@Subsection{Oxygen}

There are many reasons we need oxygen, but only one is continuously important to the pilot aloft in an aircraft:  without lots of it, our brain does not work right.

An example of how subtle and insidious hypoxemia can be was published in the January, 2000, @I{ASRS Reporter} (See http://asrs.arc.nasa.gov). @begin(Quotation, Spacing = 1.1 line, Above = 0)
While at FL250 on an IFR flight plan, ...I could hear the oxygen escaping and thought the regulator had not sealed on the portable tank behind the passenger seat.  As I had changed tanks within the past 15 minutes, I attempted to tighten the regulator, but to no avail.  I recognized hypoxia coming on, pulled power back, disconnected the autopilot, and lost consciousness.  I became conscious at 17,000 feet.  The plane was descending and in a bank.  I leveled the plane and declared an emergency and told the controller I had lost my oxygen supply and had lost consciousness.  I landed at the nearest airport, [where] I saw that the line to the regulator had come off. @end(Quotation)

The obvious teaching points of this experience: @begin(Itemize, Above = 0, Spacing = 1.1 line)
Hypoxic incapacitation develops very rapidly when supplementation is lost.

Hypoxia must be @I{recognized} before incapacitation occurs: time is short, folks!

This pilot used the "benign spiral mode" for a safe descent while unconscious, by disconnecting the autopilot and throttling back.  How does your own ship behave when you release the controls?  Would you remember quickly to configure your ship for this mode if you sensed incapacitation coming? @end(Itemize)

In late 1999, the golfer Payne Stewart left Florida in a LearJet and crashed in South Dakota.  Reports are clear that there was no response from the crew to ATC's transmissions and no activity to manipulate the airplane's controls, as it flew in a straight line until its fuel was exhausted.  The only thing that could have caused the rapid and complete simultaneous incapacitation of pilots and passengers in this aircraft is hypoxia, as carbon monoxide is not available to the cabin pressurization and ventilation system of this jet.  Above FL 300 incapacitation occurs quickly with a sudden decompression.

More often the impairment of hypoxia is incomplete, and early hypoxia is unrecognizable.  This is always a risk at lower altitudes, between about 8000 and 22000 ft.

Some pilots think that their skills are so well honed that they can fly automatically.  Well, that's almost true, but the "automatic" stuff is located in cerebellar and cerebral memory, and that's your brain, my friend.  Without oxygen its pilot light goes out, and you start doing clumsy and dumb things -- unfortunately without feeling either dumb or clumsy.

We all know that the higher we fly, the less oxygen is available.  We could spend a couple of pages happily exploring the physics of the atmosphere and why the FAA regulations are too strict and why they're not strict enough.  But before we do that, there's something more important to bring up.

@I{The oxygen in the air around you may not be available to your brain.}

That is, you may be happily flying around without oxygen at 12,499 feet, blissfully unaware that your brain has only as much oxygen as you'd expect it to have at 18,000 feet.  (Just to pick some numbers arbitrarily.)

This is true because the oxygen in the air you breathe must somehow be @I{delivered} to your brain.  There are a number of physical conditions that can impede this delivery; you may not be aware that you have one of these conditions, or you may underestimate the importance of one you do have.  If you're not sure, talk to a doctor who understands your disease and altitude issues.

Let's get technical for a couple of paragraphs and simply list the fences that oxygen has to cross in order to fuel your brain's fires. @begin(Enumerate, Above = 0, Below = 0, Spread = .5 line, Spacing = 1.1 line)
@I{Your lips or nose}.  Seems tautological, doesn't it?  But here's your first chance not to get the oxygen you think you're getting: You have hayfever or a cold; your nose is stuffy; you've become a mouth-breather for awhile.  We don our oxygen canula as usual.  What do we do?  We put it across our stiff upper lip, the little prongs tickling our nose hairs.  The oxygen goes where?  Why, it wanders around our face instead of flowing in as we breathe, and perhaps a little of it will be entrained with the air rushing into our mouth...if there's no draft in the cockpit.

We assume, since this an essay on human physiology, that @I{the equipment} is actually delivering oxygen, that the bottle is full enough, the regulator is on, there's no frozen spit or dew in the line and no kinks, and so on.  But your physiology does need this stuff to be working, or you may not have any physiology afterward.  It's worth checking to be sure.

@I{Your mouth, throat, and trachea} we assume are working.  Diseases of the major airways that would hinder oxygen delivery would pretty much motivate you to go to hospital; flying would be the last thing on your mind, so we can dispense with a whole list of things.

