Hypoxia is defined as a deficiency of oxygen in the body tissues or simply oxygen starvation. Hypoxia can occur due to a variety of reasons. Normally this condition is preceded by low levels of oxygen in the blood system-a condition normally known as hypocalcaemia. Nonetheless, it is important to note that hypoxia can occur without hypoxemia.
Hypoxia can also lead to high level of carbon dioxide in the blood system and not in all cases. This condition is normally referred to as Hypercapnia. The general effect of hypoxia in most cases depends on the duration, intensity and type of hypoxia. In addition, the impact of hypoxia depends on the sensitivity of the tissues affected.
The pathogenic mechanism leading to low level of oxygen in the body tissues can be well understood by looking at the factors (both internal and external) responsible for sufficient supply of oxygen to the tissues.
These factors include: the concentration of oxygen in the air inhaled; appropriate exchange of gases in the air circulation system; the amount of hemoglobin in the blood for oxygen transfer; functioning of the cardiovascular system; and lastly the capability of the tissues to utilize the supplied oxygen.
Different types of Hypoxia
Corresponding to the above external and internal factors are different types of hypoxia. These are hypoxic hypoxia, anemic hypoxia, circulatory (ischemic) hypoxia and histotoxic hypoxia. Hypoxic hypoxia is as a result of low level of oxygen in the inhaled air (particularly in high altitudes) or disorder in the respiratory system (for example due to asthma, pulmonary edema and pneumonia among others).
Anemic hypoxia results from different anemic states or insufficient hemoglobin in the blood (Metheglobinemia). Carbon monoxide poisoning, chronic blood loss and sickle cell anemia can lead to anemic hypoxia.
Ischemic hypoxia is caused by a disorder in the air and blood circulation system or arterial blockage leading to ischemization. Last but not least, histotoxic hypoxia is caused by the inability of the tissues to utilize the supplied oxygen (Dembrovsky & Racz, 2012, p. 4).
This can be as a result of cyanide poisoning, excessive alcohol consumption and use of narcotics. Hypoxia can also be classified as chronic hypoxia, acute hypoxia and fulminant hypoxia. Chronic hypoxia results from never-ending infections on the respiratory system and blood circulatory system or blood.
Acute hypoxia is a rare condition and normally stems from heart failure, suffocation, disturbance in the respiratory system and severe form of mountain illness. Lastly, fulminant hypoxia is caused by malfunctioning or damage of aircraft pressure cabins in altitudes above ten kilometers.
In such scenario external oxygen pressure is lower than the internal oxygen pressure and as a result oxygen is drawn out of the body. This is usually followed by unconsciousness and eventually death without any form of warning (Dembrovsky & Racz, 2012, p. 4).
Medical Causes of Hypoxia
Given the above factors and different types of hypoxia it is clear that individual susceptibility to hypoxia increases with high altitude, use of drugs, poor aircraft maintain ace practices and anemic conditions (AAME Training Manual, 2011, p. 4). However, the medics attribute hypoxia to hyperventilation, low inspired oxygen, right to left shunt, ventilation-perfusion inequality, and diffusion impairment (Osborne, nd, p. 1).
Hyperventilation refers to low ventilation in the lungs (alveolar). Hyperventilation is caused by depression of central nervous system by drugs, spinal cord and brain injuries/malfunction, abnormality in the chest wall and upper airway obstruction. Hyperventilation can be corrected by either increasing inspired oxygen or physically ventilating an individual to get rid of carbon dioxide.
Low inspired oxygen results from low oxygen supply in the air or breathing circuits, for instance, breathing at high altitude, improper installation of oxygen equipments and deliberate reduction of oxygen supply by medical practitioners (Osborne, nd, p. 2).
Right to left shunt is as a result of blood bypass. This can either be anatomic shunt or physiologic shunt. Anatomic shunt is when the blood circumvents the lungs via an atomic route. On the other hand, physiologic shunt is when the blood does reach the alveolar because the alveolar spaces are filled with fluids, for instance, when a person drowns, suffers from pneumonia or pulmonary edema (Osborne, nd, p. 3).
