The Air Is Thinner Up There
Every year I see patients just back from Colorado who are concerned because they
experienced shortness of breath or palpitations while there on vacation. We go there as
well on vacation with our Labrador dog, Moose. I will try to explain why many of these
symptoms are in fact predictable and a result simply of the physics of going to altitude.
We are essentially at sea level here in Dallas at 431 feet elevation. When you arrive in
Colorado by plane you likely land in the mile high city of Denver at 5,280 feet or, if like
us, drive the first leg to Santa Fe (7,000 feet) or Raton (6,680 feet) for the first night.
From there you set out for the bigger mountains. The base elevation of some of the ski
resorts range from 6,900 feet at Steamboat to 10,780 at Keystone. The peak elevation is
more than 12,000 feet at many of the familiar names: Telluride, Vail, Beaver Creek,
Snowmass, Crested Butte, Breckenridge, Keystone and Arapahoe Basin among others. If
you are up in the mountains skiing or hiking you are probably going to be above 10,000
feet and can even be above 14,000 feet if you tackle one of the states 53 “fourteeners”,
mountains taller than 14,000 feet.
So you are up there. Not quite Mount Everest (29,020 feet) or Kilimanjaro (19,341) but if
I suddenly took you to Everest your oxygen level would be so low you likely would pass
out in minutes. But even at 10 or 12,000 feet, despite being healthy, you may not feel so
great and want to come back to see your cardiologist.
The air we breath in the atmosphere is composed principally of oxygen (21%) and
nitrogen (78 %) and that percentage remains the same no matter the elevation, from sea
level to the top of a mountain. What does change is the atmospheric pressure, which
drops almost linearly as you gain altitude. The atmospheric pressure compresses all the
gases at that altitude meaning that the greater the pressure, the more molecules of oxygen
(and nitrogen) there are in the air per square inch. That pressure is 760 mm Hg
(millimiters of mercury) at sea level, denoted as 1 atmosphere pressure; whereas at
10,000 feet it is 534 mm Hg or 0.7 atmospheres. That means that at 10,000 feet there are
only 70% of the number of oxygen molecules in the air versus at sea level. And it is the
number of molecules that impacts how much oxygen is getting into our blood. The
difference is roughly linear so at 8,000 feet we have 76% and at 12,000 feet 65% the
number of oxygen molecules, and at Everest only 30%.
So the air is thinner up there. How does that impact how we feel and react on our
vacation in Colorado? In order to understand that, we have to understand 2 concepts.
First, how the number of oxygen molecules in the air, or partial pressure of oxygen,
effects the partial pressure of oxygen that is in our arteries. And, second, how the partial
pressure of oxygen in the arteries impacts the oxygen saturation in the blood and the
amount of oxygen that is delivered to our tissues and organs
Mathematically, the partial pressure of oxygen in the air is derived by multiplying the
oxygen concentration, 21%, times the atmospheric pressure. Thus, at sea level, .21 times
760 = 159 mmHg. But by the time that oxygen gets humidified in the lungs, is diluted by
the carbon dioxide we expel and crosses into the arterial blood the partial pressure drops
to 98 mm hg. This can be measured by a test, the arterial blood gas or “ABG” and is
reported as the PaO2. If you have had it done, you will likely remember it because it
requires a pretty painful stick with a needle into the artery in your wrist. People rarely
volunteer to have it done twice. A normal PaO2 ranges from 80-100 and is usually at the
high end in a young, healthy person. The PaO2 in a patient with heart or lung disease can
be 50-70.
A much simpler and painless test to administer is to measure the blood oxygen saturation
or “sat”. This can be done with a pulse oximeter which is clipped on a finger or an ear.
My patients have this done routinely on their visits to the office and some have purchased
a monitor for their own use. This measures the percentage of hemoglobin molecules
which are carrying oxygen. The oxygen saturation, or more precisely, the oxyhemoglobin
saturation, is not the same thing as the partial pressure of oxygen. This can be confusing
but is important to understand.
Red blood cells are essentially bags full of hemoglobin and hemoglobin’s job is to bind
and carry oxygen to the tissues, release it there and return to the lung to pick up more.
Very little oxygen is dissolved in the blood, not enough to support life, but each
hemoglobin can bind and carry four oxygen molecules and provides almost all of the
necessary oxygen to the organs and tissues. Each red blood cell can carry a billion
oxygen molecules when the hemoglobin is fully saturated. If you study Figure 1 you see
the partial pressure of oxygen in the artery on the lower horizontal line and the oxygen
saturation on the left, vertical line. The shape of the curve is not a straight line, but S
shaped. As you see, the saturation remains above 90% even with a PaO2 down to 60 but
after that point there is a steep fall off, where the oxygen saturation falls precipitously
with each small decline in the PaO2. The oxygen saturation with a pulse oximeter can be
deceptive in the upper, flat portion of the curve because it will decline little despite a
significant reduction in PaO2. We look for an oxygen saturation of 94% or more to
indicate a normal PaO2.
Now let’s go back to altitude. At 10,000 feet the number of oxygen molecules is only
70% of sea level and so the partial pressure in the air is only 112 mm Hg instead of 159
and in the arterial blood of a healthy person at rest drops to 54 mm Hg. That is a number
we can encounter in our seriously ill patients with emphysema. So even if you are
healthy, and have never smoked, simple physics indicates that going to 10,000 feet has
the equivalent effect on PaO2 of having emphysema. That PaO2 level is right on the
steep part of the oxyhemoglobin dissociation curve, and instead of being 100% saturated
at sea level, you are now 86% saturated. So your tissues, your muscles, brain and heart,
are only being delivered 86% of their normal oxygen
Figure 1: This plots the oxygen saturation on the vertical line versus the partial pressure
of oxygen in the blood on the bottom, horizontal line. Notice the S shape.
The numbers I have discussed so far are for healthy people, with healthy lungs, who are
at rest and not exercising and who are not anemic. But what happens if you are not young
and healthy and instead of starting with a PaO2 of 98 your Pa02 is a still normal but
lower 80 at sea level? Then at 10,000 feet your PaO2 drops to 46 mm Hg and your
hemoglobin oxygen saturation drops to 78%. If you walked into my office with those
numbers in Dallas we would be slapping an oxygen mask on you and shipping you to the
emergency room. Similarly, if you are anemic any reduction in your oxygen saturation is
magnified because you have fewer red blood cells and the reduced oxygen carrying
capacity of your blood takes an additional hit. You can calculate your predicted PaO2 at
various altitudes on Figure 2 if you know what your PaO2 is here in Dallas.
Figure 2: Draw a straight line from your PaO2 at sea level to the altitude to which you
will travel and the middle vertical line will show the PaO2 that will result. Then you can
plot that on Figure 1 to arrive at the oxyhemoglobin saturation that will produce.
The body responds to this oxygen deprivation via two short term mechanisms which are
actually determined by the PaO2 level rather than the oxyhemoglobin saturation. This
level is detected by peripheral chemoreceptors on the carotid body, a group of cells next
to the carotid artery in the neck. Detecting lower oxygen levels, the carotid body signals
the body to try to augment oxygen delivery to the tissues. One mechanism is to increase
the amount of blood you pump by increasing your heart rate. Second, you increase the
amount of air you exchange by increasing how frequently and deeply you breath.
Even if you are healthy, it is normal for you to feel that your heart is beating faster and
you are breathing more rapidly and deeply as you arrive at your Colorado mountain
destination. These are the symptoms which mainly provoke concern in my patients. As
you go to 10,000 feet your heart rate will increase by 10-30% at rest and at any level of
exertion compared to in Dallas. At Pikes Peak, an increase of resting heart rate of 40-50%
is normal. And for any level of submaximal exercise, the increase in rate is proportional.
