Cannula vs Mask Above FL180

[petrolero]

Unpressurized aircraft pilots are supposed to switch to oxygen masks above FL180.

This rule was written before the advent of small pulse oximeters. I don't fly above 10k without checking sats and maintaining >91%, but...

Has anyone found masks above FL180 necessary? My a/c is only certified to FL200 and i have found cannula wholly satisfactory up there. But some unpressurized ac are certified to FL270.

Has anyone tried maintaining their O2 sats >91% up in the mid flight levels?

It really makes no difference to your body how its O2 is delivered as long as it has enough!

 

[Henning]

Yes, it makes a difference even above 16,000'.

 

[FORANE]

It really makes no difference to your body how its O2 is delivered as long as it has enough!

This is true but one must understand that we are giving oxygen due to decreasing atmospheric pressure. If you look at the table below at the altitude and mmhg columns you can calculate the partial pressure of oxygen for any given altitude. Oxygen is approx 21% of any given total pressure. So at sea level the partial pressure o2 is 760 x 21% = 160 mmhg. At 10,000 it is 522 x .21 = 109. At 20,000 it is 349 x .21 = 73.

A nasal cannula can deliver inspired o2 fractions at 20-40%. Notice at 40% max delivery at 20,000 you get 349 x .4 = 139. 139 mmhg is still less than sea level atmospheric oxygen partial pressure.

A simple face mask can deliver 40-60% at 5-10 lpm oxygen flow and a non-rebreather mask can deliver up to 90% at higher oxygen flow rates.

So, at higher altitudes such as above 18,000 a cannula is simply not capable of the required delivery rates.

 

[Dr. O]

It is even more complicated than the chart suggests.
Even at 100% FIO2 the rate at which O2 can be absorbed across the alveolar membrane decreases with decreasing ambient pressure. There is a point at which you must have a pressure mask, not just a mask.
The other thing that altitude chambers have clearly shown is that decreasing blood oxygen levels in the central brain are not sensed by the person. Watching the pilot trainees in air force videos try to copy a clearance with low oxygen is hilarious - yet they think they are doing fine.
Those who fly 18,000 and up with a nasal cannula are kidding themselves.

 

[spdracer888]

Unpressurized aircraft pilots are supposed to switch to oxygen masks above FL180.

I thought anytime over 12,500' for more than 30 min and over 14,000' you are required to have supplemental oxygen. Does the reg calls for different delivery methods and for higher FLs for unpressurized AC?


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[Alexb2000]

I fly the teens and 20's all the time. I have a pulse oximeter and I've tested 10-15 different people and their reactions (all middle aged plus). I use the oxygen boom cannulas and they seem to be able to keep up with most people at 180. I was just at 190 and using a cannula felt great O2 sat. in the mid-90s. However, I generally follow the basic guidance and switch to a mask above 180. Some people in particular NEED a full mask. I have almost instantly brought someone into the low 90's with a mask that was low 80's with a cannula. In general I am hesitant to fly at those altitudes without people that know what they're doing, just to much going on flying the plane and manage others O2. IMO the most important thing is to keep passing that oximeter around, testing and retesting everyone.

Also, around 250 most people will have an elevated heart rate due to the reduced pressure. I am very careful not to take anyone there that isn't generally in very good health.

 

[Henning]

I thought anytime over 12,500' for more than 30 min and over 14,000' you are required to have supplemental oxygen. Does the reg calls for different delivery methods and for higher FLs for unpressurized AC?


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Yes, but there are two typical ways, and a third extra high altitude, methods of delivering oxygen. There are cannulas, those tubes that blow O2 up your nose, then there are masks which cover mouth and nose (what I use typically above 16,000') then there are pressure masks which you should pretty much have above 23,000'. There are also constant flow, pulsed flow, demand, and diluter variants of different systems.

 

[spdracer888]

Yes, but there are two typical ways, and a third extra high altitude, methods of delivering oxygen. There are cannulas, those tubes that blow O2 up your nose, then there are masks which cover mouth and nose (what I use typically above 16,000') then there are pressure masks which you should pretty much have above 23,000'. There are also constant flow, pulsed flow, demand, and diluter variants of different systems.

Good to know. Thanks for the explanation .



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[Jaybird180]

This whole thread is an oxymoron.




Sorry, just had to get that off my chest.

 

[petrolero]

Yes, it makes a difference even above 16,000'.

Not according to my measured O2 sats. I was easily and consistently satting in the 90s at FL190 as was my entire family... on cannula.

Rules and formulas are one thing... measurements are another.

 

[petrolero]

This is true but one must understand that we are giving oxygen due to decreasing atmospheric pressure. If you look at the table below at the altitude and mmhg columns you can calculate the partial pressure of oxygen for any given altitude. Oxygen is approx 21% of any given total pressure. So at sea level the partial pressure o2 is 760 x 21% = 160 mmhg. At 10,000 it is 522 x .21 = 109. At 20,000 it is 349 x .21 = 73.