Ventilation-perfusion inequality is the ventilation-perfusion variance at the individual alveolar-capillary level for the ample exchange of gases. Diffusion impairment is a very rare medical condition.
It leads to the low exchange of gases between the alveolar and pulmonary veins. Diffusion impairment is usually caused by thickening of the wall separating alveolar and pulmonary veins or condensed pulmonary blood transit duration. Both conditions are common among patients with lung diseases, especially interstitial diseases (Osborne, nd, p. 4).
Stages of Hypoxia among Aircraft Handlers
There are four distinct hypoxia stages. The duration of each stage varies and depends on individuals and external conditions. The medical attendants in any aircraft should be on very watchful on any sign pertaining to hypoxia. The four stages of hypoxia are asymptomatic or indifferent stage, compensatory stage, deterioration or disturbance stage and critical stage.
At the asymptomatic or indifferent stage individuals are normally unaware of the consequences of hypoxia. The main symptoms at this stage are night vision and color vision impairment. These symptoms can be observed at relatively low altitudes and are most common among pilots operating at hours of darkness. Oxygen supply in the arteries is between ninety to ninety five percent.
Compensatory stage usually takes place at altitudes between ten thousand to fifteen thousand feet. Generally, the human body is capable of staving off additional effects of hypoxia by accelerating the respiratory and cardiac rate. The oxygen level in the body at this stage is between eighty and ninety percent (AAME Training Manual, 2011, p. 5).
At the deterioration stage, individuals are not able to compensate for oxygen loss. Unluckily, the signs and symptoms of this stage are not common. As a result, the necessary precautions are always ignored.
Signs and symptoms associated with deterioration or disturbance stage include difficulty in breathing, lack of coordination, cyanosis, lethargy, headache, itchiness, excitement, unfriendliness, lack of sensation, weakened vision, poor judgment and difficulty in performing uncomplicated undertakings.
Oxygen supply is between seventy and eighty percent. Last but not least, critical stage is the last stage leading up to an individual demise. At the critical stage, individuals are totally incapacitated. The main symptoms are unconsciousness, convulsion, cardiac arrest and eventual death. Oxygen level in the body is less than seventy percent (AAME Training Manual, 2011, p. 6).
Precautionary and curative measures for hypoxia
The precaution and cure for hypoxia in the aerial environment can be achieved in two fundamental ways: providing enough oxygen and by flying an aircraft at a relatively low altitude. Unluckily, it is normally not possible to accomplish both.
Aircraft handlers are normally required to soar at a secure altitude. For this reason, aircraft medical escort are supposed to exchange a few words with pilots before and during flights regarding the impact of elevation.
They should advise the pilot on the best altitude to fly the plane during flights taking into consideration weather and terrain. In addition, they should supply sufficient amount of oxygen in readiness for any eventuality. Sometimes it becomes very challenging to determine the sufficient amount of oxygen especially when the patient is already on oxygen even before a flight (AAME Training Manual, 2011, p. 7).
Oxygen needs at different altitudes
The above table is generally used to guide aircraft users to establish oxygen requirements at different altitudes. The table shows the concentration of oxygen required to sustain oxygen saturation at various heights. Each column shows the flow rate of oxygen needed to maintain the saturation level above 90 percent from sea level (denoted as SL) to ten thousand feet.
In general, it is prudent to keep the oxygen saturation level for patients who do not have chronic obstructive pulmonary disease (COPD) above ninety percent. The empty spaces in the table represent altitudes where artificial oxygen saturation is not required. Most patients can succumb to oxygen deficiency in less than a day. Hence, patients suffering from cardiovascular diseases and hypoxia should receive constant supply of oxygen.
Additionally, COPD patients should be kept at their normal oxygen saturation level. Some of the most common oxygen delivery devices used in aircrafts include: nasal cannula, simple face masks, non-rebreather mask, bag valve mask and transport ventilators. Oxygen normally passes through the humidifiers to enhance longevity (AAME Training Manual, 2011, p. 8).
