Why Can't "Fluid in Lungs" Just be "Sucked" Out?

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One aspect of COVID-19 I read about is getting fluid in the lungs.

This is possibly a very dumb question from a completely non-science/non-medical person, but why can't fluid in there just be "sucked out" medically?

If the fluid is preventing breathing or causing other damage, why can't we medically just remove the fluid? Thanks.
 

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  • #2
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There are several reasons:
- the exudate collects in alveoli and bronchioli, which are extremely small structures. Getting a suction tube in to an alveolus would be practically impossible
- even if you managed that, there are hundreds of thousands of alveoli to suction
- if you simply try to apply negative pressure to the bronchi to suction out the viscous fluid, this would collapse the alveoli, and cause bleeding in to the alveoli through the extremely small membrane separating the cavity from the blood vessels (that extremely thin wall is what facilitates the gas exchange)
- If the alveoli collapse, and their walls stick together, it requires a lot of pressure to re-open them (If you wet a balloon, and let the inside walls stick together, then you need to apply a lot of pressure to unstick those walls to blow up the balloon again)

So the current treatment is based on either continuous, or bi-level positive airway pressure ventilation. This means, the ventilator keeps a certain air pressure in the lungs even when exhaling, to ensure that the exhudate-filled alveoli don't collapse and stick together during exhalation.
 
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Suctioning of the airways is done for all ventilated patients. It removes the mucus build up from the endotracheal tube, but it doesn't remove any fluid from the lungs.
 
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  • #5
atyy
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The article's description makes it seem that some of the mucus is secreted from the lungs. Is that incorrect? What is the source of secretions that build up in the endotracheal tube?
 
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jim mcnamara
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@Runesmith @atyy Please do not forget citations when you make declarations of facts you know. It helps all of us. Good thread. Thanks.
 
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The article's description makes it seem that some of the mucus is secreted from the lungs. Is that incorrect? What is the source of secretions that build up in the endotracheal tube?
The mucus that accumulates in the ET tube is secreted by goblet cells in the bronchial epithelium, and pushed upwards towards the larynx by actively waving hair-like projections on epithelial cells, called celia. This secretion is natural, and is a defense against pathogens and particulate material that get deposited on the bronchial walls. As the ET tube has no celia, the secretions accumulate there, and can cause occlusion of the tube, aspiration, and/or infection by opportunistic bacteria. That's the reason why the ET tube needs to be suctioned regularly, and this removes only the naturally occurring secretions, not the exhudate in alveoli.

The suctioning of the ET tube is done with a suction tube with a much smaller diameter than the ET tube itself. So it just sucks out fluid where the tip is pointing to. It never creates a negative pressure inside the lungs. Negative pressure strong enough to suck out fluid would cause massive alveolar damage resulting in hemorrhage in the lungs.

*I am a physician (MD) by training, although I am currently not in practice - I am engaged in biomedical research.
 
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  • #8
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Be careful when people say "fluid in the lungs". It may be what has been described. But people also say this when the problem is fluid in the lung tissue (edema); like swelling. This reduces the diffusion of O2 from the alveolar space to the capillaries. The common terms can be confusing.

There have been some indications that this is a problem with severe CoVID-19 cases and explains why they don't do well on ventilators. There is more to lung function than filling them with O2.
 
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  • #9
Tom.G
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