Hypoxia and Hyperventilation - ATPL 040 — Human Performance and Limitations

The previous lesson ended with haemoglobin saturation sliding down its altitude ladder. This one names what waits at the bottom. Hypoxia is the shortage of oxygen in organs and tissues, and its aviation form arrives without pain, without breathlessness at first, and with the treacherous habit of dismantling the very judgement needed to recognise it. Its twin in the exam room is hyperventilation, which feels alarmingly similar while being, chemically, almost its opposite: not too little oxygen but too little carbon dioxide.

The two major forms of hypoxia

The syllabus defines two major forms by where the transport chain fails:

Hypoxic hypoxia — a reduced oxygen tension in the arterial blood. The flight-deck cause is the low partial pressure of inspired oxygen at altitude (hypobaric hypoxia), and it can also arise from hypoventilation or impaired gas exchange in the lungs. This is the aviator's hypoxia.
Anaemic hypoxia — a reduced oxygen-carrying capacity of the blood: too few red cells, too little haemoglobin, or haemoglobin that is abnormal or occupied, as after blood loss, in anaemia, or with carbon monoxide on board. The air can be perfect and the tissues still starve.

Medicine adds further varieties (stagnant circulation, poisoned tissues), but the two above are the forms the syllabus asks for, and the distinction to hold is the failure point: thin air in the first, thin blood in the second.

Symptoms: why hypoxia is dangerous

Hypoxia announces itself with symptoms that are easy to list and hard to notice from inside. Subjectively: breathlessness, headache, dizziness, nausea, a feeling of warmth about the face, dimming and blurring of vision, confusion or a misplaced exhilaration and euphoria, sleepiness and weakness. Objectively, an observer might see over-breathing, yawning, tremor, sweating, pallor and cyanosis, the blue lips and fingertips of desaturated blood, along with poor judgement, slurred speech and incoordination, ending in unconsciousness. The danger hides in the order: judgement and self-criticism degrade early, so the hypoxic pilot is typically the last person aboard to suspect hypoxia, and the euphoria actively argues against the diagnosis. Nothing hurts; that is the trap.

Compensation and the three thresholds

A healthy body fights back with faster breathing and a faster pulse, and up to about 10,000 to 12,000 ft it compensates well enough for ordinary purposes. The syllabus cuts the climb into three named thresholds:

Reaction threshold, 7,000 ft — the first measurable physiological reactions appear, though performance remains essentially intact; night vision is the classic early casualty, its decline beginning even lower, from about 5,000 ft.
Disturbance threshold, 10,000 to 12,000 ft — compensation no longer fully covers the deficit: complex mental work degrades, and from about 12,000 ft short-term memory begins to be affected, matching the steepening saturation curve of the previous lesson.
Critical threshold, 22,000 ft — rapid, severe impairment: consciousness is now a question of minutes, then seconds.

Time of useful consciousness

Above the critical threshold the clock becomes the whole story, and the syllabus gives it two names. Time of useful consciousness (TUC) is the time from the onset of hypoxia until purposeful activity, donning an oxygen mask, is lost; effective performance time (EPT) is the near-synonymous span during which the pilot can still perform duties effectively. TUC varies among individuals, but the approximate values for a person seated at rest are:

20,000 ft: about 30 minutes.
30,000 ft: 1 to 2 minutes.
35,000 ft: 30 to 90 seconds.
40,000 ft: 15 to 20 seconds.

Any activity shortens these times drastically: the same decompression that leaves a seated pilot half a minute at 35,000 ft leaves a walking cabin-crew member far less. The values are for the resting case, which is the syllabus's case.

What makes it worse, and what oxygen buys back

The severity of hypoxia at a given altitude is not fixed. It is determined by altitude and the rate of ascent, the duration of exposure, physical activity (exercise raises oxygen demand), cold, illness and fatigue, alcohol and smoking, and individual tolerance. Several of these stack against the same pilot on the same bad day, which is why the thresholds are bands, not walls.

Breathing 100 per cent oxygen rewinds the ladder. At mean sea level and at 10,000 ft, pure oxygen delivers more than sea-level air does, so saturation runs at its ceiling. At 30,000 ft on 100 per cent oxygen the body is still roughly at sea level in oxygen terms, since sea-level equivalence holds up to about 34,000 ft. At 40,000 ft, 100 per cent oxygen is worth about 10,000 ft of breathing air: the top of the compensation zone, and the practical limit, because above that only positive-pressure breathing can force enough oxygen across, which is why 40,000 ft figures in oxygen-equipment rules.

Hyperventilation

Now the twin. Hyperventilation is breathing in excess of the body's metabolic need, and its chemistry follows from the previous lesson's control loop: breathing is driven by carbon dioxide, and over-breathing washes carbon dioxide out of the blood faster than metabolism replaces it. The blood turns alkaline, vessels in the brain constrict, and the symptoms arrive: dizziness and light-headedness, tingling in the fingers, toes and lips, visual disturbance, a feeling of unreality, muscle cramps stiffening towards spasm, and in the extreme, unconsciousness, at which point breathing slows and the condition corrects itself.

The causes may be psychological or physiological: anxiety, fear and stress on one side; pain, heat, vibration, illness, and hypoxia itself on the other. That last item is the diagnostic knot, because a hypoxic pilot over-breathes too and the two symptom lists overlap heavily. The syllabus's countermeasures are a fixed list worth quoting: breathe slowly, close one opening of the nose, speak loudly, place a paper bag over nose and mouth. Each one either slows the washout or returns exhaled carbon dioxide to the lungs, and deliberate slow speech doubles as a self-test: forming loud, steady sentences forces a calmer breathing rhythm. In flight the practical rule cuts the knot: at cabin altitudes where hypoxia is possible, treat the ambiguous case as hypoxia first, oxygen and descent, because the hypoxia clock runs faster than the hyperventilation one and oxygen does no harm to an over-breather.

What the exam asks

The threshold altitudes and the TUC table are pure recall and heavily examined; the hypoxia-versus-hyperventilation discrimination is the standard scenario item. The overview, following the learning objectives:

Define the two major forms of hypoxia and their common causes

State the symptoms of hypoxia

State the compensation limit and name the three physiological thresholds with their altitudes

State the altitude at which short-term memory begins to be affected

Define TUC and EPT and state the seated-at-rest TUC values

List the factors that determine the severity of hypoxia

State the equivalent altitudes when breathing 100 per cent oxygen

Define hyperventilation and describe the role of carbon dioxide in it

List the causes (psychological and physiological), signs and symptoms of hyperventilation

List the countermeasures against hyperventilation