Types

Four types of hypoxia

Hypoxia is the general term for insufficient oxygen supply to tissue. Medicine distinguishes four types by cause:

1. Hypoxic hypoxia

Cause: the oxygen partial pressure in inhaled air is too low — Hb cannot be sufficiently loaded with O₂.

Main aviation cause: altitude. With increasing altitude P_O₂ decreases → less O₂ in alveoli → lower Hb saturation.

Other causes:

• Lung disease (asthma attack, pneumonia) → poor gas exchange,
• Pressurised-cabin failure,
• Anti-ice system failure → iced air intakes.

Effect:

• At 10 000 ft: light symptoms (headache, slow thinking).
• At 14 000 ft: marked judgement impairment.
• At 18 000 ft: TUC only 20–30 minutes.
• At 30 000 ft: TUC only 1–2 minutes.

Prevention: supplemental oxygen at altitude (see EASA recommendation) or pressurised cabin.

2. Hypaemic (anaemic) hypoxia

Cause: Haemoglobin in the blood is reduced or blocked — even with normal O₂ partial pressure too little O₂ can be transported.

Subtypes:

a) Reduced Hb — anaemia:

• Iron deficiency, vitamin B12 deficiency,
• After blood loss (injury, blood donation!),
• Chronic disease.

Consequence: less Hb → less O₂ transport. Even at 98% saturation total O₂ carried is insufficient.

b) Hb blockade — carbon monoxide (CO):

• CO binds Hb 200–250× more strongly than O₂ → Hb becomes carboxyhaemoglobin (HbCO).
• HbCO transports no O₂.
• A CO concentration of 0.02% (200 ppm) in the cockpit can produce 20% HbCO — equivalent to hypoxia at 10 000 ft.

CO sources in PPL aircraft:

• Defective heater muff (around the exhaust manifold): exhaust enters the cabin — the main cause of CO poisoning in light aircraft.
• Tobacco smoke.
• Pre-flight at the ground behind a running engine.

CO symptoms (like hypoxia plus):

• Headache,
• Nausea,
• Cherry-red skin (rare in PPL — more often dark/blue-grey skin tone).

Immediate action on CO suspicion:

1. Heater off immediately,
2. Fresh air (open window or vent),
3. Descend to lower altitude,
4. Land at the next opportunity, medical exam.

Detectors: CO detectors in the cockpit (chemical indicator strips or electronic alarms) are standard or carried in many aircraft.

3. Stagnant hypoxia

Cause: Blood flow to an organ or tissue is reduced — even with normal Hb and partial pressure the tissue is not supplied.

Main aviation causes:

a) High G forces:

• Positive G pushes blood into the lower extremities → reduced cerebral perfusion.
• 4–5 g for a few seconds → grey-out (loss of colour vision),
• 5–6 g → black-out (complete sight loss),
• 6+ g → G-LOC (G-induced Loss of Consciousness).

b) Heart failure or shock:

• Severe shock or cardiac failure reduces cardiac output.

c) Cold extremities:

• In cold, peripheral circulation withdraws → cold fingers/toes respond slowly.

Prevention:

• Avoid G manoeuvres if not trained.
• Learn anti-G manoeuvres (breathing, abdominal tensing) in aerobatic types.
• Warm clothing in winter.

4. Histotoxic hypoxia

Cause: The tissue cannot use the available O₂ — even with normal Hb and circulation, cellular respiration is disturbed.

Main aviation cause: alcohol.

• Alcohol disturbs the enzyme reactions that use O₂ in the mitochondria.
• Rule of thumb: 1 oz alcohol (28 g) raises the effective hypoxia altitude by about 2 000 ft.
• After drunkenness has worn off, residual effects last up to 24 hours ("hangover effect").

Other causes:

• Cyanide poisoning (smoke in a cockpit fire),
• Severe drug effects.

Regulatory alcohol limits:

• EASA: 0.2 ‰ blood-alcohol limit for pilots.
• FAA: "8 hours from bottle to throttle" and no residual effects.
• Obligation: no residual effect — even after 8 h alcohol may still impair after a heavy session.

Summary — four types