Hypoxia
The pilot was cleared to climb to FL200, but as he continued, his speech became slurred. He acknowledged an ATC instruction to check his oxygen and instruments and to descend to 15000ft, but he did not comply. After that, there was no response to radio calls. A C-130 made a night intercept, but detected no occupants or signs of aircraft control. After being airborne for about three hours, the plane descended and was observed entering a fog bank near the crash site. The pilot was found alive 6.5 hours later. The oxygen tank was servicable, empty, valve at "off" position, but the oxygen masks were still packaged" -- extract from NTSB report.
Definition
Hypoxia is the lack of oxygen to the tissues that can result in impairment of function.
Types of Hypoxia
Signs and Symptoms
Summary
Hypoxia
Man requires oxygen to survive. The atmosphere provides this oxygen at all levels, however the partial pressure of oxygen decreases as total air pressure decreases with altitude. At sea level on a standard day the partial pressure of oxygen is 160mm Hg and as we climb this drops off until at 10000ft the partial pressure has dropped to 55mm Hg. If we look at the figure below we can see that 55mm Hg equates to a haemoglobin saturation of 85%. It is below 85% saturation that the body's functions start to degrade and thus where the semi-magic number of 10000ft is derived above which hypoxia is possible.
Despite the above hypoxia is not cut and dried at 10000ft. There are a number of types of hypoxia and each effect is cumulative.
Hypoxic Hypoxia
This is the most common form and is caused by low breathing rates or a low oxygen partial pressure in the inspired gas (eg. at higher altitudes)
Hypaemic Hypoxia
This is caused by a reduction in the oxygen carrying capacity of the blood and can be caused by anaemia, haemorrage, carbon monoxide (sticks to haemoglobin 250 times more readily than O2), drugs or blood donation.
Stagnant Hypoxia
A reduction in blood flow through the tissues is the actual definition and this can be caused by fainting, cold temperatures (due to constriction of blood vessels (as an aside cold also increases metabolic rate)), cardiac failure, and G-Forces.
Histotoxic Hypoxia
This is the inability of the tissues to absorb the oxygen and can be caused by carbon monoxide, alcohol, cyanide, and narcotics.
Given all the above it can be seen that despite the fact that a healthy person on a standard day is unlikely to become hypoxic below 10000ft, the person who is a smoker, who had a big night before, and took a few Codral because of a sniffle before going flying, and is a bit cold at altitude is susceptible to hypoxia at a much lower level.
Personality changes - Similar to the early stages of intoxication, most become happy although a few become beligerant.
Impaired Judgement - One of the most dangerous effects of hypoxia. The individual will not notice degradation of his performance, nor that a problem exists. The individual can be told that he will will die if he does not descend and he will happily accept that!
Mental and Muscular Incoordination - Simple problem solving becomes difficult, reasoning and decision making is affected. Fine muscle movement (eg. fingers) become difficult and speech slurs.
Memory loss - Short term memory loss is onset at the early stages of hypoxia.
Sensory Loss - Vision may tunnel or "spots" may appear in the field of view. Hearing degrades as does general orientation.
Time distortion - The individual will perceive a long period of time to pass slowly.
Hyperventilation - As the body senses the decrease in O2 levels it will increase the respiration rate to try and counter the situation.
Cyanosis - The extremities, particularly the fingertips and lps may assumea blueish tinge due the reduced oxygen being transported by the haemoglobin.
Finally semi-consciousness, unconsciousness and then death will result.
Hypoxia is INSIDIOUS. It will creep up without you knowing it. Without formal training it is difficult for an individual to identify his own hypoxic symptoms as they are very individual. The best advice is not to fly above 10000ft without supplemental oxygen and if flying near 10000ft bear in mind the various signs and symptoms. As a passenger often you are in a better position to identify the degradation in performance of those around you so keep on eye on your pilot!
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