Hypoxia in aviation is a problem of altitude.It is very important that pilots are alert on how the human body responds to flying at high altitudes when they transition from their light training airplanes to complex high performance aircraft’s that are capable of operating at high altitudes and high speeds.Before we get to know what hypoxia is, we need to understand the relationship of oxygen in our body with altitude.
Oxygen and Altitude
Oxygen is a colourless and odourless gas that makes up 21% of the earths atmosphere by volume. The brain weighs approximately 2% of our body weight but consumes almost 20% of the oxygen that our body needs for its normal functioning.Brain cells die if they don’t receive oxygen for 2 minutes.
Alveoli are tiny sacs in the lungs of a human body where the exchange of oxygen and carbon dioxide takes place through breathing in and out. Therefore it is clear that at anytime the concentration of oxygen should be high and that if carbon dioxide should be low in the alveoli.
According to International Standard Atmosphere (ISA) conditions, the standard pressure at mean sea level is 1013 hectopascal (760 mm Hg). As mentioned above if 21% of the atmosphere is filled with oxygen , it means out of the 760 mm, 160 mm is oxygen. This is what we call partial pressure of oxygen in the air. The human body takes its oxygen from alveoli in the lungs where the partial pressure of oxygen is relatively less (103 mm Hg) as compared to the partial pressure of oxygen in the atmosphere (160 mm Hg).
As we move up to higher altitudes, the amount of partial pressure of oxygen generated by the alveoli also starts reducing. Human beings living at higher altitudes like 10-12 thousand ft above mean sea level, their alveoli produces partial pressure of 55 mm Hg oxygen which is considered to be the minimum for normal operations.
Hence as we go above 10 000 ft, oxygen needs to be added to the cabins air supply .The oxygen that gets added is sufficient to maintain a partial pressure of oxygen that is similar to what humans require on ground (103 mm Hg). A stage is reached where 100% oxygen supply is required to maintain the 103 mm Hg of partial pressure ( what we breathe on the ground)
This stage is at approximately 33,700 feet. However this does not mean that we cannot go above 33, 700 feet, as we discussed above for our normal operations in the body we require a minimum of 55 mm Hg of partial pressure ( what we would breathe at 10 000 feet) . With 100% oxygen supply, this is reached at 40 000 ft .
Hypoxia ( HYPO= less , OXIA=oxygenation) is a condition where not enough oxygen makes it to the cells and tissues in the body. This can happen even though blood flow is normal. Hypoxia can lead to many serious, sometimes life-threatening complications.The most common causes of hypoxia in aviation are: flying, non-pressurized aircraft above 10,000 ft without supplemental oxygen, rapid decompression during flight, pressurization system malfunction, or oxygen system malfunction.
Types of Hypoxia and their Causes:
- Hypoxic Hypoxia (H) :The reason for this type of hypoxia is the result of low levels of oxygen in the bloodstream. The main reason for this condition occurring in aviation is altitude. As we have discussed in detail above that with increase in altitude, there results a fall in atmospheric pressure and the consequent resultant drop in the partial pressure of oxygen leading to Hypoxic Hypoxia.
- Anemic Hypoxia (A) :In this type of condition , the lungs are working perfectly but the red blood cells which are the carriers of oxygen in the blood decrease. The reason for the decrease in the count can be due to heavy bleeding, anemia, some type of cancer and lastly the intake of carbon monoxide.
- Stagnant Hypoxia (S) :In stagnant hypoxia, there is enough oxygen available to breathe but the blood flow is compromised for some reason like a heart attack and the blood is not received by the cells of the body tissues to support their metabolism.Stagnant hypoxia also occurs when the body is exposed to cold temperatures because the blood flow is decreased to the extremities.
- Histotoxic Hypoxia (H):In this category of hypoxia, there is enough oxygen to breathe and the oxygen is also being carried by the blood but the cells do not accept it. For pilots, some of the primary factors causing hystotoxic hypoxia are alcohol, narcotics and cyanide.
Signs and Symptoms of Hypoxia:
Hypoxia is easy to succumb to the human body and affect each body with varying intensities. even though the condition does not have an alarming warning system to protect us against us the treat, the signs and symptoms are easily recognizable if we identify them. As pilots we need to km=now that no matter the type or cause of hypoxia, the signs and symptoms for each type do not differ a lot and neither do they affect our flying skills in separate ways.
The signs and symptoms are mentioned below:
- The onset of hypoxia can be accompanied by a feeling of well being, known as euphoria.
- Impaired Judgement
- Tingling in hands and feet
- Memory and Muscular Impairment
- Sensory Loss
- Cyanosis (a bluing of the body extremities)
- Tunnel Vision
- Fornication ( a feeling of ants under the skin)
- Hyperventilation (over breathing)
- As hypoxia keeps intervening with reasoning, it gives rise to unusual fatigue and finally loss of consciousness or death.
Time of Useful Consciousness (TUC)
This is the period of time from interruption of the oxygen supply, or exposure to an oxygen-poor
environment, to the time when an individual is no longer capable of taking proper corrective and
protective action.It is also know as Effective Performance Time (EPT). We are clear by now that as altitude increases the risk of hypoxia increases and due to that our time for useful consciousness will decrease. Hence, altitude and time for useful consciousness are inversely proportional.The chart below will make it even more clear:
Immediate and Preventive Actions
- One way to avoid the risk of hypoxia for a pilot to make sure that his aircraft is correctly pressurized above 10,000 feet.
- In case the aircraft cannot be pressurized, carry supplemental oxygen (don your oxygen mask).
- If both the above options are not available, it is safe to be flying below 10 ,00 feet and even if we fly above that altitude due weather or terrain, we limit our time to a maximum of 1 hour if flying between 10 to 14,000 feet and 30 minutes if flying between 12 to 14,000 feet.
- The most important action will always be to use supplemental oxygen above 10,000 feet in day and 5,000 feet at night ( vision gets impaired at a lower altitude in the night due to hypoxia).
- The most effective way to prevent hypoxia is through education and experience.When pilots are trained in the proper use and care of their pressurization systems and supplemental oxygen equipment, and are aware of their personal hypoxia signs and symptoms, they are safer and better prepared to meet the challenge of flying in an oxygen-poor environment.
Fact of the Week
Boeing’s Everett Site is heralded as having the largest manufacturing building in the world, producing the 747, 767, 777, and the 787 airplanes. Thousands of aerospace employees in Everett support aircraft fabrication and production, product development, aviation safety and security and airplane certifications. Other production areas at the site include the paint hangars, flight line and delivery center. Originally built in 1967 to manufacture the 747, the main assembly building has grown to enclose 472 million cubic feet of space over 98.3 acres.
In January 1967, the first production workers arrived at Everett, and on May 1, 1967, the major assembly buildings opened their doors for the first time. Thousands of people from all over the world visit the Everett site every year. VIP visitors have included U.S. presidents, international dignitaries, CEOs, astronauts and other celebrities.Click the link to know more about this huge airplane manufacturer building https://www.boeing.com/company/about-bca/everett-production-facility.page
We are done for this week. I hope you gained some new insight from this post and if you did please share with it with your fellow aviators and aviation lovers. I encourage you to like this post as it gives me great confidence from your’e interest in these topics. If you have any suggestions please feel free tom drop in a comment or an email, I would be happy to reply. Until next week , stay safe and stay healthy.