17.hypoxia. Hypoxia
Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. Because the human body and brain require an uninterrupted supply of oxygen to maintain normal function, even mild hypoxia produces measurable degradation in vision, judgment, coordination, and short-term memory. Pilots are particularly vulnerable because the partial pressure of oxygen decreases with altitude, and the onset of hypoxia is typically insidious — symptoms are often unrecognized by the affected pilot.
There are four classical types of hypoxia, distinguished by the point at which the oxygen pathway is disrupted:
- Hypoxic hypoxia (also called altitude hypoxia) — A reduction in the partial pressure of oxygen in the inspired air. This is the form pilots encounter at altitude, where atmospheric pressure (and therefore the pressure driving O₂ across the alveolar membrane) is reduced even though the percentage of oxygen in the air remains 21%.
- Hypemic hypoxia — The blood is unable to carry sufficient oxygen to the cells. Causes include anemia, blood loss, and most importantly for pilots, carbon monoxide (CO) poisoning, in which CO binds to hemoglobin roughly 200 times more readily than oxygen. Smoking and donating blood also reduce oxygen-carrying capacity.
- Stagnant hypoxia — Oxygenated blood is not moving efficiently to the tissues. Caused by poor circulation: shock, heart failure, prolonged G-loading (G-LOC), or extreme cold restricting blood flow to extremities.
- Histotoxic hypoxia — The cells are unable to use the oxygen delivered to them. Caused by alcohol, narcotics, cyanide, and certain drugs. As a rule of thumb, one ounce of alcohol can equate physiologically to about 2,000 feet of altitude.
Symptoms of hypoxia vary among individuals but commonly include cyanosis (blue fingernails and lips), headache, dizziness, tingling or warm sensations, sweating, reduced visual field, sleepiness, euphoria, and ultimately unconsciousness. A particularly dangerous feature is that the affected pilot often experiences a sense of well-being (euphoria) and is convinced everything is fine even as performance deteriorates.
Time of useful consciousness (TUC) is the time available to a pilot, after oxygen supply is lost or becomes inadequate, to make rational decisions and take corrective action. Approximate TUC values during normal activity are:
- 22,000 ft: 5–10 minutes
- 25,000 ft: 3–5 minutes
- 28,000 ft: 2.5–3 minutes
- 30,000 ft: 1–2 minutes
- 35,000 ft: 30–60 seconds
- 40,000 ft: 15–20 seconds
- 45,000 ft and above: 9–15 seconds
Rapid decompression cuts these times roughly in half because the pressure differential forces oxygen out of the lungs.
Regulatory and prevention strategies are built around the fact that, although the body can compensate for short exposures up to about 12,500 feet, performance begins degrading well before symptoms are noticed. Per 14 CFR 91.211, the required pilot supplemental oxygen use is:
- 12,500 ft to 14,000 ft MSL — required pilot oxygen for any portion of the flight at those altitudes lasting more than 30 minutes.
- Above 14,000 ft MSL — required pilot oxygen at all times.
- Above 15,000 ft MSL — oxygen must be provided to each occupant.
For unpressurized aircraft, the FAA recommends pilots use supplemental oxygen above 10,000 feet during the day, and above 5,000 feet at night, because the rods of the eye are especially sensitive to oxygen deprivation and night vision can deteriorate by up to 25% at 5,000 feet.
Recognition and treatment. A pilot who suspects hypoxia in self or a passenger should immediately:
- Don supplemental oxygen and ensure 100% flow.
- Descend below 10,000 feet MSL — the most reliable cure.
- Check the oxygen system: pressure, mask seal, tubing connections, and flow indicators.
- If carbon monoxide is suspected (CO detector indication, exhaust smell, heater in use), shut off the heater, open vents, and land as soon as practical.
- Breathe deeply and avoid smoking before and during flight.
Example: A pilot cruising at 13,500 feet MSL on a long cross-country flight without supplemental oxygen begins to feel slightly euphoric and notices it is taking longer to interpret chart symbols. Color discrimination is reduced and a mild headache develops. Although the pilot feels capable, performance is in fact significantly impaired. The correct response is to immediately descend to or below 12,500 feet — preferably below 10,000 feet — and use supplemental oxygen if available.
Because hypoxia symptoms are subtle and individualized, the FAA strongly encourages pilots to undergo physiological/altitude chamber training (offered through FAA CAMI and military programs) so they can recognize their own personal symptom signature before encountering it in flight.