A nasal cannula can deliver inspired o2 fractions at 20-40%. Notice at 40% max delivery at 20,000 you get 349 x .4 = 139. 139 mmhg is still less than sea level atmospheric oxygen partial pressure.

A simple face mask can deliver 40-60% at 5-10 lpm oxygen flow and a non-rebreather mask can deliver up to 90% at higher oxygen flow rates.

So, at higher altitudes such as above 18,000 a cannula is simply not capable of the required delivery rates.

This is what prompted the question.

My experience is that cannula can maintain sats just fine at least as high as FL200. I'm wondering if others have had a different experience.

 

[petrolero]

It is even more complicated than the chart suggests.
Even at 100% FIO2 the rate at which O2 can be absorbed across the alveolar membrane decreases with decreasing ambient pressure. There is a point at which you must have a pressure mask, not just a mask.
The other thing that altitude chambers have clearly shown is that decreasing blood oxygen levels in the central brain are not sensed by the person. Watching the pilot trainees in air force videos try to copy a clearance with low oxygen is hilarious - yet they think they are doing fine.
Those who fly 18,000 and up with a nasal cannula are kidding themselves.

That point is approximately FL270 based on what I've read. That's why unpressurized ac don't get certified for higher than that. The mask that the FAA requires is not a pressure mask, just a mask that encloses nose and mouth.

Was I kidding myself that my sats were 91 to 93%? No. I measured it.

But again, I was only at FL190 which is only 1000 ft above the supposed FL180 'cannula = death' level (as some seem to imagine it). So my question is whether the cannula fails at higher altitudes than that and has anyone actually attempted to maintain sats in the 90s using cannula above FL200?

Also I live in Denver so my body should be a little more acclimatized to elevation than average.

 

[MachFly]

I uses to use cannulas at 18K (highest I've been without pressurization), didn't find any significant issues. Granted it wasn't very comfortable but that's okay. I haven't used a mask outside of a hyperbaric chamber, so my experience with that is a bit limited. Given the option however, I'd take a mask over cannulas, more efficient and more comfortable.

 

[dans2992]

Also, around 250 most people will have an elevated heart rate due to the reduced pressure. I am very careful not to take anyone there that isn't generally in very good health.

Is the elevated heart rate simply due to low O2 saturation, or something else?

 

[Alexb2000]

So my question is whether the cannula fails at higher altitudes than that and has anyone actually attempted to maintain sats in the 90s using cannula above FL200?

Also I live in Denver so my body should be a little more acclimatized to elevation than average.

I spent an half an hour at 210 with a cannula, just because I didn't feel like switching just to top a little weather. I stayed about 91-92%. Like you I live about half the year at 8,000', so I'm sure that has an effect.

Is the elevated heart rate simply due to low O2 saturation, or something else?

I'm no physician, but I've seen this pretty consistently. Sat. with a full mask is low to mid-90's so perfect, but with a slightly elevated heart rate. I just chalked it up to reduced pressure, IDK.

 

[petrolero]

I'm no physician, but I've seen this pretty consistently. Sat. with a full mask is low to mid-90's so perfect, but with a slightly elevated heart rate. I just chalked it up to reduced pressure, IDK.

I bet being in a small aircraft can account for most of it!

 

[Alexb2000]

I bet being in a small aircraft can account for most of it!

Maybe that's why my passengers cry so much after we land.

 

[petrolero]

Maybe that's why my passengers cry so much after we land.

Yours too, eh?

 

[Clark1961]

That point is approximately FL270 based on what I've read. That's why unpressurized ac don't get certified for higher than that. The mask that the FAA requires is not a pressure mask, just a mask that encloses nose and mouth.

Was I kidding myself that my sats were 91 to 93%? No. I measured it.

But again, I was only at FL190 which is only 1000 ft above the supposed FL180 'cannula = death' level (as some seem to imagine it). So my question is whether the cannula fails at higher altitudes than that and has anyone actually attempted to maintain sats in the 90s using cannula above FL200?

Also I live in Denver so my body should be a little more acclimatized to elevation than average.

I've been at 18 and fine with a cannula. Blood sat >90%. The problem I had was O2 flow was cranked way up to keep the sat up. I consider my practical limit with a cannula to be about 16 just because of O2 consumption.

 

[Skip Miller]

This rule was written before the advent of small pulse oximeters. I don't fly above 10k without checking sats and maintaining >91%, but...

Not according to my measured O2 sats. I was easily and consistently satting in the 90s at FL190 as was my entire family... on cannula.

When is the last time you checked the calibration of the pulse oximeter? That is the device you are trusting to be correct.... potentially a single point of failure. "I feel good" isn't a good diagnostic test for hypoxia, either.

My credentials in this argument: None. I have never been above 12.5 in a GA plane.

-Skip

 

[Clark1961]

When is the last time you checked the calibration of the pulse oximeter? That is the device you are trusting to be correct.... "I feel good" isn't a good diagnostic test for hypoxia, either.

My credentials in this argument: None. I have never been above 12.5 in a GA plane.

