Southwest Plane Accident, Alveolar PO2

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Ronin786

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I was reading this article about how most passengers in the recent Southwest flight that lost an engine + window were wearing their oxygen masks incorrectly:

Southwest Airlines Flight 1380 suffered a major engine failure on Tuesday, forcing its pilot to make an emergency landing.

Shrapnel pierced the airplane's fuselage, blew out a window, and caused the cabin of the airplane to depressurize. One passenger died.

The pilot, Tammie Jo Shults, who used to fly US Navy fighter jets, guided the airplane, which took off from New York and was bound for Dallas, to a landing in Philadelphia.

Oxygen masks dropped from the cabin ceiling during the incident, according to a public Facebook post by Marty Martinez, a passenger on the flight.

Bobby Laurie/Twitter; Marty Martinez/Facebook
Bobby Laurie, a former flight attendant who now hosts a TV show, shared one of Martinez's photos on Twitter along with a reminder that people should cover their nose and mouth with an oxygen mask in an emergency.

"PEOPLE: Listen to your flight attendants!" Laurie said. "ALMOST EVERYONE in this photo from @SouthwestAir #SWA1380 today is wearing their mask WRONG."

Why you need oxygen if an aircraft cabin loses pressure
Flying at high altitudes with a hole in an airplane is, to put it lightly, dangerous.

At altitudes above 15,000 feet, people struggle to breathe and keep enough oxygen in their blood. They can lose consciousness within minutes — a condition called hypoxia.

Symptoms of hypoxia include "nausea, apprehension, tunnel vision, headaches, fatigue, dizziness, blurred vision, tingling sensations, numbness, and mental confusion," according to the Aircraft Owners and Pilots Association.

The problem isn't the percentage of oxygen in the air, which stays relatively constant at about 21% until about 70,000 feet — it's the lack of air pressure.

The correct way to wear an oxygen mask during an in-flight emergency.
Shutterstock
High pressure makes air dense, which helps force oxygen through lung tissue and into the bloodstream. Insufficient pressure lowers air density, thereby decreasing the amount of available oxygen.

Adding a flow of 100% oxygen helps counter this physiological problem. But you have to wear and use the mask correctly.

If you don't cover both your nose and your mouth with the mask, you may not get enough oxygen into your bloodstream, putting you at risk of losing consciousness.

How correctly wearing an oxygen mask could save your life — and those around you
The Southwest flight's engine failure happened when the plane was about 31,000 feet in the air, based on passenger reports.

Shults descended the airplane to an altitude of 10,000 feet shortly after, according to flight-tracking data from FlightRadar24.com, and landed the aircraft about 12 minutes after an emergency was declared.

According to a chart from AOPA on "time of useful consciousness," the passengers would have had about 30 seconds to get their masks on after the window blew open.

Aircraft Owners and Pilots Association
That flow of oxygen is crucial in emergencies, as passengers who are passed out won't be able to evacuate. And if there's any kind of fire or smoke condition, an unconscious neighbor slumped in an aisle could mean the difference between life and death. That's why masks are designed to deploy immediately.

Southwest public-announcement flashcards posted on Quizlet indicate passengers get these instructions before every takeoff:

"If needed, four oxygen masks will drop from the compartment overhead. To activate the flow of oxygen, pull down on the mask until the plastic tubing is fully extended. Place the mask over your nose and mouth and breathe normally.

"Secure the mask with the elastic strap. Although oxygen will be flowing, the plastic bag may not inflate. Continue wearing the mask until otherwise notified by a crew member. If you are traveling with children or anyone needing special assistance, put on your mask first."

An investigation into the incident by the National Transportation Safety Board is underway.

In response to Business Insider questions about Southwest's use of oxygen masks on Flight 1380, a company representative said via email: "We aren't ready to engage that level of detail at this time as we are focused on the immediate needs of the NTSB investigation."

Though concerns about flying and airplane safety permeate popular culture, it's much safer to travel by plane than by car.

The person who died on the Southwest flight on Tuesday was the first fatality in a US passenger airline accident in over nine years. In that time, there were almost 100 million US flights that carried billions of people, according to Bloomberg.

DbAQtDTVMAAVo-i.jpg:large


This got me to thinking, according to the article, the plane depressurized at 30,000 feet. Clearly nobody is getting 100% FiO2 given their terrible non-rebreather technique. Why didn't anybody pass out from hypoxia?

The article also states that they descended to 10,000 feet within 5 minutes, which doesn't explain it. They also had a broken window, so no way to pressurize the plane.

Thoughts?

