Definition
Hypoxia (n.) can be defined as "a deficiency of oxygen reaching the tissues of the body" (Merriam-Webster Online Dictionary, 2005 [13]).
The term hypoxia also describes the state that occurs when oxygen available to the tissues in the body is insufficient to meet their needs (Green et. al., 1996 [9])
Theoretical frame
Hypoxia can be suffered at any time and place where oxygen deficiency occurs. The main interest for researchers, however, is human performance in those environments where oxygen deficiency is natural rather than created (for example, at high altitudes). Mountaineering and aviation qualify as such environments. Laboratory research using breathing mixtures to induce hypoxia have also being used as a controlled environment for mimicking hypoxia at high altitudes.
Research has found that psychological functioning, especially cognitive functioning, can be impaired with altitude. In fact, 3,000 to 3,500 metres (9,843 to 11483 feet) and above may be the altitude zone where mos adverse impairment take place (Richards, Cleland & Zuckerman, 2006 [19], also Nelson, 1982 [15], and Cahoon, 1972 [3]).
Medical Explanation of Hypoxia
The loss of energy and impaired ability to think clearly can be explained; the reduced Oxygen intake causes decreased in partial pressure of the oxygen supply by the blood. The reduced Oxygen also causes the decreased in metabolism and lack of energy production. The formula of the metabolism is Food+O2 (energy Production)= CO2 + H2O . As it can be seen in order to produce energy to move and think, human's body need oxygen. Thus, the lack of oxygen supply to the body and brain can severly downgrade the performance of human ability both physically and mentally.
Symptoms of Hypoxia
The signs and symptoms of Hypoxia can be various to individuals however, in most cases people will suffer from these symptoms
Visual effect : night vision reduced, peripheral vision decrease, clour vision dissipate
Physical effect : Muscle incoordination,slurred speech(drunken), cyanosis
Mental effect: loss of judgment, poor problem solving, fatigue , personality change, loss of memory, poor decision making, fixation and loss of consciousness
(This image is a direct link to an online resource by Diffusio2n Pharmaceuticals LLC, 2008 [6])
The factors that reduces tolerance to hypoxia
- altitude, time, exercise, cold, illness, fatigue, drugs/alcohol, smoking
Altitude - The greater the altitude the more rapid the onset
Time - The longer the the time of exposure, the greater the effect
Exercise - Exercise increases the body's demand for oxygen
Cold - Energy is required to generate heat to heat up our body to compensate for the low temperatures and thus increases demand for oxygen
Illness - Illness similarly increases the demand for oxygen
Fatigue - Fatigue lowers the threshold for hypoxia symptoms
Drugs/alcohol - Drugs and alcohol can depress brain functions, thus reducing the tolerance of altitude
Smoking - Smoking produces carbon monoxide, which binds to haemoglobin with a greater affinity than oxygen, reducing the amount of haemoglobin available for oxygen to transport
Taken from Human performance and limitations in aviation (3rd ed.) (Campbell & Bagshaw, 2002, p.23 [4])
In aircraft , when the pressurised oxygen supply stops the time that human can sustain conciousness are
FL 180 = 10-15 mins
FL 250 = 2-3 mins
FL 300 = 45-75 seconds
FL 450 = 9-12 seconds
Classical signs and symptoms
Apparent personality changed - change in outlook and behaviour with excitement or hostility and the loss of inhibitions.
Impaired judgement - loss of self-criticism with the individual oblivious to performance being reduced significantly.
Muscular impairment - slow decision making and poor fine muscular control leads to coordinated movements becoming difficult.
Memory impairment - short term memory is lost early, making drills difficult to complete unless trained in long-term memory.
Sensory loss - vision, especially for colour, is affected early, then touch, orientation, and hearing, all of which become impaired
Impairment of consciousness - as hypoxia progresses the individual's level of consciousness drops until he becomes confused, then semi-conscious, and unconscious. Unless he is rescued he will die and at high altitude death can occur within a few minutes.
