Most clinical education related to this area of medicine concerns hyperbarics or increased pressure. Human exposures to increased pressure occur while diving and during hyperbaric oxygen therapy. Hypobaric (reduced pressure) exposures occur on commercial plane flights where the cabin pressure equals about 2438.4 meters (8000 feet); however, professional aviators, particularly military personnel, mountain climbers, research subjects, and astronauts are exposed to much greater extremes in low-pressure environments. The 2 main concerns about hypobaric exposure relate to absolute pressure upon the human body and the total oxygen available to the body.
A solid understanding of the gas laws is foundational to understanding how pressure changes affect humans. The gas laws describe the relationship between temperature, volume, and pressure for a given amount of gas. For example, Charle's law states that for a given pressure, the volume is proportional to the temperature, for example, a gas expands when heated. However, what happens if gas cannot expand, for example, when trapped inside the middle ear or a nasal sinus? Boyle's law states that for a given temperature (note that humans have a relatively stable temperature while alive) the volume is inversely proportional to the pressure. Barometric pressure at sea level is around 14.7 pounds per square inch (psi) with variation depending on the weather. Finally, Dalton's law notes the total pressure of a mixture of gases equals the partial pressure exerted by each gas. This concept is important given that the air humans breath is a mixed gas of nitrogen (approximately 78%), oxygen (approximately 21%), and trace other gases.
If the human body is exposed to a low enough absolute pressure then surface fluids (tear film, saliva, and the air-exposed surface of alveoli) will begin to boil thanks to the gas laws mentioned above. This occurs at around an altitude of 60,000 feet (approximately 11.4 miles) depending on exact atmospheric conditions. This altitude has been named "Armstrong's limit" or "Armstrong's line" and is named after an early aerospace medicine physician, Harry G. Armstrong. When blood boils, this is called "ebullism" and is the trapping of gases from the blood under the skin. Ebullism is painful but recoverable to full function as human experience has shown.
Reduced oxygen levels in the body can occur for various reasons. Hypoxia is the general term for low oxygen content in the blood or at the tissue level. Hypoxic hypoxia is hypoxia secondary to low oxygen levels in the blood or at tissue levels and has several causes. In this article, "hypoxia" means hypoxic hypoxia due to low oxygen availability in the environment.
Humans require oxygen supplementation as they ascend in altitude. While there is some variation from person to person, the effects of hypoxia are accepted to begin at 3048 meters (10,000 feet). These effects include reduced light reception and decreased the ability to distinguish colors. As a person ascends in altitude, their body compensates with increased depth of respiration, increased rate of respiration, and increased heart rate in an attempt to maintain oxygen delivery to the tissues. Further ascent leads to extreme fatigue and reduced mental capacity. Exposure to atmospheric conditions in approximately 7620 to 10,363 meters (25,000 to about 34,000 feet) results in death if supplemental oxygen is not used. By 34,000 ft 100% oxygen in a tight-fitting mask will deliver near ground level oxygen to the tissues. Pressure suits or pressurized cockpits must be used beyond this level to maintain near sea level oxygenation to the tissues.
Hypoxia can occur without notice. Slow loss of cabin pressure or loss of the oxygen source during flight may lead to a gradual onset of the effects of hypoxia. This occurs occasionally and is a perilous situation for pilots as the early symptoms of hypoxia are mild but, if not corrected, they will become disabling. For some, early hypoxia leads to euphoria and an inability to recognize the hazard. This results in pilot deaths even today, mostly in the military but also in civilian aviation as well.
Flight systems are designed to recognize hypoxia hazardous situations and alert pilots to the potential danger to counter this subtle threat. Additionally, military air forces require hypoxia training either through altitude exposure in low-pressure chambers or via normal pressure/reduced oxygen gas exposures. The FAA does not require hypoxia training in commercial aviation but it is highly encouraged, and many companies educate their aircrew through one of the hypoxia training systems mentioned above.
Sudden loss of pressure is not a subtle event when there is a significant pressure difference between the inside and the outside of an aircraft. Pilots are required to have "quick-don" masks readily available while piloting for these situations. While this is a dangerous situation overall, it does not pose the subtle threat of a slow loss of pressure.
The treatment for hypoxia is oxygen. For the loss of cabin pressure at high altitude, return to below 3048 meters (10,000 feet) as fast as safely possible. In large, multiplace aircraft, aircrew are to wear masks and breath 100% oxygen. Symptoms will resolve almost immediately. Passengers in passenger planes should put on the deployed yellow masks when there is a loss of cabin pressure. In small military tactical aircraft, aircrews wear masks all the time, and when the need arises, they switch to 100% oxygen. These small aircraft also will descend to 10,000 feet or less if the situation allows.
Mountain climbers also risk altitude related illness. High altitude sickness is a term used to describe pathologic conditions that occur that occurs when unacclimated individuals are exposed rapidly to elevations above 1500 meters (5000 feet). "Rapid" means a change that happens within a few hours. Change that takes days will allow one to acclimate to the new altitude and prevents high-altitude illnesses. Illnesses associated with altitude exposure include acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE). Definitive treatment of these conditions is moving to a lower altitude, but this may not be necessary in mild AMS or HAPE cases. AMS may be treated with rest, hydration, NSAIDs, and not moving any higher until symptoms have resolved. AMS is considered an early form of HACE. If AMS worsens or if one suspects HACE then an immediate descent of 762 to 914.4 meters (2500 to 3000 feet) or more is extremely important to avoid severe consequences. HACE and HAPE have caused death and should be treated as soon as the condition is suspected. Additionally, there are portable, single-place altitude chambers available and utilized by professional organizations. Pharmacologic therapy of HACE and HAPE differs and is beyond the scope of this article.