The Civil Aviation Medical Examiner and You


Aviation Hypoxia
by J. Robert Flood, MDCM, CCFP (EM), Senior Consultant, Clinical Assessment, Civil Aviation Medicine, Civil Aviation, Transport Canada

In aviation medicine, as in other areas of general aviation, we look at safety management in terms of risk assessment. Whenever we discuss risk, we try to keep things in perspective. In life, we must get out of the habit of worrying excessively about mere possibilities, like catching bird flu, and focus more on real probabilities, such as the consequences of not using seatbelts or not getting the influenza vaccination. Things are no different when it comes to aviation and hypoxia. Hypoxia is still very much a threat in the aviation environment, as we are reminded from time to time by the doomed Helios aircraft last year and Payne Stewart’s accident some years ago.

If you only fly below 10 000 ft in the daytime or 5 000 ft at night, and never intend to go above these levels, then your risk from this hazard is manageable and your reading for today is complete. All other pilots should keep reading and keep thinking about the risk.

Most pilots will recall from their training that the risk to flight safety from hypoxia is incapacitation, and that the most common causes of hypoxia are either a sudden decompression or a slow, unrecognized loss of pressure at altitudes above 10 000 ft. We know that the effects of hypoxia vary from person to person and can also vary in the same person under different circumstances, so it can be difficult to precisely state a flight level at which hypoxic symptoms will occur. It is far more important to be aware that the problem can occur, recognize its symptoms, and know the effects that hypoxia will have on your flying skills. In this article, I will discuss the risk of hypoxia and its management.

Before getting to the technical aspect, think about this simple scenario that was presented in a recent hypoxia article in an aviation journal. It described a pilot flying a Grumman in the Lake Tahoe area at an altitude of 11 500 ft for a period of 30 min, followed by a few hours flying at 9 500 ft. The environment under the bubble canopy was warm, and the pilot experienced heightened anxiety, palpitations, light-headedness, and generally felt unwell. Some symptoms persisted later into the day. Clearly, the pilot should have been on supplemental oxygen for at least part of the flight. But were other factors at play here? Could all the symptoms have been attributable to hypoxia? How much of an effect can hypoxia have at 11 000 ft? Why and when should we be concerned? What do the rules say about intervals above 10 000 ft in terms of time on oxygen? And what other effects can hypoxia have on healthy subjects at the common cabin altitude of 8 000 ft? Can some of the issues encountered in flight, such as blood clotting, fatigue and air rage, be related to mild degrees of hypoxia?

Definition

Hypoxia, by definition, is a lack of sufficient oxygen for the body to operate normally. It is actually a state of dysfunction due to inadequate oxygen in the blood passing to the tissues and/or cells of our bodies. Mankind has known about this problem long before we took our first tentative leaps towards the sky. Oxygen is required for all the body’s cellular activity. Some organs are more demanding than others. The brain and heart require large amounts of oxygen from the circulation, and cannot function efficiently when blood oxygen levels fall. Throughout the atmosphere, the concentration of oxygen remains the same (a little over 20 percent). The key is the partial pressure of oxygen and the effect from increasing altitude. At low altitudes, where the atmospheric pressure is higher, the partial pressure of oxygen is adequate to maintain brain function at peak efficiency. As one ascends to higher altitudes, atmospheric pressure declines, and with it, so does the partial pressure of oxygen. For example, at 18 000 ft, the partial pressure is half of that at sea level. At 10 000 ft above sea level (ASL) all pilots will experience mild hypoxia and some will become symptomatic.

Signs and symptoms

The most threatening feature of hypoxia is that it can have an insidious onset as one climbs to higher altitudes. Secondly, it may be accompanied by a feeling of well-being, known as euphoria. Even minor degrees of hypoxia impair night vision, motor skills and slow reaction time. The body’s physiological responses can include an increased rate and depth of respirations and an increase in heart rate. With prolonged exposure to a hypoxic environment, oxygen supply to the brain is reduced and changes in functioning can start to occur. Early signs and symptoms of cerebral hypoxia include headache, nausea, drowsiness and dizziness. More serious hypoxia interferes with reasoning, gives rise to unusual fatigue, and finally, can produce unconsciousness and death. You can imagine flying at altitude without oxygen and starting to feel giddy, light-headed and out of it. Unless you have pre-programmed yourself to consider hypoxia, you could find yourself in trouble. As well, knowing what corrective actions to take would be of benefit.

In the case of rapid decompression, the human’s ability and time to react has been closely studied. The term time of useful consciousness (TUC) has been coined and reflects the time from decompression to the loss of effective performance. At 40 000 ft, TUC has been measured at around 20 seconds, so donning an oxygen mask and starting a rapid descent cannot be delayed. Crews working in pressurized cabins at high altitude must be aware of oxygen system performance, should a rapid decompression occur. Above 33 000 ft ASL, the partial pressure of oxygen in the air, even supplemented by 100 percent oxygen, is inadequate to avoid hypoxia, so descent is essential.

Hyperventilation

Hyperventilation is a related concern, with symptoms that may be difficult to distinguish from those of hypoxia. Some circumstances may lead to a condition of breathing at a faster rate than normal. This rate in excess of the body’s oxygen requirement can reduce the carbon dioxide in the blood, resulting in an acid-base imbalance in the blood, and leading to symptoms of dizziness, malaise, tingling and anxiety, which may mimic hypoxia.

The regulations

As a reminder, Civil Aviation Regulation (CAR) 605.31, states that we:

do not require supplemental oxygen below 10 000 ft ASL;

require oxygen for the entire period of flight exceeding 30 min at cabin-pressure-altitudes above 10 000 ft ASL, but not exceeding 13 000 ft ASL; and

require oxygen for the entire period of flight at cabin-pressure-altitudes above 13 000 ft ASL.

So what is the solution?

Simple:

Don’t t fly above 10 000 ft ASL without supplemental oxygen or pressurization, and when you do, follow the regulations.

Fly a well-maintained aircraft.

Fly healthy-any lung problem puts you on the down slope of the oxygen curve, and decreases the threshold for hypoxia.

Don’t smoke.

Avoid self-imposed stresses. Hypoxic symptoms can be more pronounced under stress, and anxiety may lead to hyperventilation. Monitor your rate and depth of breathing.

Remain aware. Pilots operating at higher altitudes should be alert for unusual difficulty completing routine calculations, and should take corrective action if difficulties are noted.

If you do a lot of flying at higher altitudes, get some hypoxia familiarization. The effects of hypoxia can be safely experienced under professional supervision. This may be done with an altitude chamber or a mask set-up, which provides a lower oxygen concentration. This will help you learn to recognize your own symptoms of hypoxia or hyperventilation. A pressure chamber offers the additional opportunity to experience rapid decompression, the effects of trapped gases, and related human factors.

And what are the take-home messages?

Hypoxia is a constant and dangerous companion while flying at higher altitudes. Although the onset and severity of symptoms may vary with individuals, no one can escape the effects of hypoxia, even patients and air medical flight crew.

Awareness, education and experience will reduce the risk of encountering hypoxia and result in safer flying.

And what about the issues of blood clotting, fatigue and air rage? You’ll have to keep reading the Aviation Safety Letter for future updates!

Dressed for survival?


Date modified: