6 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, ASRS Reporter (See http://asrs.arc.nasa.gov).
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.
The obvious teaching points of this experience:
- Hypoxic incapacitation develops very rapidly when supplementation
is lost.
- Hypoxia must be 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?
In late 1999, the golfer Payne Stewart left Florida in his 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 the 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.
More often the impairment of hypoxia is incomplete, and early hypoxia
is unrecognizable.
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.
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
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.
1. 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.
2. We assume, since this an essay on human physiology, that 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.
3. 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.
4. 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:
- 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.
- Gas Exchange: After fresh air is brought in, gases must
diffuse across the membranes that separate blood from air in
the 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, a
person who feels fine on the ground will discover they have
inadequate reserve at altitude.
5. Blood abnormalities may hinder oxygen from being delivered to
the tissues where it is actually used in energy production.
- Anemia: Anemia is a shortage of hemoglobin, the chemical
that transports oxygen from lung to tissue. Hemoglobin is
packaged in 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.
- Carbon Monoxide: Glider pilots don't have to worry about
carbon monoxide (CO), of course. Except for motorgliders.
Or the ride to the airport in that old car. Or flying tow.
Or 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.
Carbon monoxide poisoning is just a form of hypoxia, one
that can't be "fixed" by going to a lower altitude.
- 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 have other effects on vascular
dynamics. The retina is especially dependent on oxygen:
night vision deteriorates above 8000 ft normally, but as low
as 5000 ft in smokers.
Smokers have, on the average, about a 10% decrease in
delivery of oxygen to tissues, and the effects of a single
cigaret last for an hour, not counting the carbon monoxide,
which is retained for days.
Also, nicotine has a sedative effect; nicotine withdrawal
enhances anxiety, so a cigaret smoker who is aloft a long
time is more susceptible to hyperventilation and other
anxiety-related problems.
6. Oxygen release: Oxygen must be released from red blood cells
and diffuse into tissues. 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 Hyperventilation, page two, below.)
7. 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.
6.1 The atmosphere and hypoxia
"Hypoxia" means "low oxygen." If your cells lack oxygen, we say 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 hypoxic environment, usually high altitude. Hypoxemia includes
the root "heme," meaning "blood;" if the oxygen content of your blood
is low, you are hypoxemic. This is just to be pedantic and precise
about terms, so if I misuse them below you can snicker or send me a
tart letter.
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 gradually with altitude. There 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.
This is probably why the crew and passengers in Payne Stewart's jet
never had a clue something was wrong.
There 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 2) are very similar.
6.2 Hypoxic symptoms
A symptom is something you 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,3 we don't notice this happening, because we
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3. Smokers have reduced night vision even at sea level. Sorry about
that.
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 the horizon tilt crazily...
6.3 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 O2
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.
Altitude Atmosphere O2 Pressure Blood O2 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% O2 121 mm Hg 91%
(This is about the best a canula can do)
Figure 2: Partial Pressure of Oxygen and Hemoglobin Saturation With Altitude
6.4 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 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 $350 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.
6.5 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.
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 CO2, in the form of [HCO3]- as a buffer to regulate pH.
When we breathe either more deeply or more rapidly than necessary--
hyperventilate--we blow off CO2, 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 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.
The chief symptom is shortness of breath. There is really no
physiologic reason whatever for a healthy pilot, sitting in a cockpit,
to become short of breath. QED, 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 put a paper bag over our head, is
breath-holding. Simply hold your breath for a few seconds over and
over again, 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 air for ten to twenty minutes, until all the symptoms
are gone completely.