There comes a time in almost every pilots career when the need arises to transit high terrain or overfly weather at an altitude in the low to mid teens.
Sometimes the climb is planned, such as crossing a mountain pass in the west or the Appalachians in the east. Sometimes its not, like when trying to overfly building cumulus clouds on a summer morning. In either case, the flight to higher altitude is generally intended to be brief and the need for supplemental oxygen is often written off.
These flights seem to start out smoothly. Once you climb, you wonder why you dont fly high more often. Things are going well – very well, in fact. You realize you havent felt this well in years. You start to giggle, and then you sense fatigue and a feeling of slight dizziness and tingling. Youve fallen prey to the trap feared by many pilots who fly at high elevations – hypoxia.
Hypoxia is a deficiency in the delivery of oxygen to the blood, tissues, and cells significant enough to create physiological impairment. Any pilot with normal respiratory function who travels above 10,000 feet agl in an unpressurized aircraft without supplemental oxygen may be subjected to the signs and symptoms of hypoxia. At night, flight above 5,000 feet can result in hypoxia-induced deterioration of night vision.
Those altitudes can be even lower for those who smoke, drink, are obese or have lung diseases or anemia.
Breaking Down the Breathing Chain
When an individual is subjected to environmental flight stress, all organ systems are affected – both physically and psychologically. Although all organ systems work together synchronously, the respiratory and circulatory systems seem to undergo the most rapid and direct effects while flying. In the lungs it all boils down to respiration, defined as the ability to exchange oxygen from the atmosphere with carbon dioxide produced as a by-product from the body.
Anything that slows down the ability to transport or utilize oxygen in the body can contribute to a hypoxic state. When a pilot climbs to altitude, the partial pressure of atmospheric oxygen falls and slows the flow of oxygen into the lungs. The ability for blood to transport oxygen using the red pigment hemoglobin is affected by anemia (low blood counts) or by carbon monoxide exposure (such as from chronic cigarette exposure or a cabin heater leak).
Any condition that interferes with the normal circulation of blood to the cells can also cause hypoxia, such as extended positive g-force while flying, or poor heart pumping function (called congestive heart failure). Some substances, such as alcohol, narcotic medications, cyanide and certain over-the-counter remedies all disrupt the ability for the cells of the body to utilize the oxygen once it is delivered.
All living cells in the human body require oxygen to function, some more than others. While some organs have the ability to store an emergency supply of oxygen, the brain and spinal cord require a steady feed. In fact, roughly 20% of the oxygen taken in is used to supply the brain. Consequently, in a hypoxic state, the brain is one of the first organs to be affected and the cortex (where higher reasoning input is processed) is the first part of the central nervous system to have its function deteriorate. A pilot in a hypoxic environment, will find that judgment and cognitive skills are compromised significantly.
Sniffing Out the Problem
Unfortunately, the person who is experiencing hypoxia is often the last to notice it. The clinical signs of hypoxia include poor judgment, fatigue, inability to coordinate fine movements, and rapid breathing. However, the symptoms, or sensations a pilot can detect while experiencing hypoxia, are unique to each individual. Different individuals exposed to hypoxic conditions will develop similar symptoms; however, the symptoms occur with varying orders and intensities. Its important to recognize your own individual symptoms if you should develop hypoxia, since over time the sensations tend to occur in the same order for any individual.
The symptoms of hypoxia come on slowly, so the slightest distraction from flying the aircraft may place the pilot and passengers in a hypoxic scenario that may not be easily corrected. Headaches, nausea and dizziness are most evident.
A sense of inflated well being or euphoria has the potential to be the most dangerous hypoxic symptom, as the oxygen-starved pilot will tend to forego reasonable safety precautions and make potentially fatal in-flight decisions – including continuing on as if nothing was wrong. Visual field impairment and unconsciousness are end-stage symptoms of hypoxia.
There are two variables used to determine an individuals hypoxic limit – Effective Performance Time, or EPT, and Time of Useful Consciousness, or TUC.
The EPT refers to the amount of time available for a pilot to competently perform their flying responsibilities in an environment lacking adequate oxygen. The TUC is the time available to an individual in an oxygen-deprived environment before they are no longer capable of taking protective or correcting action.
Of course, these times will vary depending on several factors. The more physically fit a pilot is, the more resistant they are to hypoxia. On the other hand, fear and anxiety can increase the bodys demand for oxygen and thereby increase the risk of hypoxia.
Strenuous activity also is associated with significant loss of ability to withstand hypoxia. For example, if you lifted rapidly a 20-pound dumbbell for 15 repetitions at 22,000 feet without supplemental oxygen, you would become unconscious in roughly 4 minutes instead of about 10 minutes if flying at rest.
The speed of ascent also matters; the faster the climb, the shorter the TUC. Pilots who are fatigued prior to flying, have low blood glucose levels, drink alcohol before flying, or use over-the-counter medications (such as certain antihistamines) are more prone to the effects of hypoxia.
Another population of air travelers at risk are those who are slightly hypoxic on the ground from underlying lung disease or anemia. Often, they become quite fatigued and weak during flight, symptoms exacerbated by the exertion of carrying suitcases and the stress of travel. Passengers with underlying heart disease are at higher risk for heart attacks.
Pilots should remember the importance of avoiding hypoxia when transporting family and friends who may have underlying cardiopulmonary disease. A quick query of passengers who are traveling with you about previous or current health conditions may avoid a potential emergency during the flight.
Although hypoxia can add a potentially life-threatening variable to flying, it is easily preventable. Avoiding high altitudes without a pressurized cabin is perhaps the easiest way to stay safe. The FAA recommends that any flight with a cabin altitude at or above 10,000 feet during the day or 5,000 feet at night should be performed using supplemental oxygen.
Supplemental oxygen can be fitted into the airplanes systems, with outlets at each seat, or may be as simple as a portable tank with masks attached to a regulator. Different kinds of systems have different limitations, but for most general aviation aircraft a simple continuous flow system will work.
Depending on the design of the mask, a continuous flow system will work to altitudes of 25,000 feet, sometimes higher. In this system, a flow knob sets the desired flow of oxygen, and it feeds continuously through the mask or nasal cannula.
Fortunately, hypoxia is easily treated with 100% oxygen and resolves in seconds once treatment is initiated. As a pilot, it is important to recognize your individual hypoxic symptoms.
If possible, attend an aviation safety course that features an altitude chamber flight. During the chamber flight, you will be subjected to conditions that simulate your hypoxic symptoms while in flight and youll know what to expect should it ever happen to you in flight.
Whatever your aircraft, hypoxia is a risk that can be anticipated and prevented. Simple flight planning, awareness of your individual hypoxic symptoms, and use of supplemental oxygen when appropriate can eliminate hypoxia as a potentially lethal hazard.
-by Clayton T. Cowl
Clayton T. Cowl is an FAA-designated medical examiner at the Mayo Clinic in Rochester, Minn., and a commercially rated balloon pilot.