Taken from the book Human Factors for Pilots (Green et. al., 1996 [9])
Supporting evidence
The following studies have been cited by Richards et al (2006 [19]) as supporting evidence of the effects of hypoxia on human performance:
- Mountaineering: it has been found that climbers after a high-altitude expedition showed:
- mild impairment in concentration, verbal learning and memory and cognitive flexibility (Bonnon, Noel-Jorand & Therme, 1995 [2]; Regard, Oelz, Brugger & Landis, 1989 [18])
- decreased memory performance (Cavaletti & Tedici, 1993 [5])
- decline in visual and verbal learning and memory (Hornbein, 1992 [10])
- deterioration of the ability to learn, remember and express information verbally (Townes, Hornbein, Schoene, Sarquist & Grant, 1984 [20])
- mild impairment in either short-term memory or conceptual tasks (Regard, Landis, Casey, Maggiorini, Bartsch & Oelz, 1991 [17])
- impairment in grammatical reasoning and in pattern comparison (Kennedy, Dunlap, Banderet, Smith & Houston, 1989 [11])
- improper evaluation of danger or poor judgments leading to accidents (Nelson, Dunlosky, White, Steinberg, Townes & Anderson, 1990 [16])
- Laboratory setting with induced hypoxia. When compared to performance in normal circumstances, it has been found that participants under hypoxia conditions showed:
- slower response in a memory task (Fowler, Prlic & Brabant, 1994 [8])
- Aviation. It has been found that pilots showed:
- deficits in recalls (readbacks) of information with high memory loads, at moderate altitudes (3,810 to 4,572 meters - 12,500 to 15,000 feet) (Bartholomew, Jensen, Petros, Ferraro, Fire, Biberdorf et al, 1999 [1])
- steadily decrease in mental arithmetic performance as altitude increases (Wu, Li, Han, Wang & Wei, 1998 [21])
Risk Assessment and Management for Passengers
Millions of people travel with airlines annually. This too includes passengers with cardiovascular and respiratory illnesses. Hypoxia then is one of the most known consequences at high altitude. It cannot be overcome by sheer human skill or talent. Any individual, weak or strong will succumb to hypoxia if conditions become susceptible. It therefore becomes very vital that one recognizes the symptoms of hypoxia to be able to manage the situation and make decisions on it, especially for people who are already having cardiovascular and respiratory illnesses.
Environmental conditions at high altitude
Barometric pressure is known as pressure on an object from the atmospheric layers above. This pressure decreases exponentially as distance from the earth’s surface increases. According to Dalton’s law, the barometric pressure exerted by non-reacting gases, is equal to the sum of partial pressures of a separate component. The proportion of atmospheric oxygen as we all know remains constant at 21% in the air, at altitudes below 100,000 meters. Thus, the partial pressure of oxygen (PO₂ = Barometric pressure x 0.21) falls dramatically with pressure at higher altitudes. PO₂ at seal level is 159 drops two times at 5496m. For every 300m, PO₂ drops a further 4-5 mm Hg.
When barometric pressure decreases, this leads to expansion of other surrounding gases according to Boyle’s law. At 5486 m (0.5 atm), the volume of an enclosed gas would be doubled as oppose to if it were to be at sea level (1 atm). Clinically, this means gas trapped in our sinuses, cochlea (middle ear), gastrointestinal tract, pleural cavity and eyes will have implications to our body.
Aircrafts normally cruise between 22,000 to 44,000 feet (6,500m to 13,500 m). Hypoxic and hypobaric conditions are lethal at altitudes such as these of cabin are not pressurized. Pressurization works in a way that air is compressed in the aircraft to obtain a cabin pressure equivalent to that found at altitudes 5000 feet to 8000 feet (1500m to 2500m) giving the term “cabin altitude”. Newer aircrafts today fly at higher altitudes exposing passengers to a more positive hypoxic environment. Commercial air carriers normally maintain a cabin altitude below 8000 feet (2500m). The FAA requires supplemental oxygen to be accessible for passengers at flight altitudes above 10,000 feet (3000m). In addition, humidity is very low inside the cabin, under 25%, and if supplied for longer periods, oxygen should be humidified.
(Mortazavi, Eisenberg, Langleben, Ernst, Schiff, 2003 [14]))
Aviation accidents
King Air 200 Crash 1979
In 1979, a training flight involding 2 pilots on a British Super King Air 200 was carried out somewhere south of England at FL 300. The pilots failed to recognised the onset of hypoxia before losing consciousness. The aircraft then, orbiting on autopilot with the crew already dead drifted towards French airspace and crashed shortly after fuel ran out. Reports indicated that the oxygen supply switch was not on. Detailed report concludes that when the crew put on masks after dumping pressurisation at FL 310, they only breathed in ambient air therefore succumbing to lost of consciousness quickly. (Flight Global, 1981 [7])
Refuting evidence
The following studies have been cited by Richards et al (2006 [19]) as refuting or non-conclusive evidence of the effects of hypoxia on human performance:
- Mountaineering:
- verbal fluency was not significantly affected by testing location (Lieberman, Protopapas and Kanki, 1995 [12]), although this study did not compare high-altitude measures with baseline or normative data at low altitudes.
- Laboratory setting with induced hypoxia. When compared to performance in normal circumstances, it has been found that participants under hypoxia conditions showed:
- similar rate of scanning short-term memory (Fowler et al, 1994 [8])
- Aviation. It has been found that pilots showed:
- no deficits in recalls (readbacks) of information with low memory loads and no deficits in vigilance, at moderate altitudes (3,810 to 4,572 meters - 12,500 to 15,000 feet) (Bartholomew et al, 1999 [1])
Way forward (to do list)
Knowledge Management Space
- item
- explanation