@I{Your lungs} are another matter.  Folks with asthma or other chronic lung or heart disease might feel just fine on the ground, but could discover at altitude just why the doctor seemed too interested.  Here's the deal:  Your lungs do two things: @begin(Itemize, Spacing = 1.1 line, Spread = .5 line, Above = 0, Below = 0)
@I{Ventilation}:  Air must move in and out, as with bellows, in sufficient volume to exchange fresh air for (pardon the expression) dead air.  Asthma, pneumonia, emphysema, and the like hinder the lung's bellows function.

@I{Gas Exchange}:  After fresh air is brought in, gases must diffuse across the membranes that separate blood from air in the @I{alveolus}, the terminal air sacs that are the business part of the lung.  Diseases that thicken these membranes hinder oxygenation of the blood.  Also, diseases that cause uneven ventilation of the lung cause some blood not to become oxygenated as it flows through.  In either case, such a person who feels fine on the ground will discover there's inadequate reserve at altitude. @end(Itemize)

@I{Blood} abnormalities may hinder oxygen from being delivered to the tissues where it is actually used in energy production.  @begin(Itemize, Above = 0, Below = 0, Spread = .5 line, Spacing = 1.1 line)
@I{Anemia}:  Anemia is a shortage of hemoglobin, the chemical that transports oxygen from lung to tissue.  Hemoglobin is packaged in @I{red cells}.  These are freighters for oxygen.  If you don't have enough buses, you can't get the tourists to the museum.  If you have anemia, oxygen delivery to your tissues is hindered no matter how much oxygen is flowing through that nasal canula.

@I{Carbon Monoxide}:  Glider pilots don't have to worry about carbon monoxide (CO), of course.  Except for motorgliders.  Or on the ride to the airport in that old car.  Or while flying tow.  Or, what about that heater in the camper where you slept last night?  Carbon monoxide binds almost irreversibly to hemoglobin, preventing a portion of red blood cells from carrying any oxygen at all.  This has exactly the same effect as anemia.  It can take days for the carbon monoxide to completely leach out of your red cells.  We all know that carbon monoxide exposure can be fatal or incapacitating; subthreshold exposure, without symptoms, can severely decrease altitude tolerance:  exposure you're unaware of, too small to cause headache, can decrease your altitude tolerance by half.

@I{Smoking}:  Smokers of tobacco usually have carbon monoxide levels of 3 to 7 per cent, enough to reduce altitude tolerance significantly, and the hundreds of organic chemicals in cigaret smoke affect vascular dynamics: it causes veins and arteries to constrict.  The retina is especially dependent on oxygen: night vision deteriorates above 8000 ft normally, but as low as sea level in smokers.  If you doubt this, take a night flight, get settled in at any altitude you like, put on oxygen and watch the lights go on.

	Smokers have, on the average, about a 10% decrease in the blood's ability to deliver of oxygen to tissues, and the effects of a single cigaret last for an hour, not counting the carbon monoxide, which slowly decreases over several days.

	Also, nicotine has a sedative effect; nicotine withdrawal enhances anxiety, so a cigaret smoker who is aloft a long time (long enough for withdrawal to begin) is more susceptible to hyperventilation and other anxiety-related problems.  @end(Itemize)

@I{Oxygen release}:  Oxygen must be released from red blood cells and diffuse through our juices into tissue.  There are few conditions that hinder the diffusion of oxygen from red blood cells to tissues, but several things can hinder cells from releasing oxygen properly.  Most importantly, when the blood Ph becomes alkaline--called "alkalosis"--this hinders the release of oxygen to body tissues.  Hyperventilation produces alkalosis, so hyperventilation will cause hypoxia to occur at a lower altitude than you might expect.  (See @I{Hyperventilation}, page @PageRef{hyperventilation, template = "%o"}, below.)

@I{The atmosphere}.  Only after we've made sure that our supplemental oxygen system works well, and our body's gas exchange systems are functioning, does it make sense to worry about altitude.
@end(Enumerate)

@Paragraph{The atmosphere and hypoxia}

"Hypoxia" means "low oxygen."  If your @I{cells} lack oxygen, we say @I{you} are hypoxic, meaning that oxygen is failing to get into cells such as your brain cells for some reason.  Sometimes this is because you are in a @I{hypoxic environment}, usually high altitude.  @I{Hypoxemia} includes the root "heme," meaning "blood;" if the oxygen content of your blood is low, you are @I{hypoxemic.}  This is not merely pedantic, as the paragraphs before this one detailed several ways in which you can become hypoxemic, or have "tissue hypoxia" (we don't have a one-word term for this) without being in hypoxic conditions.