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I was reading this article about how most passengers in the recent Southwest flight that lost an engine + window were wearing their oxygen masks incorrectly:



DbAQtDTVMAAVo-i.jpg:large


This got me to thinking, according to the article, the plane depressurized at 30,000 feet. Clearly nobody is getting 100% FiO2 given their terrible non-rebreather technique. Why didn't anybody pass out from hypoxia?

The article also states that they descended to 10,000 feet within 5 minutes, which doesn't explain it. They also had a broken window, so no way to pressurize the plane.

Thoughts?

You don't know when exactly the picture was taken. Could have been after they descended already.

Most people should do fine at 30000 ft for a few minutes, even on room air. Hell, 80 year olds climb ****ing mountains at almost that height. Plus even with the poor technique they're probably getting FiO2 of 40ish or so, I'd guess.

I'm curious if the fatality was related to direct trauma from the shrapnel or sequelae of almost getting sucked out of the plane.

Also, if you haven't heard it yet, listen to the audio between the pilot and the ATC. Pretty amazing stuff, great example of communication/crisis management.
 
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Looking at some graphs from Nunn's:

At cruising altitude, 30,000 feet (8000m), arterial pO2 will be ~30mmHg once the cabin fully depressurises.
Time to unconsciousness from hypoxia in an unacclimatised person would be around 2 minutes. Keep in mind it will take quite a while for the whole cabin to depressurise through a relatively small hole; normal cabin pressure is kept around 8000 feet (2400m) equivalent (i.e. PaO2 ~70)

Descending to 10,000 feet will keep the PaO2 ~60mmHg without any supplemental O2.

Those oxygen generators only work for a few minutes anyway to give the pilots time to descend, so they're probably decoration by the time these photos were taken.
 
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Chronic hypoxia/altitude sickness aside, the role of RV failure (contributing to loss of consciousness) in sudden high altitude decompression events isn't really considered much, but in some segment of travelers it would seem to be in play.
 
I was reading this article about how most passengers in the recent Southwest flight that lost an engine + window were wearing their oxygen masks incorrectly:

This got me to thinking, according to the article, the plane depressurized at 30,000 feet. Clearly nobody is getting 100% FiO2 given their terrible non-rebreather technique. Why didn't anybody pass out from hypoxia?

Thoughts?

People handle hypoxia surprisingly better than you would expect. There are plenty of kids and people who show up in the ED with initial pulse ox readings in the mid 70's, who presumably were desaturated before the pulse ox was placed. Obviously they're in some duress, might be agitated, but they're not passed out.

And spend enough time in the Peds CVICU and you'll have a whole new respect for what a body can do when they're used to only satting 72%.
 
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People handle hypoxia surprisingly better than you would expect. There are plenty of kids and people who show up in the ED with initial pulse ox readings in the mid 70's, who presumably were desaturated before the pulse ox was placed. Obviously they're in some duress, might be agitated, but they're not passed out.

And spend enough time in the Peds CVICU and you'll have a whole new respect for what a body can do when they're used to only satting 72%.

Big difference between chronic hypoxemia and sudden, catastrophic loss of atmosphere. And polycythemia doesn't hurt either.
 
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Looking at some graphs from Nunn's:

At cruising altitude, 30,000 feet (8000m), arterial pO2 will be ~30mmHg once the cabin fully depressurises.
Time to unconsciousness from hypoxia in an unacclimatised person would be around 2 minutes. Keep in mind it will take quite a while for the whole cabin to depressurise through a relatively small hole; normal cabin pressure is kept around 8000 feet (2400m) equivalent (i.e. PaO2 ~70)

Descending to 10,000 feet will keep the PaO2 ~60mmHg without any supplemental O2.

Those oxygen generators only work for a few minutes anyway to give the pilots time to descend, so they're probably decoration by the time these photos were taken.

How do you figure that?

At 30,000 feet, atmospheric pressure is 252 mmhg (Altitude above Sea Level and Air Pressure) and an FiO2 of 21%.

Per the gas equation: 0.21 (252-47) - (30/0.8) is a PAO2 of 5.55. That's pretty darn low.
 
[QUOTE="Also, if you haven't heard it yet, listen to the audio between the pilot and the ATC. Pretty amazing stuff, great example of communication/crisis management.[/QUOTE]

I’ll wait for Hollywood to spruce things up a bit in the forthcoming “Tammie”
 
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How do you figure that?

At 30,000 feet, atmospheric pressure is 252 mmhg and an FiO2 of 21%.

