There are many physiological factors to consider as a pilot, and one in particular can be as insidious as it can be deadly: Hypoxia.
Hypoxia is medical jargon for “lack of oxygen.” While the cells in our body can die without enough oxygen, the most immediate threat is the effect Hypoxia has on the brain and motor functions.
The reason Hypoxia is so insidious is the way we perceive things while suffering from oxygen deprivation.
For most General Aviation pilots, Hypoxia sets in slowly, and often times we don’t realize anything is wrong.
It gives us euphoria, making us feel good, as well as slows down our reaction times and impairs our judgement, occasionally leading to catastrophic results.
What exactly are the different types of Hypoxia, how does one get it, how does it affect us, and what can we do if we’re suffering from it?
Keep reading to find out the answers.
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The 4 Types of Hypoxia
While Hypoxia means “lack of oxygen,” it is just a blanket term. There are several different types of Hypoxia, each with their own causes and effects.
There are 4 general ways a lack of oxygen can manifest, and these correspond to the different types of Hypoxia.
The 4 types of Hypoxia are:
The different types of Hypoxia are determined by their causes, such as insufficient oxygen supply, compromised transportation of oxygen, or the inability of the body to absorb oxygen.
In general, the main cause of Hypoxia for the General Aviation pilot is due to thin air, which is why the FAA mandates supplemental oxygen for pilots flying above certain altitudes.
It is required if flying for more than 30 minutes between 12,500ft MSL and 14,000ft MSL, and required for the duration of the time spent above 14,000ft MSL.
The different types of Hypoxia, including their causes and symptoms, are explained in detail below.
What is Hypoxic Hypoxia?
Hypoxic Hypoxia is the lack of oxygen available to the body as a whole.
Choking and drowning are extreme examples of ways oxygen can be cut off from the body.
For pilots, however, flying at high altitudes is what leads to Hypoxic Hypoxia.
Because the air density decreases as altitude increases, there are fewer oxygen molecules for the body to absorb.
While not as sudden or apparent as choking or drowning, the cause is the same; the body is receiving insufficient oxygen.
What is Hypemic Hypoxia?
Hypermic Hypoxia is when the body is unable to transport a sufficient supply of the oxygen that is available.
The main purpose of the blood cell is to transport oxygen and waste products back and forth between the lungs and the rest of the organs in the body.
If there are an insufficient amount of blood cells that are available to carry oxygen, this results in Hypermic Hypoxia.
A decrease in blood volume from extreme bleeding, or blood disease like anemia, can lead to Hypermic Hypoxia.
Engine exhaust contains Carbon Monoxide (CO), and if there is an exhaust leak that enters the cabin, CO poisoning can occur.
Carbon Monoxide is absorbed several hundred times more easily than oxygen, reducing the amount of blood cells available to carry oxygen to the organs.
Because of this, CO poisoning is a type of Hypermic Hypoxia, and can lead to death if not addressed quickly.
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What is Stagnant Hypoxia?
Stagnant Hypoxia is when there is a sufficient supply of oxygen in the blood stream, but it is unable to move throughout the body.
Everyone has experienced an arm or leg “falling asleep” caused by a constriction of blood flow that gives us that tingling sensation we all dread. That is a perfect example of Stagnant Hypoxia.
A constricted blood vessel or blockage can also cause the blood flow to stop, along with shock and heart related issues.
Fighter pilots experience various types of hypoxia during extreme maneuvers, but in particular, Stagnant Hypoxia is the most common.
In certain cases of high G loading, blood flow can be forced out of the brain, but the heart is not strong enough to pump against the forces.
This leads to a blackout, again caused by stagnant blood flow, resulting in Stagnant Hypoxia.
What is Histotoxic Hypoxia?
Histotoxic Hypoxia is when oxygen-rich blood is flowing normally, but the organs can’t make use of it.
Narcotics and certain poisons, like cyanide and hydrogen sulfide, can cause Histotoxic Hypoxia, but this is rare in pilots.
The biggest risk is from pilots drinking alcohol. As little as one ounce of alcohol can make the body feel 2,000ft higher than it actually is.
This is why it is imperative not to drink before (or during) flying, as the onset of Hypoxia can increase dramatically at lower altitudes than one would anticipate.
What are the Symptoms of Hypoxia?
Unless you were shot out of a cannon straight up, or experience explosive decompression, most pilots experience the gradual onset of hypoxia.
What makes it so insidious are the initial symptoms, which include euphoria and a carefree attitude, leading us to believe all is well.
A lack of judgement follows, and limbs become less responsive and coordinated flying becomes more difficult. Reaction time decreases.
