Oxygen and Respiration — Page 45, Lesson 74

I want to walk you through the effects of hypoxia — the condition of oxygen deficiency in the body — and how it progresses as you gain altitude. We'll start with the specific symptoms that appear as hypoxia sets in, then move into the formal stages or zones that classify how performance degrades. First, memory impairment. Short-term memory is the first to go, which means you'll struggle to complete drills — those memorised sequences of actions you rely on in the cockpit. This starts at approximately 12 000 ft. So if you're at that altitude without supplemental oxygen, don't expect to remember your checklist flow. Next, visual sensory loss. Vision is affected early because the light-sensitive cells of the eye — the rods and cones — are particularly oxygen "hungry". Colour perception is reduced, and peripheral vision is gradually lost. A deterioration of night vision can occur at altitudes as low as 5000 ft. That's significant: even before you feel anything else, your ability to see in dim light is compromised. As hypoxia progresses, tunnel vision develops — your field of view narrows — which means you have to make larger head movements to scan your instruments and the external environment. You can't rely on your peripheral vision to catch a traffic conflict or a gauge moving out of limits. Then we get to impairment of consciousness. As hypoxia worsens, your level of consciousness drops. First you become confused, then semi-conscious, and eventually unconscious. This is a progressive slide, not a sudden switch. Cyanosis is a visible sign. An individual who has become hypoxic at altitude is likely to be cyanosed — that means the lips and the skin under the fingernails develop a blue tinge. Why? Because much of the blood haemoglobin is in the deoxygenated state. Haemoglobin that isn't carrying oxygen turns a darker, bluish colour, and you see that through the thin skin of the lips and nail beds. Formication is a strange one — a creeping sensation on the skin, as if ants are crawling over your body. It's a sensory hallucination caused by the oxygen-starved central nervous system. Then unconsciousness, and finally death. The excerpt puts it in capital letters: UNLESS HE OR SHE RECEIVES OXYGEN THE INDIVIDUAL WILL DIE AND AT HIGH ALTITUDES DEATH CAN OCCUR WITHIN A FEW MINUTES. That's the bottom line — hypoxia is not something you can tough out. Now let's look at the formal stages or zones of hypoxia. There are four, and we'll cover the first two here. Stage 1 is the Indifferent Stage or Zone, from ground level up to 10 000 ft (3 048 m). In this zone, you don't feel obviously impaired, but effects are already present. Dark adaptation is adversely affected — and that can start as low as 5 000 ft, as we already saw. Performance of new tasks may be impaired, meaning if you're asked to do something you've never done before, you'll be slower or less accurate. There's also a slight increase in heart rate and breathing rate — your body's first attempt to compensate. Stage 2 is the Compensatory Stage or Zone, from 10 000 ft up to 15 000 ft (3 048 to 4 572 m). Here, the body's physiological automatic responses kick in to provide some protection against hypoxia, trying to maintain homeostasis — that's the body's internal balance. These responses include an increase in respiratory volume — you breathe deeper and faster — and an increase in cardiac output and blood pressure — your heart pumps harder. But here's the catch: after a short time, the effects of hypoxia on the central nervous system become perceptible. You get drowsiness, decreased judgement and memory, and difficulty performing tasks that require mental alertness or very small, precise movements. So even though your body is fighting back, your brain is already losing the battle. Let me show you the anatomy of the lungs and air passages that make this oxygen exchange possible. That figure shows the air passages in the lungs, which is where the oxygen you breathe gets into your bloodstream. When hypoxia sets in, it's because that process is starved — either because there's less oxygen in the air at altitude, or because your body can't get it to the tissues effectively. So to summarise: hypoxia starts subtly with vision changes as low as 5 000 ft, progresses through memory loss and confusion, and if uncorrected leads to unconsciousness and death within minutes at high altitude. The Indifferent Stage up to 10 000 ft gives you a false sense of safety, and the Compensatory Stage from 10 000 to 15 000 ft masks the problem with physiological responses that ultimately fail.