[thornjm]

Med student here - so I know some of the theory only. The most relevant "law" is Fick's Law of Diffusion (https://d2jmvrsizmvf4x.cloudfront.net/CTU792sKR2evT2ezizQp_f...). Doctors can increase diffusion by increasing the partial pressure of oxygen.

In a diffuse infection a patient begins to lose both lung area (due to shunting) and the thickness of the diffusion barrier increases (due to inflammation). To help overcome this you want to increase pressure and oxygen concentration.

An iron lung helps ease the work of breathing by reducing thoracic pressure and thus creates a larger pressure gradient for inspiration. However, it does not cause an absolute partial pressure of oxygen change compared to the atmosphere.

Unfortunately, to bind haemoglobin in physiological lung conditions we need partial pressure of oxygen around 100mmHg. My guess is that an iron lung does not help increase the partial pressure of oxygen so it will do little but ease the work of breathing (which is better than nothing!).

[totalZero]

It's too bad we can't put sick people in an airplane, put oxygen masks on them, shut the door, and pressurize the cabin. Like a hospital, but at the airport.

[dschuler]

Very creative idea, I wouldn’t have thought of that.

So standard pressure at sea level is 29.92inHg, a 737 MAX can sustain 39k ft altitude indefinitely I suppose, where the exterior air pressure would be 7.66inHg. The cabin is normally pressurized to the equivalent of 8k ft, giving 26.63inHg. The fuselage could withstand a pressure differential of 26.63inHg - 7.66inHg = 18.97inHg (at least, possibly more).

That’s 63% higher than regular pressure at sea level. Not bad.

If you gave each patient a full economy row, that’s about 60 patients per plane, so 48k patients across 800 grounded 737 MAXs.

[dorfsmay]

What create the air pressure in an airplane? Is it external air speed? Or compression from the engines?

If the latter, how does it work on airplanes with needless engines?

[dschuler]

I don't know much about it, but I think it's usually provided by pressurized bleed air from the engines.

There are cabin pressurization test carts for use on the ground, as well as start carts to provide bleed air for starting engines, and bleed air in turn can supply cabin pressurization in flight, although this probably requires some rigging on the ground.

Interestingly, they switched to electric compressors for the 787 [0] because those no longer provide bleed air from/to each engine to simplify the plumbing.

[0] https://aerosavvy.com/aircraft-pressurization/

[hinkley]

Partial pressure is also what makes fire burn. There’s a fine line between pumping up oxygen and creating a tinderbox.

That said, you’re describing a bariatric chamber, and they do exist (they perform surgeries in some).

[raducu]

Aren't the lungs re-inflated because of the positive pressure differential?

If your whole body is inside, there is no pressure diferential to inflate your lungs.

Am I missing something?

[enchiridion]

This is a really off the wall idea that might actually work. Can anybody get this idea some lift? Definitely a way for Boeing to get some desperately needed good PR.

[mdturnerphys]

I'll ping my friend who's been at Boeing for 25 years about it in the morning. Anyone know an MD who could review the idea?

[mdturnerphys]

Response from my friend:

Novel thought...

Theoretically, yes. The Environmental Control Systems on all our commercial airplanes positively charge the airflow at a nominal Delta P of 9.8 psi and have HEPA filtration down to 10µm, but at $100M+ per 737 MAX, delivery customers highly frowning upon such a practice and not being able to deliver them as a "New" airplanes after contamination, highly unlikely.

The A380 fleet is starting to retire prematurely only after 10 short years of service due to lack of profitability and demand due to COVID-19. Those huge retired Behemoths could serve very well as a quarantine base in remote locations.

[toolbox]

Hello! I'm just a medical student myself, but here's my take on this:

Your lungs operate on the principle of differential pressure. During normal breathing, hen the pressure inside your lungs becomes less than that of the surrounding atmosphere, air rushes in to normalize and fill it until they are again equal pressures. Gradually increasing or decreasing the pressure of the atmosphere around you (as on a plane or while diving) does not change the physical difficulty of breathing. It may slightly alter the diffusion coefficients of gasses passing through the membranes in your lungs, but this effect is mild at the pressures you could attain in an aircraft hull (and toxic at higher pressures! [0]).

For this reason, you'll notice in images of people in iron lungs, their heads are outside of the device. This allows those who cannot create negative pressure (due to damaged or paralyzed diaphragms or ribcage muscles) to follow a different path. The pressure outside of their chest becomes lower than that experienced in their lungs, forcing an expansion; in order to breath out, the pressure in the chamber is increased. In an aircraft hull a person's lungs/trachea/mouth would be exposed to the same pressure as their chests.

This of course does bring up the very valid question: What happened to all of the iron lungs after the decline of Polio?

