13.1 The oxygen dissociation curve of blood
20 40 60 80 Oxygen tension (mm Hg)
13.1 The oxygen dissociation curve of blood exposure of aviators, even breathing 100% oxygen. Exposures above this need to achieve the 40 000 feet (12 000 m) lung picture irrespective of altitude, and this is achieved by actually pressurising the gas in the respiratory tract. This process is known as pressure breathing and is beyond the scope of this discussion.
The ability of the normal healthy individual to function efficiently up to an altitude of approximately 10000 feet (3000 m) is explained by the relationship between the oxygen saturation of haemoglobin and oxygen tension. Ascent to an altitude of 10000 feet (3000 m) produces a fall in the partial pressure of oxygen in the alveoli but only a slight fall in the percentage saturation of haemoglobin with oxygen. However, once altitude rises above 10000 feet (3000 m) the percentage saturation of haemoglobin falls quickly, resulting in the condition of hypoxia. In fact, above 8000 feet (2400 m) the effects of lack of oxygen will begin to appear and a decrease in the individual's ability to perform complex tasks and a reduction in night vision can be measured.
Figure 13.1 shows the oxygen dissociation curve of blood, with the concentrations of physically dissolved and chemically combined oxygen being shown separately. The curve illustrated is the average for a fit young adult. The actual shape of the curve will be influenced by factors such as age, state of health, tobacco abuse and ambient temperature.
Summary of Oxygen Requirements
• 10000-33 700 feet (3000-10 250 m): increasing oxygen concentration
• 33 700-40000 feet (10 250-12 000m): 100% oxygen
• 40000 feet (12000 m) and above: 100% oxygen under pressure.
This is the term used to denote the body condition when the oxygen available to the tissues is insufficient to meet their needs. We have seen that this condition will become obviously manifest when the partial pressure of oxygen in the lungs falls below about 55 mmHg. The tissues of the body become deprived, each becomes less efficient, but by far the most sensitive and susceptible is the brain. Consequently the first symptoms experienced by the hypoxic individual will be those resulting from brain inefficiency. The symptoms are in general terms similar in everybody, but have minor variations in individuals.
The classical symptoms of hypoxia may be summarised as follows:
1. Personality change. This is the initial stage of hypoxia. It is characterised by a change in the person's outlook, and his or her normal inhibitive forces of common sense tend to be diminished.
2. Impaired judgement. Loss of self-criticism, euphoria or depression. This stage can be particularly dangerous for aircrew, as, far from realising something is wrong, the aircrew member may well be led into a false state of well-being, which may lead on to a careless, carefree attitude, with reckless behaviour.
3. Mental and muscular incoordination. This is the stage of finding difficulty in 'thinking straight'. Constructive, progressive, logical thought processes become difficult, while coordinated muscular movements , such as clear speech or writing, become impaired.
4. Sensory loss. Concomitantly, inputs to the body via the sense organs are also diminished, thus such things as vision, hearing and the sense of touch become impaired.
5. Memory impairment. A classical symptom of hypoxia is impairment of recall for recent events, and is in the main due to difficulty in structuring thought. Obviously this is a most important effect for aircrew to be aware of as an unimpaired memory is imperative for recall of vital actions necessary to remedy the situation by previously taught emergency drills. Fortunately this is a fairly late symptom, and provided the aircrew member has absorbed previous training, he or she should have spotted the predicament before this potentially catastrophic stage is reached.
6. Loss of consciousness. Following memory loss, impairment of consciousness followed by loss of consciousness supervenes, leading ultimately to death.
The above are symptoms experienced or felt. Other signs are directly observable. These are:
1. Cyanosis. This is due to low oxygen concentration in the blood. As a consequence the complexion takes on a bluish hue, the skin becoming dusky, and fingernails and lips in particular become blue, similar to the colour of the hands on a cold day.
2. Hyperventilation. Low oxygen tension in the blood will lead to 'air hunger', manifested by deeper breathing. There may be other causes of deeper breathing, but above 10 000 feet the probability of hypoxia as a cause of hyperventilation should always be considered.
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