Flying into Thin Air: Preventing Hypoxia

Technological improvements in reliability and performance of cabin pressurisation and Oxygen delivery systems has greatly reduced the incidents and accidents due to hypoxia. Yet, incidence of hypoxia in flight still occurs due to lack of vigilance, mechanical failure of equipment, improper indoctrination or improper use of oxygen equipment. 

Hypoxia can be prevented by ensuring that an aircrew has sufficient oxygen to maintain a range of alveolar partial pressure of Oxygen between 60 and 100 mm Hg. This oxygen level is achieved in aircraft by an oxygen system, cabin pressurisation, or a combination of the two.

Cabin Pressurisation. The most efficient method of preventing the physiological effects of hypoxia is by providing pressurisation system in the aircraft. This ensures that the occupants of the aircraft are never exposed to the pressures outside the physiologic zone. In practice there exist two types of pressurisation schedules:

Isobaric system. The system maintains a constant cabin pressure as the ambient pressure decreases. The system is capable of maintaining the cabin altitudes between 2000 to 8000 ft. This type of pressurization schedule increases the comfort and mobility of the occupants, hence is commonly used in commercial aircraft.

Isobaric differential system. In this system the pressurisation commences at a given altitude and cabin altitude is maintained at this value till a preset pressure differential is reached. With continued ascent the pressure differential is maintained. Aircraft with isobaric differential system mandatorily require Oxygen supplement with a dedicated Oxygen system. Incidentally, tactical military aircraft, utilising this type of system, are not equipped with isobaric pressurisation system because the added weight penalty would severely affect the range of the aircraft and accidental threat of rapid decompression remains due to enemy fire.

Provision of supplemental oxygen in the aircraft ensures that the occupant receives increasing quantity of oxygen in the inspired air. The aircraft oxygen system (regulator assembly) ensures that the correct percentage of Oxygen is added from the on-board Oxygen reserve to the inspired air in order to maintain partial pressure of Oxygen in lungs at 103 mm Hg. Table below shows the percentage of Oxygen supplement in inspired air to ensure maintenance of partial pressure of Oxygen in lungs equivalent to mean sea level.

Requirement of Pressure breathing.   Military aviation, especially combat flying, exposes pilots to altitudes up to 50,000 ft. However at 40,000 ft altitude and above, breathing 100% Oxygen is not adequate to offset the effects of hypoxia. This is due to decreasing ambient pressure and thus lowered partial pressure of Oxygen. Hence at and above 40,000 ft altitude, Oxygen is required to be provided at a pressure in excess of the ambient. The aircraft Oxygen system automatically delivers the additional pressure required, as per the aircraft altitude. This positive pressure by the Oxygen regulator is required to provide adequate alveolar pressure of Oxygen to ensure tissue oxygenation. Pressure breathing is not required during normal flight with isobaric differential system, since the aircraft pressurisation does not allow the altitude of the cabin to exceed beyond 6.5 Km (20,000 ft). It is essentially required in the event of an accidental failure of cabin pressurisation system.

Oxygen Paradox.   Recovery from Hypoxia usually occurs within seconds after re-establishing a normal alveolar partial pressure. Nevertheless, mild symptoms such as headache or fatigue may persist after the hypoxic episode. The persistence of symptoms seems to have a higher degree of correlation with the duration of the episode than with its severity. In some instances following the sudden administration of oxygen to correct the hypoxic insult the individual develops a temporary increase in the severity of symptoms. This is known as ‘Oxygen Paradox’. The subject may lose consciousness or develop fits for a period lasting up to a minute. Accompanying symptoms are mental confusion, deterioration of vision, dizziness, and nausea. Initially, the blood pressure falls and the rate of blood flow decreases. This is caused by reduction in carbon dioxide due to increased respiratory activity to compensate for hypoxic insult, which along with reduced blood pressure on re-oxygenation, act together to reduce blood flow to the brain. This reduced blood flow to the brain apparently intensifies the CNS hypoxic insult for a short period until the circulation improves and the carbon dioxide tension returns to a normal range.

A word about Hypoxia in Combat Flying.   In-flight hypoxia in combat aircraft can occur due to the following reasons:-

• Inadequate ground servicing
• Failure to turn on oxygen supply
• Failure of oxygen supply to demand regulator
• Failure of demand regulator to give correct concentration of oxygen
• Inadvertent break of connection in between Oxygen mask and regulator
• Inadequate seal of mask to face with ill-fitting masks
• Malfunction of mask valves
• Faulty oxygen drill

It is advisable that both the aviator and maintenance personnel must pay necessary attention to the above mentioned areas, to prevent it from resulting in an accident.

Conscientious pre-flight checks go a long way in decreasing the incidence of hypoxia. During flight, if an aircrew experiences symptoms of hypoxia s-/he  must select 100% oxygen setting on the Oxygen regulator. If the symptoms persist despite of selecting 100% Oxygen, the aircrew must suspect contamination of the source and use emergency oxygen while immediately commencing an emergency descent below 10,000 feet.

Lastly, usefulness of hypoxia indoctrination in an altitude chamber is a valuable tool in aircrew training. The premise here is that such training reinforces hypoxia awareness, allowing aircrew to become familiar with the symptoms associated with hypoxia. Thus, hypoxia indoctrination is an effective method to promote recogntion and recall of individual ‘hypoxia signature‘. Such training is mandatory in most of the Air Forces the world over, and is recommended for commercial pilots as well, in the larger interest of aviation safety.

Read More

• Hypoxia Training

Reference

1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.

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