Time of Useful Consciousness (TUC)

This is the time available for the development of hypoxia and the pilot to do something about it, i.e. it is the time of useful consciousness. It is not the time to unconsciousness but the shorter time from a reduction in adequate oxygen until a specific degree of impairment, generally taken to be the point when the individual can no longer take steps to help him/herself.

The time will depend on the individual, and will be affected by any or all of the following:

•Individual fitness

•Workload

•Smoking

•Overweight or obesity

•Decompression is progressive or explosive

The average times of useful consciousness at various altitudes are set out in the following table.

Times of Useful Consciousness at Various Altitudes

Effective Performance Time (EPT)

Effective Performance Time is always within and shorter than TUC. Its quantification, however, is not possible since it will depend upon the individual, the task in hand, physiological and mental stress, altitude and the circumstances involved. It is highly variable and individualistic. Above 40 000 ft the EPT is approximately 5-6 seconds.

Hyperventilation

Hyperventilation can be defined as lung ventilation in excess of the body’s needs and denotes an overriding of the normal automatic control of breathing by the brain. Simply, hyperventilation is overbreathing. That is breathing in excess of the ventilation required to remove carbon dioxide. Overbreathing induces a reduction in the carbon dioxide and thus decreases the carbonic acid balance of the blood. This disturbance of the acid balance has a number of effects, the major one being that haemoglobin gives up its oxygen readily only in an acid medium.

Hypoxia does cause hyperventilation but it is far from the only cause. Anxiety, motion sickness, shock, vibration, heat, high g-forces, pressure breathing can all bring on the symptoms of hyperventilation. A high standard of training breeds confidence and decreases the chances of confronting unusual and stressful situations and is, without doubt, the best means of preventing hyperventilation in aircrew.

An anxious passenger boarding an aircraft must be closely watched since hyperventilation may take place even whilst still on the ground.

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Symptoms of Hyperventilation

•Dizziness and a feeling of unreality.

•Tingling. Especially in the extremities and lips.

•Visual disturbances. Blurred, tunnelling and clouding vision.

•Hot or cold sensations. These may alternate in time and vary as to parts of the body affected.

•Anxiety. Thus establishing a vicious circle.

•Loss of muscular coordination and impaired performance.

•Increased heart rate.

•Spasms. Just prior to unconsciousness, the muscles of the hands, fingers and feet may go into spasm.

•Loss of consciousness. Hyperventilation can lead to collapse but thereafter the body’s automatic system will restore the normal respiration rate and the individual will recover.

Treatment of Hyperventilation

The classic way to treat a patient suffering from hyperventilation is to make him/her breathe into a paper bag. The sufferer is then forced to inhale the carbon dioxide that has been exhaled. The immediate effect of this is to increase the carbonic acid level to its norm and the brain consequently reduces the breathing rate.

The symptoms can, in themselves, be alarming. In all cases try to calm the patient and encourage her/him to slow down the rate of breathing.

Hypoxia or Hyperventilation?

The natural reaction to a shortage of oxygen is for the body to try to obtain more air by breathing faster and deeper. The hypoxic individual may hyperventilate in an effort to get more oxygen, but this is of little value when in an environment of low ambient pressure.

In flight it can be difficult to distinguish the symptoms of hypoxia and hyperventilation. The appropriate response of pilots must be to assume the worst and if they are at an altitude where hypoxia is a possibility they must take that to be the cause and carry out their hypoxia drills. If symptoms occur at an altitude at which hypoxia is not a consideration (below 10 000 ft) they should regulate the rate and depth of breathing to restore the normal acid/base balance of the blood and alleviate the symptoms. When flying below 10 000 ft significant symptoms of hypoxia are unlikely and hyperventilation may be assumed.

DO NOT ASSUME HYPERVENTILATION IF IT COULD BE HYPOXIA

HYPERVENTILATION - AFTER UNCONSCIOUSNESS - RECOVERY

HYPOXIA - AFTER UNCONSCIOUSNESS - DEATH

Cabin Pressurization

Cabin pressurisation systems ensure that the effective altitude to which the occupants are actually exposed is much lower than the altitude at which the aircraft is flying. Ideally the cabin should be maintained at sea level but this is impractical because of aircraft weight and fuselage strength limitations.

The pressurization of a commercial airliner flying at 30 000 ft produces an internal cabin pressure equivalent to about 6000 ft with a maximum of 8000 ft. The pressure differential across the aircraft skin is normally designed not to exceed 8-9 psi The rate of change of cabin pressure is restricted to 500 ft/min in the ascent and 300 ft/min in the descent to minimize passenger discomfort due to the pressure equalization limitations of the middle ear.

