Hypoxia Flight Physiology

Hypoxia (Flight Physiology)

Hypoxia and Flight Physiology

What is Hypoxia?

Hypoxia is a condition that is caused by a lack of oxygen. It is a common discussion in flight physiology, and occurs when people reach a high altitude without adequate oxygen supplementation. As people fly to higher altitudes, they can get altitude sickness. That can lead to pulmonary edema and high altitude cerebral edema. These can be fatal, but not everyone who develops hypoxia will die from it. The important thing to do when hypoxia occurs is to get to a lower altitude or get supplemental oxygen quickly, in order to mitigate the problematic and dangerous effects of a lack of oxygen. It is not just flight that can cause hypoxia to occur. Climbing to very high altitudes (i.e. Mount Everest) can also cause a lack of oxygen, as can certain medical conditions. The focus of this paper, however, is based on the assumption that the hypoxia is related to a lack of oxygen due to a high altitude flight. The four types of hypoxia will be discussed, along with signs and symptoms, altitudes and onset, and concluding remarks on hypoxia from a flight standpoint.

The Four Types of Hypoxia

There are four different types of hypoxia that are seen in the medical community. It is important to understand what they all mean and how they occur, because they are all caused by different things.

1. Hypoxic hypoxia is very common. This is caused by flying to a high altitude, but also by anything that decreases the oxygen in the air or stops the lungs from diffusing oxygen properly. When this takes place, there is less than 100% oxygen saturation in the blood. Over 12,500 feet in altitude will generally cause the saturation to drop to a point where the person would start to feel sick or have trouble breathing. Damaged lungs would also put a person at risk for this type of hypoxia, because lungs that have damage cannot transfer oxygen as efficiently as healthy lungs can.

2. Hypemic hypoxia is not caused by the outside air or the lungs, but rather by the blood itself. When the capacity to carry oxygen through the bloodstream is reduced, this type of hypoxia occurs. This can come from anemia, but also from carbon monoxide poisoning. Both of those conditions can be very serious, especially when they cause significant issues with the transfer of oxygen for an extended period of time. It does not matter how much oxygen is available in the air if the blood is not able to correctly transfer the oxygen throughout the body efficiently.

3. Stagnant hypoxia comes from reduced cardiac output. It can also come from the pooling of the blood during stresses such as those caused by high G. forces. As people get older, the heart often does not pump blood as efficiently. Younger people can also have difficulty if they have a heart condition of some kind that reduces the output of their heart. People who have this type of hypoxia are generally elderly or they have a medical condition they are aware of and that they are managing or treating.

4. Histotoxic hypoxia comes from the cells' inability to accept oxygen from the blood, even though the oxygen is readily available. This can happen with cyanide poisoning, but it can also happen if a person has too much alcohol is his or her bloodstream. When people have alcohol poisoning, they can also become hypoxic. As the alcohol clears the bloodstream, oxygen saturation will return to more normal levels.

Signs and Symptoms

The signs and symptoms of hypoxia are very important to take note of, because they can come on slowly or quickly, depending on the reason for and type of hypoxia in question. For slower onset, which would be seen with altitude sickness, fatigue, headaches, and shortness of breath will be the first symptoms of a problem. Many people who are becoming hypoxic also feel nauseated and euphoric. When they become severely hypoxic or the hypoxia occurs very rapidly, seizures, coma, changes in consciousness levels, and death occur. By the time the issue is realized by others, there might not be an opportunity to help the person who has been experiencing the lack of oxygen without some type of permanent damage to the brain or other organs. Being aware of the symptoms of hypoxia that comes on slowly -- as it does when flying to a high altitude -- is very important because there is time to reverse the problems that start to occur. The earlier the lack of oxygen is noticed, the better the opportunity to descend to a lower altitude or receive supplemental oxygen to avoid hypoxia.

Altitudes and Onset

At sea level, a person's blood is oxygenated at or near 97%. When that same person goes to an altitude of 10,000 feet, the oxygen saturation in the blood drops to 90%. That is still enough for most normal life functions, although people in the medical field want to see a person's oxygen saturation point at or over 93%. At 14,500 feet (on the top of Pike's Peak, for example), the saturation is closer to 80%. That will cause the majority of people to have difficulty breathing, fatigue, nausea, and other symptoms that come along with altitude sickness. At 25,000 feet, oxygen saturation in the blood has dropped all the way to 55% and the person would lose consciousness. A pulse oximeter that clips on a finger is worn by many pilots today in order to see if they are becoming hypoxic.

The main altitude concerns are as follows:

5000 feet -- the retina of the eye will not get as much oxygen as it needs, and vision will be slightly compromised. This will be noticed most often in night vision. It is easier to misinterpret features on the ground, and maps and instruments in the cockpit can also be misread more easily. While most people can still fly safely at that altitude, it is important to be more careful than one would need to be at lower altitudes.