Hypoxia – Still an Insidious Threat

Hypoxia Introduction

The Athens crash of a Helios Boeing 737-300 killing 121 and the 1999 widely publicized crash of the Learjet 35 carrying golfer Payne Stewart, serve as striking reminders that hypoxia is still very much a threat in the aviation environment. This is particularly a problem if you become focused on resolving a problem or running emergency procedures and allow yourself to become task saturated which may have occurred with the Helios crash. Identification of your hypoxic signs and symptoms, and quick response to hypoxia before you exceed your time of useful consciousness are the keys to survival.

Just in case a GA pilot like me is reading, do not think that you are immune to the dangers of hypoxia just because you rarely get above 12,000 ft. In fact, you should think about the insidious effects of hypoxia anytime you fly above 10K or above 5K at night. As a reminder, hypoxia symptoms are a result of not enough oxygen getting to the tissues. Hypoxic hypoxia caused by decreased partial pressure of oxygen in air at altitude is the condition most of us think about when flying. There is also hypemic hypoxia due to reduced capacity of the blood to carry oxygen (e.g. carbon monoxide poisoning, medication reactions, etc), histotoxic hypoxia due to tissue’s inability to take oxygen from the blood (e.g. alcohol, narcotics, or cyanide poisoning), and stagnant hypoxia due to interruption of normal blood flow (e.g. heart failure or even the physiologic effect of pulling Gs). Notably any one or more than one of these causes of hypoxia can occur in flight and their effects can be more than additive.

Factors Influencing Hypoxia

  • Altitude
  • Rate of Ascent
  • Duration at Altitude
  • Temperature
  • Physical Exertion
  • Inherent Tolerance
  • Fitness
  • Acclimatization
  • Emotional State

From the factors listed above, you can see that onset of symptoms can be highly variable between individuals and even each airman’s experience with hypoxia can vary greatly depending on the exact situation. The stages of hypoxia are illustrated in the chart in the next section.

Stages of Hypoxia

Stage Breathing Air Breathing100% O2 Arterial O2 Saturation (%)
Indifferent 0-10K 34-39K 95-90
Compensatory 10-15K 39-42.5K 90-80
Disturbance 15-20K 42.5-44.8K 80-70
Critical 20-23K 44.8-45.5K 70-60

Altitude in Thousand Feet

Of particular importance to GA is that adverse effects on dark adaptation and night vision (especially color vision) can manifest as low as 5,000 ft which can be well below airfield altitude in some cases. EKG changes have also been noted to occur at this altitude. Unfortunately there are sometimes no subjective sensations up to the time of unconsciousness.

Time of Useful Consciousness

One particular concept is Time of Useful Consciousness (TUC) which is the period of time an airman has before useful function is lost and the individual is no longer capable of taking corrective action. The table below shows average TUCs for a good frame of reference, but as noted before a great deal of individual variation can occur. Bottom line, the higher the altitude, the shorter TUC you would expect.

Times of Useful Consciousness at Rest

Altitude                Time of Useful Consciousness
FL 180                     20 – 30 mins
FL 220                     10 min
FL 250                      3 – 5 min
FL 280                     2.5 – 3 min
FL 300                     1 – 2 min
FL 350                     0.5 -1 min
FL 400                     15-20 secs
FL 430                     9-12 secs
FL 500 & above       9-12 secs

Know Your Symptoms

A rapid decompression as may have occurred in the Helios tragedy may reduce TUC estimates by as much as 50%.

As most pilots are aware, when it comes to hypoxia the best solution is to know your symptoms and remain vigilant. If available to you, an altitude chamber flight gives the airmen the chance to identify their particular reactions to hypoxia and thereby increase your chances of detecting a problem early. The treatment is to get 100% oxygen immediately and descend. Usually recovery is within 15 seconds, but impairment can last for several hours. The best decision is to land and have yourself and your plane checked out.

If you have additional questions on regarding hypoxia, please do not hesitate to consult one of the AMAS physicians.