Monday, June 8, 2009

Useful Information about Hypoxia

How the Human Body Uses Oxygen
Let's start our discussion of the effects of altitude on our bodies by first discussing how the body normally acquires, transports and uses oxygen. The importance of all those gas laws should become clearer. We're all aware that oxygen is necessary to sustain combustion or oxidation. It is necessary in the human body for the same reasons -- to support the oxidation of fuels needed to provide energy for life.
Very little of the oxygen carried by the blood is carried in dissolved form in the plasma. Most of the oxygen -- almost 98% -- is transported by the hemoglobin molecules in the red blood cells. The ability of hemoglobin to combine with and transport oxygen is dependent upon the pressure of oxygen in the surrounding environment. Higher pressures of oxygen enable the hemoglobin to take up larger quantities of oxygen. Lower oxygen pressures will result in an increasing tendency by the hemoglobin to give up oxygen. This variable combining characteristic is what allows the blood to acquire oxygen in the lungs and transport it to the tissues where it is used in metabolism. This characteristic of the hemoglobin also results in what is known as the oxygen dissociation curve (see graph below). While we've seen that oxygen pressure decreases a bit less than linearly with altitude, the ability of the hemoglobin to hold oxygen follows a much different curve. There is a big change for the worse in the hemoglobin's ability to combine with oxygen that occurs in the low twenties.

Air entering the lungs at sea level enters at a pressure of 760 mm Hg. This results in a partial pressure of oxygen in sea level air of about 160 mm Hg. (that's about 21% of 760 mm). The blood flowing through the lungs isn't exposed to atmospheric air though. Blood comes in contact with alveolar air -- the air mixture contained in the tiny air sacks of the lungs -- which is only 14% oxygen. (This is because of the addition of water vapor to the air you breath in plus the carbon dioxide that has diffused from the blood returning from the tissues.) The partial pressure of oxygen in alveolar air is about 14% of 760 mm Hg or 106.4 mm Hg. Carbon dioxide, which is 5.5% of alveolar air (as contrasted to less than 1% in the atmosphere) exerts a pressure of 41.8 mm Hg.
The hemoglobin in the blood returning from the tissues carries oxygen at a pressure of about 40 mm Hg. Graham's Law governs the diffusion of oxygen from the higher pressure of the alveolar air to the blood and the diffusion of carbon dioxide from the blood to the alveolar sacks. The opposite transfer takes place when the oxygen rich blood reaches the tissues which carry oxygen at an average pressure of 20 mm Hg. This lower pressure will allow the hemoglobin to release oxygen which will then diffuse into the tissues. At the same time, carbon dioxide is diffusing from the tissues into the blood. (An average pressure for CO in the tissues is 50 mm Hg.; however, this is dependent upon the activity level of the tissue.) Getting hypoxic yet from all this high altitude discussion??

In a normal, healthy individual, sea level pressure is sufficient to cause the blood leaving the lungs to be almost totally (97%) saturated with oxygen. At 10,000 feet the saturation has dropped to almost 90% -- still sufficient for nearly all usual life functions. An oxygen saturation of 93% is considered by medical folks to be the low limit of normal functioning. On top of Pike's Peak (about 14,500 feet and 438 mm Hg atmospheric pressure) the oxygen saturation has dropped to about 80%. Many people, if left in this rarefied air for some period, will develop mountain or altitude sickness: vertigo, nausea, weakness, hyperpnea (increased breathing), incoordination, slowed thinking, dimmed vision and increased heart rate. At 25,000 feet the oxygen saturation is only 55% and consciousness is lost. (Note that the partial pressure of oxygen in alveolar air at 25,000 feet is 14% of 281.8 mm Hg or 39.5 mm Hg -- slightly less than that normally found in venous blood returning from the tissues. Which way do you think the oxygen will diffuse at altitudes above 25,000 feet?)
Nowadays, altitude-savvy pilots are starting to carry a tiny instrument called a pulse oximeter that clips on the finger and, by passing a light beam through the vascular bed of the fingertip, measures the oxygen saturation of the blood and displays it on a digital readout. Think of it as a "hypoxia meter" that allows you to see precisely how hypoxic you are at any given time.
Types of Hypoxia
The effects of hypoxia upon flying skills and the symptoms of its onset are the same no matter what the cause of the hypoxia. It is useful, however, to look at some varying causes of this condition so we can be alert to its possible onset when of one or more of these factors is present.
Hypoxic hypoxia: is also referred to by aviators as "altitude hypoxia." This is the hypoxia that results when there is a lack of available oxygen or partial pressure of oxygen in the breathing air. This is the type hypoxia experienced when flying in an unpressurized cabin or when flying at altitude in a jet with a cabin pressurized to a cabin altitude above 5000 feet. Although strictly speaking, we are somewhat hypoxic when operating even a few hundred feet above the altitude of acclimatization, this becomes most evident when flying unpressurized aircraft. In reality, the symptoms of hypoxic hypoxia do not, in the absence of other contributing factors, become significant until about 5000 feet.