Worsening hypoxia results in narrowed vision and slowed interpretation of flight instruments, but due to the euphoria, most pilots think everything is A-OK.
Because of these symptoms, even of “mild” hypoxia, it is very easy to make a mistake that can lead to injury or death.
There are several other symptoms of hypoxia to know and understand:
- Tingling extremities
- Visual Impairment
- Cyanosis (blue lips/fingertips)
If a pilot suspects they are suffering from hypoxia, descending to a lower altitude or using supplemental oxygen will alleviate the symptoms.
All pilots will be affected by hypoxia, no matter how fit or healthy they are, although physical fitness may delay its onset.
Above 10,000ft MSL, the length of time the pilot has to make safe decisions about their condition decreases rapidly.
At 20,000ft MSL, the “time of useful consciousness” is around half an hour, but above 30,000ft MSL, it decreases to less than a minute.
Therefore, it is essential for every pilot to know the causes, symptoms, and mitigations for hypoxia.
What are the 4 stages of Hypoxia?
Hypoxia affects people on a spectrum, from no issues at all, to unconsciousness.
However, there are defined stages one goes through while suffering from hypoxia, and are normally associated with altitude ranges.
Those stages are:
- Indifferent Stage
- Complete Compensatory Stage
- Partial Compensatory Stage
- Critical Stage
Indifferent Stage (0 – 5,000ft pressure altitude)
For people in good health, there are generally no adverse effects of hypoxia in this altitude range.
Complete Compensatory Stage (5,000ft – 11,400ft)
Night vision decreases as altitude increases, from about 10% at 5,000ft and 30% by 10,000ft.
Memory issues become more apparent as task load increases.
Due to the thinner air, respiration rate may increase, along with blood pressure and pulse rate.
Motor skills are generally unaffected, with no noticeable decrease in performance.
There is generally no need to mitigate the minor effects of the Complete Compensatory Stage of hypoxia.
Partial Compensatory Stage (11,400ft – 20,000ft)
Many things happen during this stage, and vary based on the particular individual and the altitudes involved.
Breathing rate drastically increases due to the thin air in the effort to maintain a functioning cardiovascular system.
The nervous system starts to degrade, with reaction times becoming slower and movements becoming uncoordinated.
Judgement becomes poor, and cognitive difficulties and disturbances manifest.
Thinking is slowed, and self-monitoring of one’s actions is diminished, leading to possibly hazardous actions.
Instruments become difficult to read, and the pilot will have trouble concentrating, resulting in delayed or imprecise communications.
For instance, while communicating with ATC, a pilot may communicate with a delayed, lazy, dull voice.
In several instances, this has alerted ATC to the fact that the pilot was suffering from hypoxia, and allowed ATC to relay mitigating procedures to the pilot.
Mood can also be affected at this stage. Pilots may appear drunk, and can switch back and forth between jovial and depressive, or become combative and aggressive.
While vision is affected by hypoxia, generally hearing stays intact, and being able to follow simple radio commands can save pilots from tragedy.
The effects of the Partial Compensatory Stage are the most pronounced, and the quickest and best mitigation is an increase in oxygen.
Studies performed in altitude chambers shows that just a few seconds of supplemental oxygen is enough to restore cognitive function.
If supplemental oxygen is unavailable, descending to a lower altitude (below 10,000ft) will suffice.
Critical Stage (above 18,000ft)
Above 18,000ft, unconsciousness and incapacitation can occur suddenly and with little warning.
If this occurs, it is unlikely that any mitigating actions can be undertaken, and death will result.
What can I do to identify Hypoxia early?
Awareness is the best way to identify early stages of Hypoxia and to start mitigation.
All Private Pilots learn about aeromedical physiology during their training, and hypoxia is covered, although in more general terms.
There are many videos on the internet that shows the effects of hypoxia at different pressure altitudes, and watching them can be both educational and amusing.
It is a very serious manner when you’re flying left seat though, so be sure to brush up on your awareness training before heading off to high altitude areas.
For more comprehensive training, the FAA Civil Aerospace Medical Institute (CAMI) offers a training course in Oklahoma City, OK.
The course offers demonstrations of rapid decompression (from 8,000ft to 18,000ft) and hypoxia (25,000ft) in specialized hypobaric pressure chamber.
As always, an ounce of prevention is worth a pound of cure. Learning about and being able to identify the signs of hypoxia is literally a matter of life and death.
How can I prevent Hypoxia during flight?
The best way to prevent hypoxia in flight is to stay below 10,000ft MSL, where most people encounter no apparent problems.
Staying away from drugs and alcohol is another way to prevent hypoxia, because alcohol has shown to lower the bodies oxygen absorption, decreasing the altitude hypoxia can occur at.