In-flight Hypoxia: Beech King Air’s Five-hour fatal flight
On September 4, 2000, Australian Beech King Air 200 flying from Perth to Leonora crashed near Burke town killing the pilot and all the passengers five hours from the departure time. According to the report released by the Australian Transport Safety Bureau, there were two important factors that could have caused the accident. First, the plane was apparently unpressurized for a considerable duration during the flight.
Second, the pilot and all those on board were incapacitated, possibly due to hypoxic hypoxia since the plane had cruised to high altitude and there was no supplementary oxygen.
The report further stated that the pilot and the passengers were probably incapacitated due to decrease in the oxygen fractional pressure or insufficient supply of air to the lungs. This in turn led to a reduction in the amount of oxygen supply to the body (Flight Safety Foundation, 2002, p. 1).
The Beech King Air 200 was manufactured in 1976 and had accrued 18000 service hours. The records kept by the maintenance department did not show any defect on the aircraft. All the necessary check up and maintenance had been carried out on the plane before taking off.
As per the report, the last inspection was carried out on the back pressure bulkhead and no fault was found. In addition, the aircraft’s pilot had earlier reported that the plane was in good condition. Particularly, the pressure systems were all in good condition (Flight Safety Foundation, 2002, p. 2). The pilot was among the highly trained pilots in Australia and was very professional.
He had an air transport pilot license and had accumulated more than 2100 hours. The passengers on board were mainly miners returning to work. The report further emphasizes that the plane was within the stipulated weight and balance and there was no sign of excessivedangerous goods (Flight Safety Foundation, 2002, p. 4).
According to the report, the aircraft took off from Perth at 1800 hrs Australian time under serene weather conditions which prevailed for the rest of the journey. Ten minutes later the pilot was advised by the air traffic control to ascend to FL 130. Five minutes later the Pilot was again told to ascend to FL 160 before moving 67 kilometers further.
At the air traffic control further advised the pilot to climb to FL 250 and to verify the altitude. The pilot’s voice was considerable inaudible and appeared not able to take the instructions from the air control center. In the next eight minutes no human voice could be heard from the open microphone transmission. Everyone in the plane including the pilot was already unconscious (Flight Safety Foundation, 2002, p. 5).
The air traffic control data revealed that the aircraft ascended to about 35000 feet, which surpassed its service ceiling. The pilot was unaware that the aircraft was depressurizing and probably he did not put on his oxygen mask. The pilot lost control of the aircraft and 65 kilometers from Burke town 2300 hrs local time.
The aircraft investigators did not find any pre-existing defect that could have affected the pressurization and oxygen system (Flight Safety Foundation, 2002, p. 5). The report mentioned that the pilot may have failed to spot the visual warnings of high altitude while engaged in other undertakings.
The report concluded that the life of the pilot and the passengers could have been saved if the aircraft had been equipped with an audio-visual warning (Flight Safety Foundation, 2002, p. 6).
Another incident took place on June 21, 1999, 72 kilometers east of Edinburgh. According to a report by the Australian Transport Safety Bureau (ATSB) regarding this incident, the aircraft had ascended to FL 250 when one of the passengers noticed that the pilot was busy programming the GPS (Global Positioning System) receiver and not attending to air control center radio transmissions.
The pilot later became unconscious. Luckily, among those on board were Royal Australian Air Force Pilot and a navigator who took control of the plane and embarked on an emergency landing.
The Royal Australian Air Force navigator took the pilot’s oxygen mask, took a number of oxygen puffs and subsequently placed the mask on the pilot. The Royal Australian Air Force Pilot landed the aircraft successfully (Flight Safety Foundation, 2002, p. 6).
The report further states that when the pilot had regained consciousness, he noticed that the pressurization and oxygen systems had all been activated. He also realized that the engine bleed air were on the ENVIR OFF. In his report, the pilot tried to absolve himself from blame by claiming that he did not see any pressure warning and that the oxygen masks were not deployed.
However, none of the passengers remembered seeing or annulling the flash master warning. The Australian Transport Safety Bureau (ATSB) report concluded that the following were the major factors during the incident. First, both the bleeding air switches were unintentionally turned off when the aircraft had ascended to about 10000 feet.
