Friday, October 23, 2015

Oxygen Exchange

I work in an unpressurized aircraft at altitudes where the atmospheric pressure is low enough that oxygen is required. Between 10,000' and F180 (about 18,000') I wear a nasal cannula: an arrangement of tubes, including two blowing oxygen up my up my nostrils. Above FL180 I wear a mask that covers my nose and mouth. I could skip the cannula and wear the mask any time I was above 10,000' but I can't eat or drink while wearing it, so it gets old fast. Plus the design of the cannula is such that it recycles exhaled oxygen, meaning that the oxygen supply lasts a lot longer on cannulas. Thus we only use the masks if we're working above 18,000'.

Putting the mask on involves taking off my headset, putting the mask over my face, pulling the head strap into place, putting the headset back on, and fastening another mask strap behind my neck. Finally I have to swap the microphone plug for the headset boom mike with the microphone plug for the in-mask mic. I can do all this before take-off, which I will if I'm in busy airspace. I started up next to a military jet once and noticed its pilot put on his oxygen mask before start up. If I am in uncontrolled airspace, I typically put my mask on just before I climb through 10,000'. If I'm expected to be monitoring a frequency, I wait until a needy or slow-talking pilot starts to make a call irrelevant to me, and then I can get the headset off, mask on, and headset back on before the call is over, ensuring I don't miss any calls. It takes less than thirty seconds. On descent, I can use the same technique, or just leave it on until I park, likely confusing the FBO marshallers.

Sometimes the day starts out with an hour of work at at 20,000' and then progresses to three more at 15,000'. Obviously I need the mask for the high level work, but would prefer to be able to eat and drink for the rest of the flight. In this case the swap involves all of the above, plus disconnecting the mask from the oxygen receptacle, connecting the cannula, and putting the cannula in my nostrils. It looks just like the cannulas the patients in House wear, but rather than looping around my ears and hanging down in front, it just goes around my head, held up by my headset earcups, and secured in place with a baseball cap. The point of this post is that I think about perfecting the swap between oxygen supplies at altitude. Should I breathe normally, or hold my breath during the swap?

The way breathing works is that the partial pressure in the air I inhale is greater than that in the lung capillaries it is in contact with. The imbalance causes oxygen to diffuse into the capillary, until the partial pressures are equal, the way any gas does across any membrane. So if the partial pressure in the ambient air is less than that of my lung capillaries, the act of breathing will actually decrease the oxygen level in my blood, and it would be better to hold my breath while I switch between mask and cannula.

The partial pressure of oxygen in ambient air at sea level is 21% of 29.92 inches of mercury, which is 6.28 inches of mercury. I'm going to convert that 160 mm Hg, not because I'm obsessed with metric, but because Wikipedia gives me biological numbers in millimetres of mercury so I have to convert one of them in order to do the math. At 18,000' the partial pressure of oxygen in the ambient air is half that at sea level, so 80 mm Hg. The partial pressure of oxygen in lung capillaries at sea level is 20-40 mm Hg, and as the mask maintains my blood oxygen at the same saturation as at sea level, then that's the partial pressure of oxygen while I'm wearing an oxygen mask. Eighty is clearly greater than forty, so even at 18,000' breathing is better than holding my breath to maximize blood oxygen during the mask swap.

So at what altitude should I hold my breath? At 32,800' the partial pressure of oxygen in the atmosphere is 25% of the sea level pressure, which is 40 mm Hg, the top of the range for lung capillary pressure. I would say, "so up to 32,000 it's better to breathe than hold one's breath when swapping oxygen sources" but above 32,000' the time of useful consciousness without oxygen is around a minute, even with no physical activity, so one shouldn't be messing around with one's oxygen source at such altitudes. But my conclusion is that above that, hold your breath for those few seconds of scrabbling before you either get your oxygen on or forget how and pass out.

I only took biology to about grade eight, so most of what I know about the human cardiovascular system comes from lifeguard class. I welcome any corrections to this analysis, even if you're stopping by years after I wrote it.


Sarah said...
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Doug Sinclair said...

An interesting wrinkle in this is the effect of water vapour. At 37 C water has a vapour pressure of 47 mmHg. The air in your lungs is always at 37C, 100% RH, regardless of the outside environment.

At sea level in dry air the pressure is 760 mmHg and the partial pressure of O2 is 21%. So there is 160 mmHg of O2 in the air. In your lungs there is 47 mmHg of H2O and 713 mmHg of air, of which 150 mmHg is O2.

