Above 40 000 ft — Page 45, Lesson 72

I want to walk you through what happens to the human body above 40 000 ft, and specifically the condition called hypoxic hypoxia. This is a critical topic for you as a pilot because at those altitudes, the environment is completely hostile to human life without proper protection. Let’s start with the term itself. Hypoxic hypoxia is the medical name for the effects of a shortage of oxygen in the body. The word "hypoxic" means low oxygen, and "hypoxia" is the condition of oxygen deficiency. This can result from a number of causes — things like extreme anaemia, asthma, or meningitis. But for you as a pilot, the most important cause is altitude. As you climb, the partial pressure of oxygen in the air drops, and your body struggles to get enough oxygen into the blood. To understand why altitude matters, let’s look at how oxygen is carried in your blood. The oxygen is transported by a protein in your red blood cells called haemoglobin. At sea level, your haemoglobin is approximately 97.5% saturated with oxygen — that’s nearly full. At 10 000 ft, that saturation falls to 87%. And then it falls off rapidly. At 20 000 ft, your haemoglobin is only 65% saturated with oxygen. That’s a dramatic drop. The symptoms of hypoxia may develop slowly at lower altitudes, or very rapidly at high altitudes — and above 40 000 ft, the time of useful consciousness is measured in seconds. Now, let’s go through the specific symptoms of hypoxic hypoxia. I want you to remember these because they can be subtle and insidious — you may not realise you’re affected until it’s too late. First, apparent personality change. This is a change in outlook and behaviour. You might experience euphoria — a false sense of well-being — or aggression, and you lose your normal inhibitions. The real danger of this was described in 1875 by a balloonist named G. Tissandier, after his balloon flight to 25 000 ft. That flight proved fatal to both of his companions. He wrote: “But soon I was keeping absolutely motionless, without suspecting that perhaps I had lost use of my movements. Towards 7500 m (24 606 ft) the numbness one experiences is extraordinary. The body and the mind weaken little by little, gradually, unconsciously, without one’s knowledge. One does not suffer at all; on the contrary. One experiences an inner joy, as if it were an effect of the inundating flood of light. One becomes indifferent; one no longer thinks of the perilous situation; one rises and is happy to rise.” That quote is a chilling reminder that hypoxia can feel pleasant — which makes it even more dangerous. Second, impaired judgement. You lose self-criticism — you simply cannot evaluate your own performance. You are unaware that your abilities have degraded. This is made worse by short-term memory loss, which can occur at approximately 12 000 ft. So you might forget that you were about to do something important, and you won’t even realise you’ve forgotten. Third, headache — particularly if you are mildly hypoxic for a long period. This is a common symptom in less severe cases. Fourth, tingling in hands and feet. This is a sensory warning sign. Fifth, increased rate of breathing, which is called hyperventilation. Your body tries to compensate for low oxygen by breathing faster, but that can actually make things worse by blowing off too much carbon dioxide. Sixth, muscular impairment. Finely co-ordinated movements become difficult because of slow decision-making and poor fine muscular control. For example, your handwriting becomes more and more illegible. In the late stages of hypoxia, you may experience muscular spasms and convulsions. So to summarise: above 40 000 ft, you need 100% oxygen under pressure to survive. Without it, hypoxic hypoxia sets in rapidly. The symptoms include personality change, impaired judgement, headache, tingling, hyperventilation, and loss of fine motor control. And the most dangerous part is that you may feel happy and indifferent while your brain is starving for oxygen.