Hemoglobin  (is the iron-containing oxygen-transport metalloprotein in the red blood cells of almost all vertebrates as well as the tissues of some invertebrates.) The inadequate level of oxygen in the blood is called Hypoxia, depending on the cause, there are different kinds of hypoxia. In aviation this reduction in oxygen is usually due to altitude itself and this is called hypoxic hypoxia. If it’s due to the lack of blood meaning lack of hemoglobin to deliver the oxygen to your system that can be caused by loss of blood from stomach ulcer, menstrual periods or blood donation, this type of hypoxia is called anemic hypoxia. And lastly is when hemoglobin’s ability to carry oxygen is impaired by toxin such as cyanide or carbon monoxide (CO) it is called histotoxic hypoxia.

Hypoxia is insidious. Most pilots can operate safely up to cabin altitude of 10,000′ if they are in good health and do not smoke. Between 12,000 – 15,000′ increased in breathing rates will partially compensate to allow functioning for an indefinite time. Unfortunately, the first side effects ( impaired judgement and euphoria or lack of concern) may not be recognized and by themselves create a risk to flight safety.

Increased hypoxia interferes with muscular coordination, mental calculation and reasoning power. Above 15,000′ a person not acclimatized to this altitude may become severely impaired. (usually within minutes or even seconds depending on the altitude) Unconsciousness may occur before the pilot become aware of the problem.

Although unconsciousness may come on quite slowly, a more useful measure is the time of useful consciousness (TUC) or effective performance time (EPT) This reflects the period beyond which a pilot would be unlikely to take corrective or protective action and is much shorter than the onset of unconsciousness.

Rapid decompression can reduce the TUC by up to 50% owing to the forced exhalation of the lungs and the extreme rapid rate of ascent.

PREVENTION OF HYPOXIA

Aircraft oxygen system are of three types. The continuous flow system is the most common although it is wasteful of oxygen. diluter demand systems increase the percentage of oxygen with ascending altitude by a barostat to meet physiological requirements. At cabin altitude above 30,000′ even 100% oxygen is insufficient and pressure demand regulators are required to ensure that the partial pressure of oxygen supplied to the lungs is adequate.

HYPERVENTILATION

The rate at which we breathe is regulated by the amount of carbon dioxide in the lungs and blood rather than by the oxygen concentration. This is why mild hypoxia due to altitude may not cause any perception of a need for more air. The normal rate of breathing is 12 – 14 breathes per minute. If we breathe faster than this at rest we will not increase the amount of oxygen in the blood (it is already at maximum) but we will decrease the carbon dioxide. This causes the blood to become more alkaline which in turn leads to constriction in the blood vessels of the head and neck. Then the blood supply and oxygen to the brain is reduced.

Hyperventilation, an increase in the rate and depth of breathing, can be the result of stress, anxiety or fear. All these factors may be present during a difficult instrument approach in bad weather. They are also more common in inexperienced pilots who have not yet gained confidence in their ability. As the breathing rate increases symptoms begin to appear. These include a feeling of lightheadedness, numbness around the mouth, tingling in the fingers and toes and later muscular spasms. Paradoxically there is often a feeling of breathlessness that may worsen the situation.

Symptoms of hypoxia and hyperventilation are similar and that ultimately both cause a reduction of oxygen delivery to the brain. At ground level hyperventilation may be treated by calming the person in an effort to reduce the rate and depth of breathing, and by re-breathing exhaled air (usually in a paper bag) often the victim will feel nausea and lightheadedness for some time afterwards. At altitude, the strong possibility that these symptoms may represent hypoxia demands a different approach.

TREATMENT OF HYPOXIA AND HYPERVENTILATION

Because the symptoms are similar the treatment must deal with both problems safely without the pilot having to make a diagnosis.

• Below 10,000′ severe hypoxia is unlikely and the pilot should slow the breathing rate 12 – 14 breaths / minute maximum. The breath may be held briefly.
• Above 10,000′ oxygen should be used and three or four deep breaths taken quickly. If the symptoms are due to hypoxia they will improve immediately. If they do not improve the rate of breathing should be controlled as above.