Now let's say you want to go as high as you can breathing pure O2 through a mask. When the outside pressure is 197 mmHg you will have 47 mmHg of H20 and 150 mmHg of O2 in your lungs. This corresponds to an altitude of about FL330.

If you go up to where there is an outside pressure of 150 mmHg (i.e. FL400) then you will only have 100 mmHg of O2 in your lungs. Maybe enough to stay conscious, but certainly not sea-level pressures.

Aviatrix said...

So Doug, you are saying that because the partial pressure of H2O in the lungs does not diminish with altitude, it effectively displaces oxygen as the total pressure decreases? Interesting. Using your numbers, a pilot breathing unpressurized pure O2 through a mask would have the same partial pressure of oxygen in her lungs as a pilot without supplementary oxygen at 6500'. The 6500' is for dry air, so the equivalent altitude would be a bit less.

That also quantifies the dehydrating effect of breathing at altitude, even with sufficient supplementary oxygen.

Doug Sinclair said...

Yes, that's exactly what I'm saying. The H2O becomes a larger and larger fraction of the gas in your lungs with altitude. The Armstrong Limit at about FL620 is the point where the water in your lungs displaces all of the other gas leaving no room at all for oxygen. Put another way, it's the point where your surface fluids start to boil.

At 6500' the total pressure is about 600 mmHg. Your pilot's lungs contain 47 mmHg of H2O, and 553 mmHg of air, of which 116 mmHg are O2. So yes, this is roughly equivalent to what she'd get breathing pure O2 through a mask at FL400. That's certainly enough to function well -- people in Denver seem to manage.

My cellphone (Galaxy S6 Edge) has a little SpO2 gauge built into it. I wouldn't trust my life to it, but I do use it while flying. Using the Aerox cannula at FL190 I have an SpO2 of 84%. [Yes, that's 1000' above its rated altitude. I was trying to find out just how high my little Mooney would go. FL190 seems to be its ceiling.]

I absolutely agree about the dehydration. The outside air is cold, and carries very little absolute humidity. With every exhalation you dump water into your surroundings. The exact same applies to low-altitude Canadian winters.

Aviatrix said...

That is very interesting. I had thought that it was merely the decreased total pressure that made the partial pressure of O2 insufficient in an unpressurized mask, as altitude increases. Canadian air law required a pressurized mask above 30,000'.

Somewhere in here is a blog post about me trying to do my regularly scheduled run while working in Colorado. I think I ended up jogging slowly around a baseball diamond.

I used to have a very fine pulse oximeter, but it must have fallen out of my flight bag one day and rolled under a hotel bed, because it vanished. My employer supplies them now and they work, but the time to get a reading is a little longer, the ergonomics not as good and the battery life not as long. I miss mine, but still like to pretend it will turn up somewhere.

TgardnerH said...

The water vapor effect Doug is describing is definitely standard medical knowledge (my wife is just wrapping up med school, and it was discussed at length there).
The question of "hold breath or breathe" depends on how long you're holding your breath--you want to hold your breath until the air in your lungs from your last oxygen mask breath is "worse" than the air you'd take in from the cabin. From experience playing with a pulse-oximeter, I know I can hold my breath for at least a minute before the blood leaving my longs stops being very well oxygenated, and since the partial pressure of oxygen in your lungs after you breathe from your mask is supposed to be roughly what it is at sea level, I'd guess that you too have about a minute before it's even worth considering taking a breath.
However, if you really wanted to figure this out, you could do an experiment based on the fact that your oxygen saturation is a really good proxy for how "good" the air in your lungs is--at altitude (while somebody else has the controls), take off the mask and see what your blood saturation stabilizes at while you breathe cabin air. Then, put the mask back on, and after you're back to normal hold your breath while watching the oximeter, and see how long it takes before the reading is the same as what you stabilized at without oxygen. My bet would be that you can't even get there without it being very painful, since we generally have lots of oxygen in our lungs, but the drive to breathe is based on CO2 accumulation. Obviously you would want to do this reasonably low, so you don't fully black out and need somebody else to put the mask back on you, and not do this when you're supposed to be flying. Actually, the first part of this seems pretty stupid overall--maybe just keep it as a thought experiment. But the second part, where you hold your breath for a minute and watch the oxygen saturation data, that will show you if your blood keeps getting its oxygen while you hold your breath to change masks.