Hypoxia and Hyperventilation

Hypoxia is a state in which the body's tissues receive, or can use, too little oxygen to work normally. For aircrew the usual cause is altitude. Oxygen stays at about 21 per cent of the air, but the pressure that drives it into the blood falls as the aircraft climbs. The brain is the most sensitive organ. The first functions to go, judgement, self-criticism and night vision, are exactly the ones a pilot needs to notice that something is wrong.

Hyperventilation means breathing faster or deeper than the body needs, and its symptoms are easily mistaken for hypoxia. Hypoxia can incapacitate within seconds at airliner cruising levels, while hyperventilation seldom harms. EASA and FAA teaching therefore agree on one rule: wherever hypoxia is possible, treat the symptoms as hypoxia first.

Respiration and gas exchange

Respiration covers the whole chain:

1. Ventilation moves air in and out of the lungs.
2. Oxygen and carbon dioxide are exchanged in the lungs.
3. The blood carries the gases.
4. The cells use oxygen to release energy and produce carbon dioxide.

Air passes through the trachea and bronchi to the alveoli, tiny air sacs with walls one cell thick, each wrapped in capillaries.

Gas crosses the alveolar wall by diffusion. Fick's law of diffusion states that the rate of transfer is proportional to the membrane area and to the partial pressure difference across it, and inversely proportional to its thickness. Altitude reduces the pressure difference, and lung disease reduces the area or thickens the membrane. Either way, less oxygen enters the blood.

Almost all oxygen travels bound to haemoglobin in the red cells, with a small amount dissolved in plasma. Oxygen saturation is the percentage of haemoglobin carrying oxygen. It is about 97 to 98 per cent at sea level in a healthy person. Because the dissociation curve is S-shaped, saturation holds up at first and then drops steeply: to about 90 per cent at 10,000 ft and to about 65 per cent at 20,000 ft.

Carbon dioxide dissolves in blood to form carbonic acid, and the brainstem senses the resulting acidity. The drive to breathe therefore comes mainly from the carbon dioxide level, not from a lack of oxygen. The slight acidity also helps haemoglobin release oxygen in the tissues.

Partial pressure and Dalton's law

Dalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures its components would each exert alone. The partial pressure of oxygen is therefore about 21 per cent of total pressure. At sea level, 760 mmHg (1013.25 hPa, 29.92 inHg) gives about 160 mmHg. At around 18,000 ft total pressure is roughly halved, and so is the partial pressure of oxygen.

In the lungs the air is saturated with water vapour and mixed with carbon dioxide, so the alveolar oxygen partial pressure at sea level is only about 100 mmHg. At 10,000 ft it falls to roughly 55 to 60 mmHg, generally treated as the lowest value acceptable for normal operations. Breathing 100 per cent oxygen restores the sea-level alveolar value up to about 33,700 ft. At about 40,000 ft, pure oxygen gives only the equivalent of breathing air at 10,000 ft. Above that altitude oxygen must be delivered under positive pressure.

Lung volumes

Typical adult values are:

• Tidal volume, the air moved in a normal resting breath: about 500 mL.
• Inspiratory reserve volume, the extra air that can be drawn in after a normal breath: about 3,000 mL.
• Expiratory reserve volume, the extra air that can be forced out: about 1,100 mL.
• Residual volume, the air that cannot be exhaled: about 1,200 mL. It keeps the alveoli open.

Ventilation is tidal volume multiplied by breathing rate. The body raises both in response to hypoxia, and hyperventilation raises them beyond what is needed. In a rapid decompression the gas in the lungs expands and must be allowed to escape, so the breath must never be held.

Types of hypoxia

Carbon monoxide deserves special mention. It binds to haemoglobin roughly 200 to 250 times more readily than oxygen. In piston aircraft it usually enters through a leaking exhaust in the cabin heater, and it causes headache, drowsiness and dizziness. The response is to turn the heater off, open the fresh-air vents, use oxygen and land. Smokers carry raised carboxyhaemoglobin, which raises their physiological altitude by several thousand feet.

Symptoms and stages of hypoxia

Symptoms vary between people but tend to repeat in the same person. Those a pilot may feel include:

• euphoria or a false sense of well-being
• headache, light-headedness and air hunger
• tingling and hot or cold flushes
• blurred or tunnel vision.

Those an observer may notice include:

• slow reactions, poor judgement and poor co-ordination
• deteriorating handwriting and confusion
• cyanosis, a blue tinge to the lips and nail beds, which appears late and is absent in carbon monoxide poisoning
• finally, unconsciousness.