[0] https://en.wikipedia.org/wiki/Nitrogen_narcosis

EDIT: Oh I realize I didn't fully discuss the possibility of using a plane as a hyperbaric chamber [1]. The constraints around this concept also rule it somewhere outside of what I would consider feasible. Aircraft typically use bottled oxygen (I believe the 737M does) or a chemical generator, neither of which can produce continuous oxygen or fill the entire plane with it to high levels [2]. If you were able to outfit patients with individual tanks (which would also have to accommodate for increased pressure), the gains seem mild at best compared to 100% O2 in a hospital setting. Fick's Law for the membranes in your lungs is roughly

Rate of diffusion = (Area * Solubility of gas * concentration gradient) / (membrane thickness * sqrt(molecular weight of solute))

The factor that increasing pressure would modify is the solubility of the gas, which according to Henry's Law [4] is directly proportional to pressure. At absolute best (100% O2 on the plane at 2atm), you could expect to get 2x improvement in blood oxygen saturation. Unfortunately I don't believe the breathing issues that are being described can be overcome by this strategy. It's an incredibly creative concept though!

[1] https://en.wikipedia.org/wiki/Hyperbaric_medicine

[2] https://en.wikipedia.org/wiki/Emergency_oxygen_system

[3] https://clinicalgate.com/gas-exchange-between-air-and-blood-...

[4] https://en.wikipedia.org/wiki/Henry%27s_law

[enchiridion]

Thanks for explaining. Seems like it would not work like we were thinking.

Any changes you could think of that would make it work?

[jpmoral]

Why not? Maybe not in an airplane and not at an airport, but the same general idea.

[_bxg1]

We've got all those 737s...

[totalZero]

Almost 800 of the 737 Max are just sitting around collecting dust. Not to mention all the grounded wide-body aircraft.

Take out the seats, equip the cabin with ventilators, and when one outbreak gets under control, send the aircraft to the next outbreak zone.

They even have their own generators lol.

[james_s_tayler]

Would give all those pilots something to do and I'm sure it couldn't hurt fuel prices.

[bobthepanda]

The ones that have already crashed, and are totally grounded because they propose a risk to the public?

Not sure if this is supposed to be better. And who would fly all these people? Access to the cockpit is through the rest of the plane.

[totalZero]

The aircraft could fly empty to arrive at airports near outbreak areas, then be used as a hospital after it arrives and the pilot disembarks. Once the hospitals in the area gain some ability to control the outbreak, remaining patients can be sent to local hospitals, the aircraft can be decontaminated and left to sit for some period of time, and the aircraft can move to the next region.

Also, considering Boeing was anticipating FAA approval within weeks, it's fair to assume that the MCAS problem has been resolved. (https://www.reuters.com/article/us-boeing-737max/boeing-737-...)

If it were truly necessary, you could also set up an airtight septum to separate the contaminated part of the aircraft from the front part that includes the cockpit. There is an aft door (as well as emergency exits) that can be used to access the rear of the aircraft.

[mdturnerphys]

No flying needed--just need to pressurize the aircraft while on the ground.

[numpad0]

We public know what the issues was. Apart from MCAS risk, I think the planes would be mostly safe for these one-off missions.

It just has to fly handful times, from Boeing park lots to a depot in a desert and back from frontline after months/years. They are supposed to be clean before deployment, and can be cleaned by medical professionals before reflying. Or maybe cracks would develop and can't be manned after anyway, either way not much there is in terms of infection risks.

[DoreenMichele]

I imagine oxygen masks in airplanes aren't as sterile as needed.

Planes are horribly cramped and if you filled every seat you would be violating social distancing while filling them with sick people.

I can think of a few other criticisms, but those are probably the most defensible concerns.

[fredophile]

If everyone on the plane already has covid-19 then social distancing doesn't matter.

[DoreenMichele]

If only disease actually worked that way.

Covid19 is not the only germs they will be carrying. The people who are the sickest are the ones that most need oxygen supplementation. Reports suggest that most of the people dying from it already have other serious medical conditions.

This is exactly how antibiotic resistant infections get bred. You would be creating a melting pot of horribleness and cross-contamination and god-knows-what would come spewing out of it.

[krona]

The UK government has already considered this as an option. I don't know what became of it. I don't think we're quite at that stage, yet.

[ddxxdd]

I'm confused by this comment and the replies- did you mean to say depressurize the cabin?

[FreeFull]

You could always combine the iron lung with a mask that supplies oxygen.

[downerending]

I've been wondering whether CPR breaths could be used in extremis. It'd be pretty hellish, but I could imagine doing this for 24-36 hours if it was the only option to save my wife, for example.

[ars]

Could we build a huge hyperbaric building and place all the patients in there?

Would that accomplish the same goal as a ventilator?

[dredmorbius]

There's both partial pressure (O2 concentration and overall atmospheric pressure) and repiratory function. Forced-air respirators (most current models) and iron lungs (operating with partial vacuum) both address this mechanism.