Per the gas equation: 0.21 (252-47) - (30/0.8) is a PAO2 of 5.55. That's pretty darn low.

I got my numbers from Nunn's respiratory physiology, which shows blood gases from people in a low pressure chamber simulating Everest (pressure 253mmHg). These people hyperventilated to a pCO2 of 10mmHg (though I'm sure a lot of plane passengers won't manage that!).

I bet the main reason people didn't lose consciousness is the leak wasn't big enough to totally overcome the 'fresh gas flow' into the cabin from the engines - so the pressure didn't drop as much as you'd expect.
 
How do you figure that?

At 30,000 feet, atmospheric pressure is 252 mmhg (Altitude above Sea Level and Air Pressure) and an FiO2 of 21%.

Per the gas equation: 0.21 (252-47) - (30/0.8) is a PAO2 of 5.55. That's pretty darn low.

You going to believe a theoretical equation or real-world data?

Arterial blood gases and oxygen content in climbers on Mount Everest. - PubMed - NCBI

PaO2 of 20 is about right for 30,000ft for a "normal" human.

Granted, most of the people on Everest have been training and have at least some physiological adaptations that your run-of-the-mill plane passenger doesn't have. But on the other side, pretty sure the atmospheric pressure on Everest is pretty consistent and they've been on the mountain for awhile. The cabin depressurizing isn't going to be instantaneous and the passengers are starting from a higher PaO2.

Plus if you're just strapped in and holding on for dear life, you don't have to worry about your ability to perform complex tasks degrading over time. The pilot on the other hand... I wonder if the O2 system for the pilot is the same as for the passengers or maybe they have legit masks? Can you keep the cockpit pressurized separately from the cabin?
 
You going to believe a theoretical equation or real-world data?

Arterial blood gases and oxygen content in climbers on Mount Everest. - PubMed - NCBI

PaO2 of 20 is about right for 30,000ft for a "normal" human.

Granted, most of the people on Everest have been training and have at least some physiological adaptations that your run-of-the-mill plane passenger doesn't have. But on the other side, pretty sure the atmospheric pressure on Everest is pretty consistent and they've been on the mountain for awhile. The cabin depressurizing isn't going to be instantaneous and the passengers are starting from a higher PaO2.

Plus if you're just strapped in and holding on for dear life, you don't have to worry about your ability to perform complex tasks degrading over time. The pilot on the other hand... I wonder if the O2 system for the pilot is the same as for the passengers or maybe they have legit masks? Can you keep the cockpit pressurized separately from the cabin?

Equation. The data could easily have been fudged.
 
You going to believe a theoretical equation or real-world data?

Arterial blood gases and oxygen content in climbers on Mount Everest. - PubMed - NCBI

PaO2 of 20 is about right for 30,000ft for a "normal" human.

Granted, most of the people on Everest have been training and have at least some physiological adaptations that your run-of-the-mill plane passenger doesn't have. But on the other side, pretty sure the atmospheric pressure on Everest is pretty consistent and they've been on the mountain for awhile. The cabin depressurizing isn't going to be instantaneous and the passengers are starting from a higher PaO2.

Plus if you're just strapped in and holding on for dear life, you don't have to worry about your ability to perform complex tasks degrading over time. The pilot on the other hand... I wonder if the O2 system for the pilot is the same as for the passengers or maybe they have legit masks? Can you keep the cockpit pressurized separately from the cabin?
If the alveolar gas equation is incorrect, then the foundation of pretty much most of our understanding of respiratory medicine is deeply flawed. Not to mention, the article you linked mentions a mean PCO2 of 13.3, which was probably aided by urinary changes as well. That and other adaptive physiology of mountain climbers and people who live at altitude makes it less applicable to this situation.
 
What possible reason would they have to fudge the data?

Was jk about fudging the data. I meant there are many reasons for why the study's #s could be so off from alveolar equation. Your n = 10 patients for the study. When you have so many sources of error, id rather trust the more established equation.

Is the ABG machine even approved for use at 30000 ft?
 
Sweet... blind “belief” in math over empiric data. Almost religious.
Well

The empiric data is in athletes / mountain climbers who have gradually acclimated to altitude over weeks or months. Maybe comparing what's observed empirically in these people, with their very different Hb, 2,3-DPG, PaCO2, etc to a planeload of couch potatoes who "acclimated" to that altitude in the time it took for the plane window and their eardrums to blow out, requires some "belief" too. :)
 
Sweet... blind “belief” in math over empiric data. Almost religious.

If the data doesn't apparently fit the equation, it doesn't mean that the equation is wrong. Or the data. It may be the brain applying one to the other. (We see the latter in the statistics of medical research papers all the time.)

