Hypoxia, DCS & barotrauma

040 Human PerformanceChapter 3

Altitude and pressure changes affect the body through hypoxia, hyperventilation, decompression sickness, and barotrauma.

Schematic

Hypoxia progresses through indifferent, compensatory, disturbance, and critical stages with altitude.

Schematic

Above ~10,000 ft think hypoxia; below ~10,000 ft suspect hyperventilation — key sign difference is cyanosis vs clammy skin.

Types of hypoxia

Type	Cause	Example
Hypoxic	Insufficient O₂ in inspired air	Altitude (Dalton’s law)
Hypaemic	Reduced O₂ carrying capacity	Smoking, CO, anaemia
Stagnant	Inadequate blood circulation	Heart attack, +Gz
Histotoxic	Cells cannot use O₂	Alcohol, drugs

Key fact

Hypoxia can be noticed from ~6,000 ft (night vision). Short-term memory impairment from ~12,000 ft. Does not occur below ~3,000 m (~10,000 ft) in normal conditions.

Key fact

Carbon monoxide binds haemoglobin 200–250× more strongly than O₂. Colourless, odourless — from incomplete combustion. One cigarette pack/day reduces O₂ transport 5–8%.

Hypoxia stages

Stage	Altitude (approx)	Effects
Indifferent	0–10,000 ft	Night vision ↓ from 5,000 ft; cognition ↓ from 8,000 ft
Compensatory	10,000–15,000 ft	Body compensates; effects after 10–15 min
Disturbance	15,000–20,000 ft	Compensation fails
Critical	20,000 ft+	Life-threatening

Key fact

Signs (others see): cyanosis (blue skin), impaired judgement (most dangerous sign), poor coordination. Symptoms (you feel): euphoria, tingling, reduced vision. NOT aches/pains (that’s DCS).

Time of Useful Consciousness (TUC)

Altitude	TUC (approx)
25,000 ft	2–3 min
30,000 ft	1–2 min
35,000 ft	30–90 s
40,000 ft	15–20 s

Key fact

Activity reduces TUC. At 33,700 ft, breathing 100% O₂ simulates sea level — effective up to ~40,000 ft where positive pressure O₂ is required.

Hyperventilation

Key fact

Hyperventilation — breathing too fast/deep → CO₂ washed out → blood too alkaline → O₂ stays bound to haemoglobin → cells starved of O₂.

Key fact

Symptoms similar to hypoxia but: tingling, clammy skin (not cyanosis). Below 10,000 ft, suspect hyperventilation rather than hypoxia. Jogging does NOT cause pathological hyperventilation (running is voluntary).

Key fact

Treatment: slow steady breathing, breathe into a paper bag to restore CO₂.

Decompression sickness (DCS)

Key fact

Henry’s law — nitrogen absorbed under pressure; on rapid ascent bubbles form in tissues/blood. Primary symptom: “the bends” (joint pain). Also creeps (skin itching), chokes (breathlessness), staggers (neurological).

Key fact

Effects may appear up to 12 hours after decompression. Most likely above 18,000 ft unpressurised; significant risk above 25,000 ft. Wait 24 hours after scuba >30 ft; 12–18 hours after shallow dives. Snorkelling — no wait.

Exam trap

Exercise does NOT prevent DCS. Wait 12 hours minimum after diving before flying (24 h after deep dives).

Trapped gas (dysbarism / barotrauma)

Key fact

Dysbarism — medical problems from gas expansion when ambient pressure falls (Boyle’s law). Affects middle ear, sinuses, teeth, gut.

Key fact

Eustachian tube equalises middle ear on descent (harder than climb). Blocked by cold/infection → vacuum on descent → pain, possible eardrum rupture. Relieve: swallow, yawn, Valsalva, Frenzel.

Key fact

Aerodontalgia (barodontalgia) — trapped gas in teeth; worse on climb (expansion). Does not occur in desert (dry) conditions typically cited for absence of moisture-related dental issues.

Key fact

Gastrointestinal expansion — severe pain possible above 25,000 ft; relieved by belching, descending. Vegetables produce gas — dietary consideration.

Risk factors summary

Smoking, alcohol (1 oz ≈ +2,000 ft physiological altitude), obesity, age, scuba diving, rapid decompression, unpressurised flight >10,000 ft without supplemental O₂.