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Aeromedical Factors
Chapter 9
9-2
Environmental and Health Factors Affecting Pilot Performance
A number of health factors and physiological effects can be linked to flying. Some are minor, while others are important enough to require special consideration to ensure safety of flight. In some cases, physiological factors can lead to in-flight emergencies. Some important medical factors that a pilot should be aware of include hypoxia, hyperventilation, middle ear and sinus problems, motion sickness, stress and fatigue, dehydration, and heatstroke. Other subjects include the effects of alcohol and drugs, anxiety, and excess nitrogen in the blood after scuba diving.Hypoxia
Hypoxia means “reduced oxygen” or “not enough oxygen.” Although any tissue will die if deprived of oxygen long enough, the main concern is usually with getting enough oxygen to the brain, since it is particularly vulnerable to oxygen deprivation. Any reduction in mental function while flying can result in life-threatening errors. Hypoxia can be caused by several factors, including an insufficient supply of oxygen, inadequate transportation of oxygen, or the inability of the body tissues to use oxygen. Four forms of hypoxia based on their causes are: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.Hypoxic Hypoxia
Hypoxic hypoxia is a result of insufficient oxygen available to the body as a whole. A blocked airway and drowning are obvious examples of how the lungs can be deprived of oxygen, but the reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots. Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. As the aircraft ascends during flight, the percentage of each gas in the atmosphere remains the same; however, there are fewer molecules available at the pressure required for them to pass between the membranes in the respiratory system. This decrease in number of oxygen molecules at sufficient pressure can lead to hypoxic hypoxia.Hypemic Hypoxia
Hypemic hypoxia occurs when the blood is not able to take up and transport a sufficient amount of oxygen to the cells in the body. Hypemic means “not enough blood.” This type of hypoxia is a result of oxygen deficiency in the blood, rather than a lack of inhaled oxygen, and can be caused by a variety of factors. It may be because there is not enough blood volume (due to severe bleeding), or may result from certain blood diseases, such as anemia. More often it is because hemoglobin, the actual blood molecule that transports oxygen, is chemically unable to bind oxygen molecules. The most common cause of hypemic hypoxia is carbon monoxide poisoning. Hypemic hypoxia can also be caused by the loss of blood from a blood donation. While the volume of blood is restored quickly following a donation, normal levels of hemoglobin can require several weeks to return. Although the effects of the blood loss are slight at ground level, there are risks when flying during this time.Stagnant Hypoxia
Stagnant means “not flowing.” Stagnant hypoxia results when the oxygen-rich blood in the lungs is not moving, for one reason or another, to the tissues that need it. One form of stagnant hypoxia is an arm or leg “going to sleep” because the blood flow has accidentally been shut off. This kind of hypoxia can also result from shock, the heart failing to pump blood effectively, or a constricted artery. Cold temperatures can also reduce circulation and decrease the blood supplied to extremities. Histotoxic Hypoxia
The inability of the cells to effectively use oxygen is defined as histotoxic hypoxia. “Histo” refers to tissues or cells, and “toxic” means poison. In this case, plenty of oxygen is being transported to the cells that need it, but they are unable to make use of it. This impairment of cellular respiration can be caused by alcohol and other drugs, such as narcotics and poisons. Research has shown that drinking one ounce of alcohol can equate to an additional 2,000 feet of physiological altitude. There are other issues concerning the use of alcohol in relation to flying in general; those will be discussed later in this chapter.
Symptoms of Hypoxia
High altitude flying can place a pilot in danger of becoming hypoxic. Oxygen starvation causes the brain and other vital organs to become impaired. One noteworthy attribute of the onset of hypoxia is that the first symptoms are euphoria and a carefree feeling. With increased oxygen starvation, the extremities become less responsive and flying becomes less coordinated. The symptoms of hypoxia vary with the individual, but common symptoms include:
• Cyanosis (blue fingernails and lips),
• Headache,
• Decreased reaction time,
• Impaired judgment,
• Euphoria,
• Visual impairment,
• Drowsiness,
• Lightheaded or dizzy sensation,
　
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