50g of mannitol leads to at least 0.75-1L of UOP within the next hour. Unless your patient was severely dehydrated.

    As an aside, I've seen a few anesthesia providers not using a filter or the fluid warmer for mannitol, not to mention infusing it into the arm that they cannot see. I mean, not that I'm against phlebitis +/- painful edema of the extremity or anything...
     

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