The only organ that matters while soaring, as far as oxygenation goes, is your brain.  The glider is the only soaring bird with a removable brain, and we'd not like you to take yours out of commission while still aloft.  High altitudes are a low-oxygen environment because the "partial pressure" of oxygen--the fraction of atmospheric pressure that represents oxygen--decreases with altitude.  As the amount of oxygen available to our brain cells decreases, they don't work  as well.

Brain efficiency decreases @I{gradually} with altitude.  There @I{is} an on-off switch, called "consciousness," but decreased performance begins as low as 5000 ft msl, and there's a long gradual slide before we wink out somewhere above 20,000 ft msl.  As I read the literature, night vision declines first, then complex problem-solving such as mental arithmetic, then procedural mistakes begin to occur, then judgment declines.

The sensation of shortness of breath is caused by many things, but not by lack of oxygen.  Anything that makes the lungs stiff causes us to feel short of breath, and exercise or excitement do, too.  But slowly decreasing the oxygen level in your blood is just like putting a frog in a pan of cold water over a fire.  It's comfortable until the end.

There @I{are} symptoms of hypoxia that precede unconsciousness, but they are subtle and vary considerably between persons.  In order to recognize hypoxia, you must try it out yourself and see what your own symptoms are.  As you'll discover, the symptoms of hypoxia and hyperventilation (page @PageRef{hyperventilation}) are very similar.

@Paragraph{Hypoxic symptoms}

A symptom is something you @I{notice} about your body.  Although night vision is reduced by 10% at 5000 ft msl and by 28% at 10,000 ft msl (3000 M), in nonsmokers,@Foot{Smokers have reduced night vision even at sea level.  Sorry about that.} we don't notice this happening, because we don't have anything to which we can compare our vision.  Cognitive symptoms -- slow or erroneous thinking -- begins when our arterial blood is about 87% saturated with oxygen.  Incapacitation is likely if the saturation falls below about 65%.  The actual point of incapacitation varies considerably between people, and real men don't admit they're incapacitated at any point short of being comatose.  But your ability to add and subtract goes away long before you lose your grip on the control stick or see strange colors in the clouds.

@Paragraph{Clues you can Use}

Here's where you're on your own to experiment.  The symptoms of mild hypoxia are so subtle and differ so much from one person to another that you must simply go up without oxygen and see how you feel.  Yet at moderate altitudes without oxygen (the exact range depends on acclimatization and individuality) our brains simply aren't in fine tune.  Your mind isn't as nimble at 14,000 msl as are at sea level.

Tiredness or poor night vision develop at altitudes of 5000 or 6000 feet msl, but these aren't reliable clues to hypoxia.  The safe way to do discover your own symptoms is with a ride in an altitude chamber.  Judgment and fine motor control are impaired when the blood O@-[2] saturation is below about 85% in the healthy, unacclimatized pilot.  It's judgment that matters most at this point.  Hypoxia is a little like getting smashed: the bystanders are much more clear about who is impaired and by how much than the drunk.

At 14,000 or 16,000 msl, fine motor control is less important, as at altitude in smooth weather, piloting an aircraft is about as demanding as sitting in front of your TV twiddling the remote.  But navigating and complex strategic decisions are different matters entirely.

A ride in a pressure chamber is the safest way to find out how to detect your own symptoms of hypoxia.  Another way is to fly dual with a pilot who has oxygen.  Headache, nausea, dizziness, sleepiness, or fatigue are common symptoms.  Some pilots repeatedly calculate compass bearings as a check on mental function, with the very logical theory that if you can mentally subtract 170 from 340 without hesitation, the brain is at least working well enough to navigate.

@begin(Figure)@U{
Altitude	Atmosphere	O@-{2} Pressure		Blood O@-{2} Saturation}

Sea level	760 mm Hg	160 mm Hg		     98%

5000 ft msl	624 mm Hg	131 mm Hg		     94%

10,000 ft msl	523 mm Hg	110 mm Hg		     87%

FL 180		404 mm Hg	 85 mm Hg		     72%
	with 30% O@-{2}		121 mm Hg		     91%
   (This is about the best a canula can do)

@Caption{Partial Pressure of Oxygen and Hemoglobin Saturation With Altitude} @end(Figure)

@Paragraph{How Much Oxygen is Enough?}

Good question.  The answer is, "enough is enough."  Enough, that is, to keep your brain working satisfactorily.

