Aerospace Pressure Effects

Issues of Concern

Hypoxia can occur without the subject being aware of its effects. 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 become disabling. For some, early hypoxia leads to euphoria and an inability to recognize the hazard. For a pilot or other aircrew, if the symptoms of hypoxia are not recognized, the problem may culminate in a crash.  

Because of this potentially subtle problem, flight systems are designed to recognize hypoxia-hazardous situations and alert pilots to potential dangers so they can take action and correct the situation. Additionally, military air forces require hypoxia training through altitude exposure in low-pressure chambers or via normal pressure or reduced oxygen gas exposures. The Federal Aviation Administration does not require hypoxia training in commercial aviation. Still, it highly encourages it, and many companies educate their aircrew through one of the hypoxia training systems mentioned above.

Sudden pressure loss is not a subtle event when there is a significant pressure difference between the inside and the outside of an aircraft. Pilots must have "quick-don" masks readily available while piloting in these situations. While this situation is dangerous, it does not pose the subtle threat of a slow pressure loss. However, with a sudden and dramatic reduction in oxygen, one may lose consciousness in a few minutes. The time between sudden decompression and loss of the ability to perform a useful activity is called the "time of useful consciousness" or TUC. Beyond this time, one can no longer act to save themselves or others (ie, they are unconscious). Any action to correct the situation must occur before the TUC runs out. Commercial airliners routinely fly in the above 10,000 meters. At these altitudes, the TUC is less than one minute. This is why pilots are required to have "quick don" masks, which greatly extends the TUC.[7][8]

Sudden pressure changes also challenge the air-filled middle ear space, especially the paranasal sinuses. The pressure changes are typically asymptomatic if the sinuses have adequate ventilation through openings or ostia. However, there can be debilitating pain if the ostia are blocked due to mucosal inflammation or other obstructions. If this happens to a pilot, it can lead to dangerous or deadly situations. As noted, gases of a fixed volume contract or expand when increasing or decreasing in altitude. Barotrauma to the sinuses, aerosinusitis, can occur during ascent or descent. On the descent, trapped air in the sinuses creates a relative negative pressure gradient, which causes damage to the sinus mucosa, resulting in edema, worsening obstruction of the sinus ostium, and worsening pain as the descent continues. This is termed sinus squeeze. During the ascent, the expanding gas in the sinuses causes a build-up of pressure that causes worsening pain as the ascension continues. This is termed reverse squeeze.[9][10]

Analogous issues can occur regarding the middle ear space, most commonly related to eustachian tube dysfunction. The eustachian tube's function is to equalize the pressure between the environment and the middle ear space. Dysfunction can result in both acute and chronic issues, including pain, dysequilibrium, and hearing loss. It is usually the increase in negative pressure during decent that is the key issue, as the eustachian tube orifice has to open against a pressure gradient vs in ascent, the air more easily escapes from the middle airspace into the environment through the eustachian tube. Mucosal damage can occur; however, it is referred to as middle ear barotrauma or aero-otitis in either situation. Middle ear barotrauma can cause fluid to accumulate through extrusion from the mucosa and create a middle ear effusion, which can cause hearing loss and occasionally vertigo. These symptoms occurring at the wrong moments in the aircrew can be catastrophic.[11][12][13]

Clinical Significance

For the loss of cabin pressure at high altitudes, the hypoxia treatment is 100% oxygen. In large, multiple aircraft, the aircrew must wear masks and breathe 100% oxygen. Passengers in passenger planes should wear the deployed yellow masks when cabin pressure is lost. Symptoms resolve almost immediately. The aircrew should simultaneously return to below 3048 meters (10,000 feet) as fast and safely as possible. In small military tactical aircraft, aircrews always wear masks; when needed, they switch to 100% oxygen. These small aircraft descend to 10,000 feet or less if the situation allows.

Barosinusitis is a strong consideration for pilots when taking preventive measures. There is a saying amongst aerospace medical professionals that holds value, "Never fly with a cold." Indeed, the best treatment for sinus barotrauma is prevention, especially considering that the most likely cause is mucosal inflammation/edema, which blocks the sinus ostia. Causes include viral, bacterial, or fungal sinusitis, allergic rhinosinusitis, polyps, papillomas, benign or malignant growths, or aberrant anatomy such as concha bullosa. The use of intranasal or oral decongestants can be very useful both in prevention (use before flight) and in the treatment of acute symptoms. Also, to treat acute symptoms, a steroid burst may be considered to aid in decreasing mucosal edema, and intranasal steroids and sinus irrigations are often recommended. Diagnosis and treatment of the underlying cause should be addressed, and antibiotics may or may not be necessary. Surgical intervention with an otolaryngologist trained in sinus surgery should be considered in cases of recurrent barosinusitis.[10][14]

Passengers and aircrew with symptoms of aero-otitis typically resolve once they equalize pressure in the ears and any mucosal edema or middle ear effusion resolves without treatment in a few days or weeks. Performing the valsava maneuver before symptoms start and repeating the maneuver multiple times can often prevent pressure build-up in the middle space and is recommended. For cases of recurrent aero-otitis or other symptoms associated with eustachian tube dysfunction, interventions such as eustachian tube dilation or myringotomy (opening the eardrum) with the insertion of pressure equalization tubes may be considered. Treatment with intranasal steroids or intranasal decongestants has been debated, with recent data showing that there is no difference. Still, many practitioners consider these as first-line medical treatments.[15][16][17][18]

Mountain climbers also risk altitude-related illness. High altitude illness (HAI) is a term used to describe pathologic conditions that occur when unacclimated individuals are exposed rapidly to elevations above 2500 meters (8000 feet). "Rapid" means a change that happens within a few hours. Change that takes days allows one to acclimate to the new altitude and prevent 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. Prophylactic use of acetazolamide has been shown to decrease the risk of AMS. AMS may be treated with rest, hydration, and nonsteroidal antiinflammatories, and it should not move any higher until symptoms have resolved. AMS is considered an early form of HACE. If AMS worsens or one suspects HACE, 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, portable, inflatable hyperbaric chambers are available and utilized by professional organizations that can be used when the descent is impossible. Pharmacologic therapy of HACE and HAPE differs and is beyond the scope of this article.[19][20][21][22]