Treatment of pneumothorax: "nitrogen washout"?

Discussion in 'Current Medical Students' started by Rich' Trash, Sep 22, 2007.

  1. Rich' Trash

    Rich' Trash New Member

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    Im a HCA on a surgerical ward (we have alot of pneumothoraces). We've just had a new registrar start on the ward who suggested we use a technique of nitrogen washout (putting patient on high flow oxygen) for treatment of a small pneumothorax. It was the first time the technique has ever been used on this ward, the nurses said.

    Can anyone explain to me what a 'nitrogen washout' is exactly? And why its so uncommonly used?
     
  2. M Clayton

    M Clayton Moderator type bloke

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    Air is mostly nitrogen, right?

    Well, if you have a pneumothroax while breathing room air, most of the volume in the pleural space is, logically, going to be Nitrogen.

    By breathing 100% oxygen, you lower the level of Nitrogen within the alveoli and, thus, the nitrogen in the pleural space will diffuse across (down the gradient).

    The volume of gas within the pnuemothorax should then fall. It won't resolve the pneuomothorax completely but it's a recognised treatment for small vol collections.
     
  3. Rich' Trash

    Rich' Trash New Member

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    Thanks Mark (avid reader of your blog btw ;) )

    My knowledge of anatomy is wrong I think.. because I thought the pleural space was a closed unit, and that there was no transport mechanism between alveoli and the space.
     
  4. M Clayton

    M Clayton Moderator type bloke

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    There's always going to be a limited amount of gas exchange going on. Don't get me wrong, nitrogen washout is a crap way to treat a pneumothorax but it's always an option in those where you'd not bother treating anyway (ie, small vol of air in a healthy patient)
     
  5. auden

    auden New Member

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    presumably it works because nitrogen is either smaller than oxygen or more easily dissolved in the membrane so it diffuses faster than oxygen? otherwise wouldn't the gas volume remain the same and just change in composition because there would be diffusion both ways?

    Libs
    confused, as usual
     
  6. M Clayton

    M Clayton Moderator type bloke

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    I'm happy to admit that my physiology fails me at this stage and I can't explain why what you're proposing doesn't happen auden!

    All I know is that nitrogen washout works (sort of!) to reduce small pneumothoraces. I imagine that the fact that oxygen is utilised when nitrogen isn't has something to do with it but I really don't know for certain.

    What we need is a respiratory bod to give us the definitive answer. As a surgical house officer, I mainly concern myself with overloading post op patients... ;)
     
  7. Rich' Trash

    Rich' Trash New Member

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    My guess would be because there is a higher pressure exerted on the air so the nitrogen, or any gas, is forced into the lungs (conc gradient allowing) more readily than oxygen, or any gas, out of the lungs into the pleural space.
     
  8. Touche

    Touche New Member

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    Half the answer's there. Putting someone on high flow oxygen lowers the nitrogen in the pleural space and replaces it with oxygen. Oxygen is absorbed more easily than nitrogen in the pleural space and thus the pneumothorax resolves quicker (1).

    Er... wouldnt really call it crap. There is evidence that it does work. If I had a 25% spontaneous pneumothorax I'd much prefer a week of O2 therapy than an FY1 sticking a needle/drain into my pleural space to save a few days (I put in around 20 drains as a PRHO in resp med, unsupervised after 2 if I recall correctly). ALL patients with pneumothoraces back when I worked on a resp firm went on high flow O2 if there was no contraindication. Every little helps! In small pneumothoraces with no breathlessness, BTS guidelines suggest discharge can be considered without any interventions with early follow up and advice to seek help if any worrying features develop (2). So your registrar's conservative management in this case is perfectly valid.

    Because you're on a surgical ward where sexy drains would be reached for over a not so sexy oxygen mask? (I'm surgically inclined so I can take the piss a little!). I doubt it's the first time it's been used on that ward anyway (since the evidence is over 35 years old), it's just not often called "nitrogen washout" and people (particularly busy nurses who have other things to worry about) often don't understand the reason behind high flow O2 in a patient with a pneumothorax past the obvious.