For example, is there much water vapor at that altitude? Let's just assume that the water vapor partial pressure decreases proportionally with the atmospheric pressure, so 760 - 47 becomes 252 - 15. Also, as somebody so wisely pointed out, there is a huge difference between mountain climbers who are chronically exposed to hypoxic air (and develop compensatory mechanisms) and couch potatoes who are not trained for it. Because there is a huge difference between 40/0.8 and 13.3/0.8.

So the maligned equation could look like this: PalvO2 = 0.21 x (252-15) - 13.3/0.8 = 50- 15 = 35 mm Hg. Minus a 5 mmHg alveolo-arterial difference, we get a PartO2 of around 30, not much different than observed.

Let's apply the same equation to our couch potatoes, and we get 0.21 x (252-15) - 40/0.8 = 0. If one hyperventilates to 30 mmHg of pCO2, it's still like 10 mmHg of alveolar oxygen pressure. That's almost pointless anyway, because even 30 mm Hg would kill a person who's just not acclimatized to that level of hypoxia.

Anyway, as long as we don't know the water vapor partial pressure at that altitude, the equation is useless, and we are just speculating.
 
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If the data doesn't apparently fit the equation, it doesn't mean that the equation is wrong. Or the data. It may be the brain applying one to the other. (We see the latter in the statistics of medical research papers all the time.)

For example, is there much water vapor at that altitude? Let's just assume that the water vapor partial pressure decreases proportionally with the atmospheric pressure, so 760 - 47 becomes 252 - 15. Also, as somebody so wisely pointed out, there is a huge difference between mountain climbers who are chronically exposed to hypoxic air (and develop compensatory mechanisms) and couch potatoes who are not trained for it. Because there is a huge difference between 40/0.8 and 13.3/0.8.

So the maligned equation could look like this: PalvO2 = 0.21 x (252-15) - 13.3/0.8 = 50- 15 = 35 mm Hg. Minus a 5 mmHg alveolo-arterial difference, we get a PartO2 of around 30, not much different than observed.

Let's apply the same equation to our couch potatoes, and we get 0.21 x (252-15) - 40/0.8 = 0. If one hyperventilates to 30 mmHg of pCO2, it's still like 10 mmHg of alveolar oxygen pressure. That's almost pointless anyway, because even 30 mm Hg would kill a person who's just not acclimatized to that level of hypoxia.

Anyway, as long as we don't know the water vapor partial pressure at that altitude, the equation is useless, and we are just speculating.

Though vapor pressure should be the same since it is not affected by attitude
 
Everyone's fretting about the alveolar gas equation being disproven - but it's really not!

For a constant FiO2, there's only one number we can change in the equation - the pCO2. Get that low enough and you can fit more O2 in.

Fit, healthy patients who are not acclimatised to altitude have been put in a low pressure chamber to have their gases measured (at standard atmospheric pressure). I'll give you the numbers directly from the experiment (found in Nunn's respiratory physiology):

Pressure chamber ambient pressure: 253mmHg (so partial pressure O2 = 53)
pO2 30.3
pCO2 11.2

Let's plug that into the equation: .21 * (253 - 47) - 11/0.8 = pAO2 29.51

The equation works.

For example, is there much water vapor at that altitude?

The main determinant of saturated vapour pressure is temperature. However, the alveolar gas equation assumes that alveolar air is warmed and 100% humidified by the nasal/oral mucosa to body temperature , which is why it remains static at 47mmHg.

I understand the point about your average plane passenger not being able to drive their pCO2 down to 10mmHg. Young people can definitely get there - we've all seen DKA patients with pCO2s in the single digits. However they can certainly get close for a short amount of time, and more importantly cabin pressure isn't getting all the way down to ambient pressure from such a small hole because the pressure is still being topped up by bleed air from the engines.
 
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I have no knowledge of the details of the situation on the plane, but, is it possible that the lady getting sucked into, and filling, the hole created actually saved the rest of the passengers by preventing further decompression of the cabin until the point where the pilots could descend? One of the ones who struggled to pull her from her position where she was wedged in the window said he was not strong enough to do so by himself. It sounds like she was likely wedged into that tiny space pretty tightly such that the amount of pressure equalization from the atmosphere to the cabin was very small and very slow, as opposed to if a hole had been blown in the side of the plane and it was big enough for her to be sucked out completely. Not to sound crude, but was she a human cork that saved the other passengers? Just a thought. I suspect it all happened in the matter of a couple of seconds.
 
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