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.

Runner's High

This story starts about five years ago when I first realized that combining my fondness for running, maps, and cool watches in one technological device was possible and affordable, even if it did barely fit on my wrist. I bought a Garmin Forerunner 305. It's essentially a stopwatch integrated with a GPS, heart rate monitor, and computer, so I can know where I am, how fast I'm going, and how hard my body is working to achieve it, all at once. And then I can upload the resulting data to my laptop and stare at it in tables and graphs and maps to come to conclusions like "I get really tired when I run up mountains" and "I should run faster." I used to come to such conclusions without the technology, but now I have data to back it up. It's ludicrous how much more fun it is to go running when you get statistics at the end. I don't remember how I ever went running without a box on my wrist telling me if I was doing it right or not.

I took my GPS watch in my flight bag everywhere I went, so that if I had time to go running at the end of the day, I'd have the motivation of knowing how far and fast I was going, and also so I could activate the "Return to Start" feature. Return to Start gives turn-by-turn directions to get me back to the hotel, regardless of how identical the suburban streets, wooded trails, or gravel road turn-offs appear to be. I don't know that I would be still wandering around a highway truck stop somewhere without it, but there's more oxygen deprivation in my brain after fifteen kilometres of pushing my pace than after five hours of sucking supplemental oxygen through my nose at 17,000'. It's nice not to have to worry which forks I took in the trail network of an unfamiliar town.

I have my data all in Garmin's Training Center software: years of runs from Alaska to Florida and many where I have to zoom out on the little map several times before I figure out which little town with limited map information is traced by the record of my exertions. I like to review this data and use the Training Center "Compare" feature to virtually race the years-ago Aviatrix. (I'm pretty sure I can still take her). But while I've become sleeker, faster and more resilient with the years, the watch hasn't fared as well. It became harder to charge, and reluctant to connect to the computer to give up its data. The beeping sounds stopped working, and then it started shutting off in mid-workout. It's well out of warrantee, and the model is discontinued, so I had to choose a replacement. I could probably export the data in a standard format. Garmin is so prevalent in the GPS field that any manufacturer that couldn't convert or use their formats might be a little silly. But my first choice was just to stick with Garmin.

Their website is a mess of poor feature explanations. It doesn't suggest an upgrade path from the 305 to another product. I didn't find an overview on what the focus of the Forerunner, vivo-, or fēnix lines is, nor could I see any coherent logic to the Forerunner model numbers and suffixes.

I thought I'd had an e-mail conversation about this back when the Forerunner was still available and one could send it in to a Garmin-approved dealer for a flat-rate "repair" (obviously a replacement with a reconditioned model, but it was worth the $75 for me back then). Mining my e-mail archive for "GPS watch" found me all my old blog posts that mention "GPS" and "watch". The insane detail I included allowed me to relive some interesting flights and revisit all manner of places. No wonder people who weren't on the flights to begin with enjoyed reading them. This has somewhat inspired me to find a way to keep blogging about them. Perhaps I'll talk to the owner about the blog.

I tore myself away from my former adventures to google up second hand explanations and information on features and backwards compatibility of Garmin products. There are SO many models. Finally I decided to artificially limit my choices by going to a store and making my selection from only what they had. I took my flaky old Forerunner with me, and on the bus ride there, noticed that the crack between the layers of the case was quite wide. With no prying or wiggling, I lifted the face of the watch right off the back, revealing its electronic innards. Well no wonder it wasn't staying on, charging and connecting. I was lucky it hadn't fallen apart during a run. I feared I had killed it by carrying it with me everywhere, including into the flight levels in an unpressurized airplane.