Cabin Decompression

Loss of cabin pressurization can occur in flight. The rate of loss may be slow, with the crew recognizing the problem and making appropriate height reductions before the passengers are aware of anything amiss. Very occasionally there is rapid decompression perhaps due to the loss of a window or door, or a failure in the fuselage.

Occupants, crew and passengers, will rapidly be exposed to the full rigours of high altitude: hypoxia, cold, decompression sickness. Oxygen can be supplied to all occupants but for only a limited period.

THE AIRCRAFT MUST RAPIDLY DESCEND

TO 10 000 ft OR MSA WHICHEVER IS THE HIGHER

In cases of rapid decompression the altitude of the cabin may actually rise to above the pressure altitude. The Venturi effect of air passing over the fuselage can suck air out of the cabin; this can make up to 5000 ft difference in pressure terms.

Another effect of decompression at height is that, due to the sudden drop in temperature within the aircraft, windows and cockpit windshields will be prone to misting or fogging.

It is most important to emphasize that crew protection must be the highest of priorities. Should decompression take place it is critical for the crew to individually don oxygen masks and check flow as quickly as possible. Any delay caused by helping other members of the crew or passengers could have catastrophic results for all the occupants of the aircraft.

Decompression Sickness (DCS)

As we have seen, the gas making up the major part of the air - nitrogen - is dissolved in the blood to a small extent but plays no part in the normal bodily processes. It may however cause severe problems if the nitrogen should come out of solution as small bubbles. It can be likened to the bubble formation in fizzy drinks when the top of the bottle is opened and the pressure allowed to drop. If this occurs in the human body and nitrogen bubbles are formed in the blood, the process leads directly to DCS.

Body exposure to reduced pressure can lead to DCS since the body is normally saturated with nitrogen. When ambient pressure is abruptly reduced some of this nitrogen comes out of solution as bubbles. Any ascent to altitudes over 25 000 ft is normally associated with DCS

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however it is more likely the higher and longer the exposure to altitudes above 18 000 ft. It is unlikely to occur below 14 000 ft.

Ultimately the individual may collapse and in rare cases DCS may occur or persist after descent and go on to cause DEATH. Hypoxia and cold increase the risk as does age and excess body mass/obesity.

The primary symptoms are:

Joints. Bubbles in the joints (shoulders, elbows, wrists, knees and ankles) cause rheumatic-like pains called the bends. In aviation the shoulder, wrist, knee and ankles are most commonly affected. Movement or rubbing the affected parts only aggravates the pain but descent usually resolves the problem.

Skin. Nitrogen bubbles released under the skin causes the creeps when the sufferer feels that a small compact colony of ants are crawling over, or just under, the skin.

Respiratory system. This is known as the chokes. Nitrogen bubbles may get caught in the capillaries of the lungs blocking the pulmonary blood flow. This leads to serious shortness of breath accompanied by a burning, gnawing and sometimes piercing pain.

Brain. The bubbles affect the blood supply to the brain and the nervous system. This effect is known as the staggers. The sufferer will lose some mental functions and control of movement. In extreme cases chronic paralysis or even permanent mental disturbances may result.

The secondary symptom is:

Post descent collapse. This may occur up to four hours after the primary symptoms when nitrogen bubbles have combined and therefore not gone back into solution and have reached the heart.

DCS can be avoided by preoxygenation before exposure to high altitudes, thus reducing the body store of nitrogen as much as possible.

DCS in Flight and Treatment

If the symptoms of DCS appear in any passenger or crew member, the pilot should commence an immediate descent to a level at which the symptoms are relieved. The aircraft should land as soon as possible. Meanwhile the sufferer should be kept warm and rested and put onto a 100% oxygen supply. Urgent medical assistance must be sought on landing even if the patient appears to have recovered.

Flying and Diving

DCS is rare but the incidence is greatly increased for individuals who have been diving, using compressed air, shortly before a flight. The pressure that a 30 ft column of sea water exerts is the same as that exerted by the atmosphere at sea level (i.e. 760 mm Hg). Therefore a person at a depth of 30 ft is exposed to a pressure of 2 atmospheres (1 atmosphere caused by the air above the water and the other by the water itself).

In scuba diving, air under pressure is used and this increases the amount of nitrogen in the body. On subsequent ascent this may come out of solution giving rise to DCS. The following rules must be strictly observed by both crew and passengers. Failure to adhere to these rules

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results in incidents each year in which individuals develop DCS in flight at altitudes as low as 6000 ft.

DO NOT FLY WITHIN 12 HOURS OF SWIMMING USING COMPRESSED AIR

AND

AVOID FLYING FOR 24 HOURS IF A DEPTH OF 30 FEET HAS BEEN EXCEEDED.

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