So if you work out on a treadmill to a heart rate of 130, the same workout at 10,000 feet
might produce heart rates of 143 to 170, which may feel very uncomfortable in the sense
that you feel your heart is pounding while doing something you do much more easily in
Dallas.
Similarly, your rate and depth of breathing increase. At Everest peak, climbers will breath
4 times the volume of air per level of work than they do at sea level. At rest at 10,000
feet, instead of breathing a normal 14 times per minute, you may find yourself breathing
24 times a minute. Again, this can induce a sense of uncomfortable breathlessness.
Another very noticeable difference at altitude is the sense that “I just can’t do it.” You
can’t perform to the same level of sustained exercise that can be done here at home. This
also has been studied and measured in a cross section of subjects from those who are
sedentary to elite athletes and is true across the board. This can be explained by the
maximal oxygen uptake, the VO2max. As oxygen is the fuel which our engine burns, this
is a measure of how much fuel is able to be used when you are at maximal exercise. It is
a product of how much blood the body can pump per minute times the number of oxygen
molecules per unit of blood that are extracted by the muscles and tissues in that minute. It
is how much oxygen you are able to burn when your engine is at full throttle. That
number decreases by 9% at 6,000 feet, 14% at 9,000 feet, 24% at 12,000 feet and 32% at
15,000 feet and an astounding 80% at Everest peak. The degradation in VO2max is
actually greater in trained athletes, who may notice the change in performance capacity
beginning at altitudes of only 1800 feet whereas a decline for most of us becomes
measurable at 4500 feet. This is probably because they have such a high capacity to start
with. The decline is 1% per every 100 meter altitude above 1500 meters so each 1,000
feet of elevation can produce a perceptible change in how much you are able to do.
One little known fact is that just flying to Colorado, or anywhere for that matter, exposes
you to an altitude equivalent to 6,000 to 8,000 foot elevation. Commercial airline cabins
are pressurized so that when you fly at 35,000 feet or more all of us don’t lose
consciousness. But the cabins are not pressurized to sea level because that would place an
undue outward pressure strain on the frame of the airliner. Even healthy people may
experience shortness of breath and a general ill feeling on a commercial airline flight due
to that lower atmospheric pressure. One of the attributes of the new 787 jetliner is that,
because it has a carbon fiber frame, it can be pressurized to that lower level of 6,000 feet
to minimize those symptoms. Some precautions are necessary however because for
patients with impaired oxygen levels at sea level, the stress even of 6,000-8,000 feet can
be enough to tip things over to produce serious respiratory distress. In general, it is
recommended that if your sea level PaO2 is 70 or less you should be considered for
wearing oxygen during your flight. To know whether you have that lower PaO2 without
having a painful blood gas test, a finger pulse oximeter test can be performed. If your
oxygen saturation is 94% or greater your PaO2 is likely above 70 and if below 90% likely
below 70. Intermediate levels may require additional testing.
Going to altitude can produce disturbing symptoms of breathlessness, rapid heart action
and exercise intolerance which to which patients are unaccustomed but may simply be
normal. From my perspective as a cardiologist, it is important to see how you responded
before and after the trip while at a lower altitude. If the symptoms that were experienced
at altitude resolve back here in Dallas and there is no other indication of heart disease on
examination it is likely that things can be blamed on your “Rocky Mountain High” rather
than your heart.
The field of human response to exercise is much more complex than I have outlined
above and I have relied upon studies performed by many researchers in the subject to
produce this distilled version. For those that are interested in reading more, there are 2
textbooks on the subject which provide very detailed information and references. Authors
Swenson and Bartsch; Title High Altitude: Human Adaptation to Hypoxia and
Authors West, Schoene and Milledge; Title High Altitude Medicine and Physiology
In addition there is a web site www.altitude.org which is useful and has a calculator to
derive predicted oxygen and saturation levels at various altitudes.