Night vision suffers first, because the retina's rods need a lot of oxygen. The FAA's Aeronautical Information Manual recommends oxygen above 10,000 ft by day and above 5,000 ft at night. Susceptibility increases with the rate of climb and time at altitude. It also rises with workload, cold, illness, fatigue, alcohol and smoking.

The classic stages of hypoxia (indifferent, compensatory, disturbance, critical) apply to a person breathing air, and their altitudes are approximate:

The time of useful consciousness (TUC) runs from the loss of oxygen to the point where effective corrective action is no longer possible, which comes well before collapse (see time of useful consciousness). The FAA uses effective performance time (EPT) as a synonym for TUC. Some European texts define EPT as the period of unimpaired performance, which can never exceed the TUC. The figures below are approximate values for a person at rest, from FAA AC 61-107B. That circular advises assuming they are roughly halved after a rapid decompression.

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A pressurisation failure can happen in three ways (see pressurisation and decompression):

• An explosive decompression takes less than about half a second, faster than the lungs can vent.
• A rapid decompression takes a few seconds.
• A slow decompression is the most insidious, because it may go unnoticed.

In the 2005 Helios Airways Boeing 737 accident, the pressurisation mode selector had been left in manual. The crew misread the cabin altitude warning and were incapacitated. The aeroplane flew on until its fuel was exhausted, and all 121 people on board died.

Oxygen equipment, rules and the PRICE check

Oxygen can be delivered in several ways:

• A nasal cannula feeds continuous-flow oxygen into the nostrils. FAA guidance limits it to 18,000 ft, and mouth breathing or talking reduces what it delivers.
• A diluter-demand mask mixes cabin air with oxygen on each breath, adding more oxygen as altitude rises. It is usable to about 40,000 ft.
• Above about 40,000 ft, pressure-demand equipment provides pressure breathing. Oxygen is forced in under positive pressure, which makes exhaling an effort and can itself trigger hyperventilation.

See oxygen systems for aircraft installations.

The FAA's PRICE check is done before flight and at intervals at altitude:

• Pressure: check the supply pressure or quantity.
• Regulator: check that it is set correctly and working.
• Indicator: check that the flow indicator shows flow.
• Connections: check that they are secure.
• Emergency: know the emergency setting, the alternative supply and the descent plan.

A pulse oximeter estimates saturation from light passing through a fingertip. It gives an objective warning before symptoms are obvious. It can mislead with cold fingers, and it reads carbon monoxide-bound haemoglobin as oxygenated.

The main supplemental oxygen rules differ between the two systems:

Hyperventilation

In hyperventilation, over-breathing washes carbon dioxide out of the blood. This state is called hypocapnia. With less carbonic acid the blood becomes alkaline, haemoglobin holds on to its oxygen more tightly, and blood vessels in the brain constrict. The brain is therefore short of oxygen even though plenty is available.

Causes include anxiety, fear, stress, pain, motion sickness, heat, vibration and pressure breathing. Hypoxia itself can also trigger it. Symptoms include:

• light-headedness and dizziness
• tingling or numbness around the mouth and in the fingers and toes
• hot and cold sensations and visual disturbance
• muscle spasm in the hands and feet
• in severe cases, unconsciousness, after which normal breathing resumes automatically.

Hyperventilation can occur at any altitude, including on the ground. The cure is to slow the breathing rate deliberately. Talking aloud, or counting, forces a slower rhythm. Rebreathing into a bag appears in older texts and in exam answers. It is appropriate only once hypoxia has been positively excluded, never as a first action at altitude.

Hypoxia or hyperventilation?

At altitude the drill is to select 100 per cent oxygen, check the system with the PRICE items and descend if in doubt. Then slow the breathing rate. Rapid improvement confirms hypoxia. If symptoms persist after a minute or two on a working oxygen supply, hyperventilation is the likely cause, and controlled breathing will cure it.

Altitude chamber training

Altitude chamber training uses a hypobaric chamber to reduce pressure to a simulated high altitude. Trainees remove their masks under supervision until they recognise their own symptoms, then replace them. The chamber can also demonstrate rapid decompression, gas expansion and the loss of night vision. Because each person's symptom pattern tends to repeat, the experience helps a pilot recognise hypoxia in flight.

The chamber carries a small risk of decompression sickness and barotrauma. Normobaric devices avoid this by lowering the oxygen fraction at ground pressure, but they cannot show gas expansion.

In the United States the FAA's Civil Aerospace Medical Institute offers physiology courses for civil pilots. 14 CFR 61.31(g) requires ground training in high-altitude physiology before a pilot acts as pilot-in-command of a pressurised aeroplane with a service ceiling or maximum operating altitude above 25,000 ft. EASA covers the subject in the Human Performance theory syllabus and does not require chamber training for licence issue.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.