Second, the cockpit warning system did not provide sufficient alert warning to the pilot regarding cabin depressurization. Third, the doors to the oxygen mask deployment were not installed correctly; hence it was not possible to deploy the masks automatically. Lastly, altitude chamber training did not provide adequate security to make sure that everyone on board could detect hypoxia (Flight Safety Foundation, 2002, p. 7).
Another episode took place on October 24, 2001, 22 kilometers southeast of Timber Creek during an aero medical flight. The aircraft had ascended to FL 125 when the medical escort alerted the pilot that the oxygen compartment had been activated. The pilot responded quickly by descending to about 10 thousand feet. Once the pilot had descended, he noticed that both the bleeding air was enlightened and the switches were off.
This means there was a problem with the bleeding air. Hence, they were not in a position to help during pressurization. A report on the incident made the following conclusion pertaining to this incident. First, the pilot failed to complete all the pressurization checks. Second, he became so occupied with GPS programming after receiving instructions from the air control center to change the track.
Third, the pilot ascended to over 10 thousand feet in an unpressurized state. Finally, the efficiency and effectiveness of the cockpit warning system were compromised by incomplete pressurization checks during pre take off and after takeoff.
The report also recommended installation of the audio-visual cabin altitude alert system in pressurized planes. As a result, installation of aural or visual signal became the aircraft certification standard both in Australia and the U.S (Ghosh & Pant, 2010, p. 2).
Despite of the developments in the performance and reliability of the aircrafts’ pressurization and oxygen systems, many incidents of in-flight hypoxia are still being experienced all over the world. This is further compounded by the fact that certain symptoms of hypoxia are very hard to detect. There are different types of hypoxia. These are hypoxic hypoxia, anemic hypoxia, circulatory (ischemic) hypoxia and histotoxic hypoxia.
Hypoxia can also be classified as chronic hypoxia, acute hypoxia and fulminant hypoxia. Hypoxia is caused by numerous factors the most common being low oxygen level at high altitude, drugs, poor aircraft maintain ace practices and anemic conditions. Hypoxia among aircraft handlers takes place in four stages: These are asymptomatic stage, compensatory stage, deterioration stage and critical or decisive stage.
The disturbance stage is very hard to comprehend and most cases go undetected. Hypoxia can be prevented and treated by providing enough oxygen and by flying an aircraft at a relatively low altitude. Unluckily, it is normally not possible to accomplish both. Aircraft handlers are always required to soar at a secure level.
In addition, sometimes it is very tricky to determine the amount of oxygen that a patient requires at a given altitude. For that reason, guide flight medical escorts were developed to help in determining oxygen requirements for patients at different altitudes.
The most common oxygen delivery devices used in aircrafts include: nasal cannula, simple face masks, non-rebreather mask, bag valve mask and transport ventilators. Oxygen normally passes through the humidifiers to enhance longevity. Most incidences of hypoxia nowadays are mainly attributed to, ignorance, negligence and system failure. For this reason, public training and awareness is necessary.
Pilots who ignore instructions from the aircraft’s control center should be punished. Pressurized planes should install audio-visual cabin altitude alert system. This should also apply to passenger planes. Last but not least, global aircraft certification should be based on such precautionary/preventive measures.
AAME Training Manual 2011, Hypoxia and Oxygenation, Alaska Air Medical Escort, Alaska.
Aeronautical Information Manual nd, Aeronautical Factors. Web.
Dembrovsky, P & Racz, O 2012, Definition and Classification of Hypoxia. Web.
Flight Safety Foundation 2002, Pilot Incapacitation by Hypoxia Cited in Fatal Five-hour Flight of Beech King Air, Accident Prevention, vol. 59, no. 10, pp. 1-8.
FSF Editorial Staff 2001, Changes Recommended in Oxygen Bottle Regulator/Shut Valves, Aviation Mechanic’s Bulletin, vol. 49, pp. 12-15.
Ghosh, P & Pant, P 2010, In-flight Hypoxia-Still a Worrying Bane, IJASM, vol. 54, no. 1, pp. 6-12.
Osborne, S nd, Causes of Hypoxemia. Web.