FAR's require the pilot of an unpressurized aircraft to use oxygen above 12,500 feet MSL "for that part of the flight exceeding 30 minutes" and to use oxygen continuously above 14,000 feet.  At 14,000 ft the pilot's blood oxygen saturation would be around 80% without oxygen, well below the level at which cognitive function fades.  This regulation makes sense as you've already seen.

If you often fly above 10,000 ft msl, consider buying a pulse oximeter, which measures blood oxygen saturation through the skin.  The @I{Nonin Onyx} is a little black block that gently embraces the end of a finger and continuously reads your saturation.  It is reliable as long as your fingers are warm.  It's not expensive considering that it could save your life when you are not certain if your oxygen system is actually delivering as it should.  It's about $400 at medical supply houses, and uses two AAA batteries.  It comes with a neck lanyard.

Despite the fact that a healthy person is unlikely to become impaired by hypoxia below 10,000 ft, the smoker who has taken a cold tablet for a stuffy nose, who is getting chilly, who may have tipped a few last night, or the pilot with a heart or lung condition, is susceptible to hypoxia at a much lower altitude.

If you're flying dual, keep an eye on each other.  Hypoxia, like drunkenness, is easier to recognize in others than in ourselves.

@Paragraph{Hyperventilation}@Tag{hyperventilation}

What causes hyperventilation?  Not neurosis, but emotional stimulation of any kind.  Hyperventilation occurs reflexively with fear, excitement, intense physical activity, etc.  Many events in a glider can cause emotional arousal, either euphoric or aversive: all cause some degree of hyperventilation.

Increased carbon dioxide in the blood is the strongest stimulus to breathing.  This is what creates the powerful drive to get to fresh air that we begin to feel in an unventilated small room.  Ironically, the brain's carbon dioxide detector has a reversal point: if your blood's carbon dioxide content falls quite low, this causes you to feel extremely short of breath.  It is this that exacerbates hyperventilation and makes overcoming it extremely difficult when it is severe.  This is not merely a psychological effect, it is physiologic: low blood carbon dioxide levels drive overventilation.

Our bodies are a complex chemical soup, dependent on millions of continuous catalyzed chemical reactions.  The catalysts--enzymes--are dependent on exact control of temperature and pH.  Cells die if the pH is below 7.0 or above 7.8; but believe me, you'll feel very ill if it's below 7.3 or above 7.6; piloting an aircraft won't seem fun.

Your body uses CO@-{2}, in the form of [HCO@-{3}]@+{-} as a buffer to regulate pH.  When we breathe either more deeply or more rapidly than necessary--hyperventilate--we blow off CO@-{2}, getting rid of this buffer and quickly making the blood alkaline (raising the pH).

Your body's respiratory center reflexively overventilates under any conditions of physical or psychological stress: excitement, fear, anxiety, euphoria, or anger; and also hypoxia, vibration, heat, or illness.  This is not something you can necessarily decide not to do.

Fortunately, most hyperventilation is not severe or disabling, and often it's appropriate, such as during illness or before or during intense exertion.  But it's seldom needed in a cockpit, as we're strapped in with little opportunity to actually exercise.  Unfortunately, there's no warning that your respiratory center is about to hit the reversal point, beyond which hyperventilation becomes self-perpetuating.

At altitude, we've got an extra problem: if the hyperventilation is due to hypoxia, the alkalosis it causes hinders release of oxygen from red blood cells in the tissues, also worsening the hypoxia.

Worst case scenario for the pilot, then, is to be at 18,000 msl or higher with a silently failing oxygen system, and hyperventilation that is unrecognized until severe shortness of breath and knifelike muscle spasms in the arms and hands, calves and feet make proper control manipulation difficult.  The home remedy--putting a paper bag over your face and breathing into it--is not something we expect the pilot in command to reach right out for as the logical solution to the problem.

The chief symptom of hyperventilation is shortness of breath.  There is really no physiologic reason whatever for a healthy pilot, sitting in a cockpit, to ever become short of breath.  QED, @I{if you feel shortness of breath while piloting an aircraft, you are hyperventilating}.  Other symptoms include tingling sensations of the lips, tongue, mouth, finger, or toes; incoordination, dizziness, lightheadedness, headache, subtle and strange visual distortions, or muscle twitching.

The solution, since we don't want to be putting paper bags over our heads in flight, is breath-holding.  Simply hold your breath for a few seconds, as long as possible, over and over again.  Meanwhile, check and turn up your oxygen, and descend if possible.  If you do have a paper bag, hold it over your nose and mouth and rebreathe your own air for ten to twenty minutes, until all the symptoms are gone completely.