The higher you go in the atmosphere, the less air there is on top of you, thus the less pressure you feel, so that instead of being all squished together to sea level pressure, the molecules are all spread out. One breath takes in less oxygen, unless I supplement it from a tank, which I do, through the aforementioned nose tubes. Anything with air sealed into it that I bring up from ground level retains the same pressure inside that it had on the ground. As it ascends, the differential between the inside pressure and the outside pressure increases, producing a force acting to expand the sealed container. The tank full of breathing oxygen is cylindrical, and its strong walls and robust valves are inspected weekly to ensure that as the pressure outside the tank decreases, the tank will not rupture, bulge or leak from the added force. Potato chips are deliberately packaged with trapped air, to protect the chips, and their bags will bulge out drum-tight, and sometimes burst, as we ascend. The gases in our intestines and ear canals expand and escape through the usual routes. I was afraid that the air sealed into my GPS watch damaged it. But then I realized that the Forerunner 305 isn't waterproof, and my Timex wristwatch, which works perfectly, is. (I'd tell you what model Timex, but it's so old that all the words have worn away, and the chunks of decorative plastic stuff have fallen off, leaving a bare case with buttons on it). A little more research showed me that plenty of people had their Forerunners come apart, sometimes falling off bike mounts and/or being run over by cars.

I got to the store and interrogated the clerk about the selection. I was almost ready to buy a Garmin Forerunner 225. It has the heart rate monitor integrated in the watch itself, with no need for a chest strap. It has a built-in accelerometer to give pace and distance information on indoor treadmill runs. "What about winter?" I asked. "Can I use it with a chest strap in the winter, when I wear the watch on the outside of long-sleeved garments?" To answer this question, the clerk went not to the Garmin website, but straight to the blog of DC Rainmaker, an obsessive sports technology reviewer. I'd encountered one of his reviews while googling for compatibility information earlier, and not noticed that everything I could ever want to know about pretty much any sports watch is on this guy's blog. The Forerunner 225 will pair with a chest strap, for people who mount it on bicycle handlebars, as well as winter runners, but it was missing a couple other features that I wanted. Indeed none of the watches they carried had the Back to Start feature, so I went home without buying any of them, and went back to DC Rainmaker's site.

Ray Maker (get it?) conveys the excitement of taking a new electronic toy out of the shipping box, gazing at its packaging, and then breaking the shrinkwrap. He includes photographs detailing the plastic bags and twist ties containing all the sub-components of the products. He does more running, biking, and swimming to test one device for a review than some buyers will log in the lifetime of their device. Every review is ridiculously detailed. Some people would say too detailed. So a little like many of my blog entries, if I did way more research. It parallels the way I'd like to blog if I had the diligence to drill down to the technological and legal root of everything I said, rather than speculating and drawing on knowledge already in my head. It made me cheer a little when I found a typo now and again, on his site, because it just goes to show that typo-free is not a requirement for an insanely awesome blog. He also displays a lot of integrity: he borrows and returns all reviewed gear despite the fact that his reviews are clearly enough work to deserve a lot of freebies. If you run or bike and like gear, you almost certainly already know his site, but if you don't you're in for a treat. If you never do anything more energetic than opening the refrigerator, but just like tech, especially GPS, you'll probably still want to read his site. The comparison tool there allowed me to quickly determine that the correct watch for my needs is the Forerunner 310XT. It wasn't at the store nor prominent on Garmin's website because it is ALSO being discontinued. I decided to get one now, through a site DC Rainmaker partners with, to compensate him for the work he does. I had some trouble with that site, so posted a comment on his blog, and as I was already thinking of blogging about the experience, I signed the comment "Aviatrix."

It turns out that DC Rainmaker is a long-time reader of Cockpit Conversation. It made me feel like a member of an elite when he asked if I was that Aviatrix. As I told him, some days I’m convinced that there are far fewer people on the Internet than is generally thought. Maybe it’s just you, me, the awesome people who comment on our blogs, and a lot of Chinese spambots.

Meanwhile I ordered a Forerunner 310XT somewhere else, bound my Forerunner 305 together with rubber bands, found it worked perfectly, glued it together with silicone sealant (as recommended by several people on the Internet), and am currently in high level discussions with my printer on why it should print the return mailing label for the 310XT. When the 305 gasps out its final beep I'll do this again, probably with a whole new set of features. I might get one of the "lifestyle" watches that track your every move, including sleeping.