    1. Northfield TC. Oxygen therapy for spontaneous pneumothorax. BMJ
    1971;4:86-88.
    2. M Henry et al. BTS guidelines for the management of spontaneous pneumothorax. Thorax 2003;58:ii39.
     
    #8 Touche, Sep 23, 2007
    Last edited: Sep 24, 2007
  9. auden

    auden New Member

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    cheers Touche - that's an excellent explanation :)

    And also cheers to Mark who made me look oh so competent the other day when we did this for a patient and the SpR asked why we were giving high flow O2 to someone with a pneumothorax ;) (I'm on a resp firm!)
     
  10. learn

    learn New Member

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    Getting a bit confused here.Once high oxygen conc is inhaled.....does the nitrogen in the pleural space diffuse into the alveoli or into the pleural capillaries?
     
  11. Martigan

    Martigan Super Moderator

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    No, it's being exhaled...
     
  12. Dr. Bailey

    Dr. Bailey New Member

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    Here is the physics/physiology:
    Every gas has a partial pressure in air. Dry air is 21% oxygen, 78% nitrogen, and 1% other gases to a total of 760 mmHg. In humidified air at 37 degrees this is reduced to 713 mmHg (the other 47 mmHg is water vapor). Every gas also has a partial pressure in blood. The pressure of oxygen in blood on room air is about 100 mmHg in arterial blood, 40 mmHg in venous. The pressure of CO2 is about 40 mmHg. The pressure of water vapor is about 47 mmHg, and nitrogen is about 556 mmHg. Summing the partial pressures in blood gives a total gas pressure of 743 mmHg on the arterial side, and 683 on the venous side. The diffusion of gas from a closed space through a membrane is determined by the permeability of the membrane to the specific gas and the pressure difference of the gas across the membrane. Assuming the pneumothorax starts out as humidified air, the initial diffusion gradients will cause oxygen to be absorbed into the blood, CO2 to be released into the pneumothorax, and nitrogen and water vapor will have no gradient. If the pneumothorax were to equilibrate completely with blood it would have a total pressure between 683 and 743 mmHg (round to 730 mmHg for further calculations) which would result in expansion of the lung and decrease in the size of the pneumothorax. What actually happens is the pneumothorax shrinks gradually, but as it shrinks it's composition changes. The easiest way to conceptualize these changes is to treat them as a series of partial pressure equilibrations followed by volume changes. If the new sum of partial pressures is 730, the volume will change by 730/760, and the partial pressure of each gas will change by 760/730. At the beginning of our second partial pressure equilibration that gives us oxygen 91 mmHg, CO2 42 mmHg, water 49 mmHg, and nitrogen 578 mmHg. These pressures give us gradients from the pneumothorax to the blood for all gases, and the pneumothorax will continue to shrink.

    If we increase the concentration of inhaled O2 it will have two effects on gas pressures in the blood: The partial pressure of nitrogen will be lower, because the oxygen displaces nitrogen from the alveoli, and the pressure of oxygen will be higher. Achieving an inhaled oxygen concentration of 40% would give a partial pressure of nitrogen in blood of 713 mmHg X 0.59 = 421 mmHg. The average partial pressure of oxygen would not be increased to 713x.04 due to oxygen uptake by tissue, so the final oxygen partial pressure on the venous side would be at most 100. Assuming equilibration between blood and pneumothorax at constant volume we have a total gas pressure of about 607 mmHg. The resulting change in volume would leave new partial pressures of: oxygen 117 mmHg, CO2 53 mmHg, water 55 mmHg, and nitrogen 498 mmHg. This results in significantly larger gradients across the membrane and more rapid diffusion of gas into the blood. Once the gas diffuses into the blood it is rapidly equilibrated with alveolar gas.

    Despite the sound physiologic basis of this treatment it has never been proven in a high quality trial. There are animal studies and small studies in adults showing increased rate of resolution, and observational studies in neonates showing no benefit.

    Nitrogen washout is appropriate for only a select group of patients:
    Stable respiratory function (unstable patients need a tube or aspiration).
    and
    In the hospital anyway (stable patients should not be hospitalized for oxygen administration).
    and
    No contraindication to oxygen administration
     

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