You bring up a good point. Smokers will get a false reading with the oximeter.

 

[jsstevens]

You bring up a good point. Smokers will get a false reading with the oximeter.

Can you elaborate on this? I've never smoked but I'm curious as to why.

John

 

[AdamZ]

I think there are a lot of factors to take into consideration including persons

1) Age
2) Physical Shape
3)Vices ie smoking
4) Disabilities or illnesses

etc.

For me at FL210 there was a noticeable difference between a cannula and a mask. I was MUCH better with the mask

 

[Henning]

Can you elaborate on this? I've never smoked but I'm curious as to why.

John

I think because it works off color, and carbon monoxide will cause a false indication.

 

[jsstevens]

I think because it works off color, and carbon monoxide will cause a false indication.

So if you're smoking while using the oximeter that would confuse it. OK, I can see that.

I was thinking the poster I responded to was asserting a smoker would confuse the oximeter even when not smoking but using oxygen. That 's what confused me.

But hopefully you are not smoking while using oxygen. That's a real bad idea.

John

 

[FORANE]

So if you're smoking while using the oximeter that would confuse it. OK, I can see that.

I was thinking the poster I responded to was asserting a smoker would confuse the oximeter even when not smoking but using oxygen. That 's what confused me.

But hopefully you are not smoking while using oxygen. That's a real bad idea.

John

John

The pulse oximeter cannot distinguish between carboxyhemoglobin and oxyhemoglobin. Carboxyhemoglobin levels are elevated in smokers and remain elevated for hours after the smoking has ceased; this level may be as high as 15%. Those levels will decrease significantly after 12 hours and generally return to normal levels within 48 hours if I recall correctly. So yes, a smoker will confuse a pulse oximeter in that the oximeter will read falsely high just as it will in the event of any other carbon monoxide poisoning event.

 

[jsstevens]

John

The pulse oximeter cannot distinguish between carboxyhemoglobin and oxyhemoglobin. Carboxyhemoglobin levels are elevated in smokers and remain elevated for hours after the smoking has ceased; this level may be as high as 15%. Those levels will decrease significantly after 12 hours and generally return to normal levels within 48 hours if I recall correctly. So yes, a smoker will confuse a pulse oximeter in that the oximeter will read falsely high just as it will in the event of any other carbon monoxide poisoning event.

Makes sense. Thanks for the explanation.

John

 

[petrolero]

When is the last time you checked the calibration of the pulse oximeter? That is the device you are trusting to be correct.... potentially a single point of failure. "I feel good" isn't a good diagnostic test for hypoxia, either.

My credentials in this argument: None. I have never been above 12.5 in a GA plane.

-Skip

There is plenty of published literature on fingertip pulse oximetry.

But I did calibrate ours against the one at the medical clinic at which my wife works. It's accurate.

 

[petrolero]

So if you're smoking while using the oximeter that would confuse it. OK, I can see that.

I was thinking the poster I responded to was asserting a smoker would confuse the oximeter even when not smoking but using oxygen. That 's what confused me.

But hopefully you are not smoking while using oxygen. That's a real bad idea.

John

You can smoke 'em a lot faster that way!

 

[petrolero]

I think there are a lot of factors to take into consideration including persons

1) Age
2) Physical Shape
3)Vices ie smoking
4) Disabilities or illnesses

etc.

For me at FL210 there was a noticeable difference between a cannula and a mask. I was MUCH better with the mask

Thanks for the info. I think you're only the 2nd person to actually answer the question in the OP.

I know there is no physiological magic to FL180. Nature doesn't work that way - it's just an FAA rule that applies to all pilots of all ages and physical conditions so you'd expect them to 'err' on the side of safety.

It's not like cannula at 17k = but cannula at FL180 = . That's silly. There is a gradient to these things and I was curious to hear from folks who'd gone higher than I can.

 

[Henning]

So if you're smoking while using the oximeter that would confuse it. OK, I can see that.

I was thinking the poster I responded to was asserting a smoker would confuse the oximeter even when not smoking but using oxygen. That 's what confused me.

But hopefully you are not smoking while using oxygen. That's a real bad idea.

John

Just makes your cigarette burn faster if you have it inside the mask.

 

[dans2992]

Cannula at FL200 was OK. Didn't stay there long though.

 

[Clark1961]

So if you're smoking while using the oximeter that would confuse it. OK, I can see that.

I was thinking the poster I responded to was asserting a smoker would confuse the oximeter even when not smoking but using oxygen. That 's what confused me.

But hopefully you are not smoking while using oxygen. That's a real bad idea.

John

As another poster pointed out, CO binds to the hemoglobin rather persistently so it causes a false reading long after the cig is out. It's another reason that CO poisoning is really bad.

 

[Palmpilot]

Interesting thread. I was involved in optical oximetry in graduate school around 1970, when we had to do it with catheters. I'm listed as one of the contributors to a paper around that time. It's great to see how much the science and technology of oximetry have advanced since then. (My career ended up going in other directions after that.)