Hypoxia is defined as a lack of oxygen in the body tissues. This can be caused either by a shortage of oxygen in the air being breathed or by a number of physiological/pathological issues affecting blood circulation or the quantity of oxygen carried by haemoglobin in the blood.
The effects of hypoxia include fatigue, confusion, euphoria, inability to concentrate, impaired decision-making, impaired psychomotor performance, loss of consciousness and, eventually, death. Hypoxia does not cause discomfort or pain so its onset can be insidious and pass un-noticed by crews who are not fully aware of its dangers.
Factors that affect the onset and severity of hypoxia include an individual’s physical fitness, cabin temperature, altitude, rate of ascent and duration at altitude. Individuals differ considerably in their ability to withstand hypoxia so in the early stages, one crewmember may be more seriously affected than the other(s).
In the context of aircraft in flight, the onset can be sudden or gradual. Sudden onset may require a rapid and instinctive response by aircrew whereas gradual onset is a matter of awareness so that an appropriate response can be made before incapacitation occurs.
The Medical Background
The blood contains haemoglobin that carries oxygen molecules from the lungs to all tissues of the body. Adequate amounts of haemoglobin coupled with adequate oxygen saturation of that haemoglobin is vital to human function.
There are four types of hypoxia:
- Hypoxic hypoxia, sometimes known as "altitude hypoxia", occurs due to the reduced partial pressure of Oxygen in inspired air.
- Anaemic hypoxia occurs when the blood’s oxygen carrying capability is reduced; this may be due to reduced haemoglobin content caused by poor nutrition or by carbon monoxide, nitrates or sulfa drugs etc. that react with haemoglobin and reduce the amount available to carry oxygen.
- Stagnant or hypokinetic hypoxia is caused by circulatory system problems such as heart failure or, in aviation, by blood pooling in the lower limbs under high g manoeuvres.
- Histotoxic hypoxia which occurs when the ability of body tissue to absorb oxygen from the blood is impeded by substances such as alcohol, narcotics and certain poisons.
All these may be encountered in flight but the most frequent and important type of hypoxia encountered by fit aircrew inflight is hypoxic hypoxia caused by breathing air at altitude.
Ambient air pressure reduces with increasing altitude and, as a direct consequence, the partial pressure of oxygen (pO2) reduces too. In a healthy individual, oxygen saturation of haemoglobin is initially little affected. Between the surface and 10,000 feet altitude, even though air pressure decreases by 25%, the saturation of haemoglobin with oxygen only declines from about 98% to 90% which makes little difference to most human functions; the exception to this is a gradual onset of significant deterioration in night vision sensitivity, which has been found to reduce by 30% by 10,000 feet altitude. (Note also that the heart is one of the most sensitive organs with respect to pO2; it extracts more oxygen from arterial blood than most other tissues, so its function can be affected when blood oxygen saturation is reduced. Significant reductions in pO2 can unmask previously unrecognised cardiovascular disease that may present a problem for both crew and passengers.)
However, above 10,000 feet altitude, the amount of oxygen in the blood begins to decrease much more rapidly, much faster than air pressure which continues to decrease at a similar rate. By 20,000 feet altitude, the concentration of oxygen in the blood is only 65% saturation and at these levels, normal human function is materially interrupted and the effects are cumulative over time. At higher altitudes, the effects worsen quickly.
Symptoms of developing hypoxia vary markedly from individual to individual; many exhibit blueness on the lips and fingertips, some may feel over-warm while others may feel cold or notice a pounding in the ears. Hypoxia training, where people experience breathing air at low pressure under carefully supervised conditions, can prove very useful in enabling an individual to understand their own personal symptoms of hypoxia. As the degree of hypoxia increases, the classic medical signs and symptoms include:
- Breathlessness/air hunger
- Excessive yawning
- Tiredness and fatigue
- Impairment of performing recently learnt task
- Impairment of mental task (learnt tasks)
- Altered sensorium, including loss of consciousness
The danger to aircrew of an insidious condition that causes euphoria and impaired mental ability without any warning signs such as pain or discomfort are self-evident!
The Technical Response
Aircraft which routinely operate above 10,000 feet altitude are pressurised to keep the aircraft cabin no higher than the equivalent of 8000 feet altitude at any actual altitude up to the prescribed AFM maximum operating altitude. The partial pressure of oxygen is equivalent to the prevailing "Cabin Altitude". The existence of an air pressure inside the aircraft pressure hull, which is never less than that outside it, implies the existence of a pressure differential between outside and inside the aircraft. Aircraft pressurisation systems operate automatically but crews must confirm correct operation monitoring cabin altitude, cabin rate of climb and descent, and differential pressure.
The possibility of hypoxia arises in two very different ways:
- Sudden loss of normal cabin pressurisation at high altitude as a result of explosive or rapid depressurisation - usually resulting from structural failure.
- Gradual and progressive onset during flight above 10,000 feet altitude in the absence of normal pressurisation. This can arise either by climb above 10,000 feet without the pressurisation system functioning, or because of pressurisation system malfunction.
Defences - Sudden Onset
The time of useful consciousness may be very short. For example, at 35,000 ft some individuals may only have as little as 15 seconds of useful consciousness - i.e. 15 seconds to make and action cogent, rational decisions - following an explosive decompression.
- For aircrew - appropriate training which ensures the instinctive response of immediate oxygen mask donning if the obvious signs of sudden decompression occur and, in the case of the pilots, ensures that there is a sequential response so that control of the aircraft is maintained. The shortest available response times before consciousness will be lost are at high altitude in small aircraft.
- For passengers - attention to the pre-departure cabin safety briefing, and recall if required, since cabin crew will not be able to assist if sudden decompression occurs.
Defences - Gradual Onset
The early symptoms of hypoxia do not include either discomfort or pain and may be more obvious to an observer than to the affected person. Blueness of the lips or fingertips and an increased rate and depth of breathing may be noticed but beyond that, a whole range of effects may apply which are dependent on the individual. The onset symptoms for Hypoxia are almost identical to those of hyperventilation and it is important not to assume that they are due to hyperventilation; hypoxia is immediately life-threatening and should always be considered as the cause of these symptoms.
Flight crew must adhere strictly to standard operating procedures (SOPs) checks of pressurisation system status, which will usually provide warning of any abnormalities before automatic system warnings are generated. If pressurisation warnings or cautions are generated, the response prescribed in the quick reference handbook (QRH) must follow without delay. When such responses are executed immediately then this may preclude a need for fight crew to don oxygen masks or for passenger oxygen masks to drop (this usually occurs at 14000 feet altitude although in some aircraft passenger mask deployment must be selected manually).
Accidents & Incidents
Two examples of the gradual onset case:
- B733, en-route, northwest of Athens Greece, 2005: 6 crew and 115 passengers perished due to lack of pressurisation. With the crew incapacitated by hypoxia, the aircraft flew on under flight management computer and autopilot control until it ran out of fuel and crashed.
- RJ1H, en-route, South West of Stockholm Sweden, 2007: Flight crew failed to notice the aircraft was not pressurised after take off until the cabin crew advised them of automatic passenger mask deployment. Incident compounded by partial failures in passenger oxygen systems, portable oxygen equipment and pressurisation warnings.
FAA - "Lessons Learned from Transport Airplane Accidents"