If flying above 12,500ft for long periods of time, a pressurized airplane or supplemental oxygen is required for the pilot.
Ensuring all oxygen/pressurization equipment is working properly is essential, and knowing and understanding the signs of failure is paramount.
Cabin depressurization is a drop in pressure inside a pressurized airplane, and can happen slowly, or rapidly, which is called explosive decompression.
Pressurized planes normally have a backup oxygen system, so in the event of a pressurization system failure, donning an oxygen mask and descending to a safe altitude is the prudent course of action.
For non-pressurized planes, there are devices that pilots can use while in flight to keep an eye on bodily oxygen absorption, such as fingertip mounted oxygen saturation monitors.
As carbon monoxide from a leaking exhaust can also cause hypoxia via CO poisoning, having a CO detector onboard is a good idea.
Personal Experience with Hypoxia
The author has had a personal experience with hypoxia during a trip to the Southwest.
Being based in the Midwest, where ground level is below 1,000ft, hypoxia is rarely an issue.
I had planned a trip to fly to Arizona and over the Grand Canyon to see its majesty from the air.
There are several VFR corridors available to pilots for flying over the Canyon, northbound being at 11,500ft MSL and southbound at 10,500ft MSL.
Traveling in a Cessna 150 that was already struggling with the high density altitudes, I decided to fly north of the Canyon and swing southbound down the Zuni corridor at 10,500ft.
I blamed it on the views and my passion for flying, but I definitely felt euphoric as I meandered over the Canyon at 85kts.
My destination was Flagstaff, Arizona, and the airport was still much higher than I was used to at 7,000ft MSL.
I made my calls correctly, and landed without issues, but I was in the Complete Compensatory Stage of hypoxia.
How did I know this? As I went inside the FBO to retrieve my rental car, the clerk asked me for my zip code, one I have had for the previous 5 years.
It was only one digit off from my former zip code but I just could not remember which zip code was the current one.
The high altitudes had definitely affected my memory, and I was beginning to get a slight headache as well.
It turns out that I had given her the wrong zip code, and that resulted in a billing error when they tried to process my credit card a few days later.
The issue was easily resolved, but it still shows how simple things can become more difficult even while suffering from mild hypoxia.
Hypoxia is a term meaning “lack of oxygen,” and there are four main types of hypoxia.
The first type is Hypoxic Hypoxia, which is caused by a lack of oxygen, either from a lack of oxygen from, say, choking, or the lack of outside oxygen caused by high pressure altitudes.
The second type is Hypemic Hypoxia, where there is enough oxygen in the environment, but the body cannot transport enough of it for the body to use.
This can be caused by a lack of blood from extreme bleeding, or can be caused by other issues, like Carbon Monoxide poisoning, which prevents the cells from absorbing oxygen.
The third type is Stagnant Hypoxia, in which there is a sufficient amount of oxygenated blood, but it is unable to be transported.
Cutting off the blood flow to a limb and having the limb “fall asleep” is one example of Stagnant Hypoxia.
The fourth type is Histotoxic Hypoxia, where there is a sufficient amount of blood being transported, but the body cannot make use of it.
This is the result of alcohol, drugs, or other poisons, which prevent the body from absorbing the oxygen.
In addition to the different types of hypoxia, there are also 4 stages of hypoxia.
The first stage is the Indifferent Stage, which normally occurs between 0 and 5,000ft.
There are generally no adverse effects during this stage for healthy people.
The second stage is the Complete Compensatory Stage, which occurs between 5,000ft and 11,400ft.
In this stage, cognitive abilities, such as memory and thinking speed, are affected. The nervous system and motor skills are generally unaffected, however.
The third stage is the Partial Compensatory Stage, and occurs between 11,400ft and 20,000ft.
This is the danger zone, where cognitive abilities decline rapidly, personality changes occur, and motor skills are impaired. Judgement is affected and can lead to fatal mistakes.
The fourth stage is the Critical Stage, which normally occurs around 18,000ft and above.
During this stage, incapacitation can occur within a few minutes, and if mitigation is unable to take place, death is inevitable.
Hypoxia can be mitigated if the effects are monitored closely and the pilots are aware of the mitigation strategies via training.
Donning an oxygen mask can restore impaired functions within a matter of seconds, and if those are unavailable, descending to a lower altitude can have the same effect.
Oxygen absorption can be monitored using fingertip devices, and Carbon Monoxide detectors can be installed for early warning.
Hypoxia can be insidious and deadly, but it is not invincible, and knowing what hypoxia is, its effects, and how to mitigate it are crucial skills every pilot needs.