Hypoxic hypoxia occurs because there is a smaller and smaller pressure differential between the pressure of oxygen in the inspired air in the lungs and the pressure of the oxygen in the blood and tissues. Remember that the combining power of hemoglobin and oxygen is influenced by this pressure differential. The greater the differential, the more efficient the hemoglobin becomes. As this pressure differential lessens, it becomes harder and harder for the hemoglobin to pick up and transport the oxygen.
Hypemic hypoxia: (also called anemic hypoxia) occurs whenever the blood's ability to carry oxygen is reduced although there is sufficient oxygen at a sufficient pressure in the inspired air. There are a variety of conditions that can cause this to happen.
Any condition that would cause a reduction in the number of healthy, functioning red blood cells (anemia or reduced production of red blood cells, blood loss, deformed blood cells, disease, etc.) will impair the blood's ability to supply the tissues with oxygen. Remember the old advertisements warning about "iron poor blood?" Iron is the functional part of the hemoglobin molecule and it is the iron which renders the hemoglobin absolutely indispensable for life. In addition to a reduction in the number of red blood cells available, anything that would interfere with the ability of hemoglobin to transport oxygen or anything that would displace the oxygen that is bound to the hemoglobin will affect the oxygen available to the cells.
The most common impairment to oxygen transport by the hemoglobin is carbon monoxide. Carbon monoxide combines with hemoglobin 200-300 times more readily than does oxygen and once bound is extremely hard to eliminate. Smokers will find that the carbon monoxide bound to their hemoglobin will lower their altitude for onset of hypoxic symptoms by 2000-3000 feet. This effect is not limited to smokers, however. Anyone exposed to a smoky atmosphere will suffer somewhat. (Remember this next time you volunteer to go along as a designated driver for a group of drinkers. Just sitting in that smoky bar for several hours is going to affect your performance the next day, even without alcohol and fatigue!) Other chemicals, among them sulfa drugs and nitrites (found in food preservatives) can have an adverse effect on the ability of hemoglobin to combine with and transport oxygen.
Histotoxic hypoxia: is a disruption of cellular respiration. There may well be sufficient oxygen of sufficient pressure in the inspired air to fully saturate the blood and hemoglobin, but the cells expecting and needing the oxygen are unable to use it due to the presence or absorption of cell toxins. The most common toxin found at the cellular level that can cause this effect is alcohol. Although other toxins, notably cyanide and some narcotics, also can cause this disruption of cellular respiration, alcohol is by far the most common culprit.
Now, we are all aware of the hazards associated with alcohol and flying and I'm not suggesting that any true professional would knowingly violate these rules and guidelines. Many pilots, however, may be impaired by alcohol at the cellular level and not be aware of the problem -- or its cause. Remember the iron poor blood mentioned earlier? Be cautious of the "tonics" or "elixirs" offered as remedies. Carefully read the labels on any over-the-counter medications or nutritional supplements you propose to ingest. Although many more manufacturers are eliminating or reducing the alcohol content of the liquid medications, you may be surprised at the percentage of alcohol some still contain. One popular vitamin supplement for "iron poor blood" contains 12% alcohol!

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