Trapped Gas - Air Expansion at Altitude

July 24, 2013

Antonio Maria Valsalva
Antonio aria Valsalva

How many people have flown on a long commercial flight and landed at their destination only to find that a toiletry container had opened and spilled its contents all over their luggage? What causes this mysterious spilling of shampoos and lotions you may ask? And what does Dr Antonio Valsalva have to do with it?Gas laws were introduced in a previous post on decompression sickness. In that post both Henry's and Boyle's Laws are discussed. In review, Robert Boyle, 17th century chemist and physicist, discovered that pressure and volume are inversely proportional at a constant temperature. This means that as pressure decreases (as it does with increasing altitude), volume increases at a similar rate. When closed containers undergo these pressure changes in flight, increasing pressure of gases inside the container can occasionally increase to that critical value causing a cap or unsecured top to pop, explaining how your toiletries were inexplicably able to run rampant ruining your new suit.[For a full discussion on the Gas Laws that apply to aviation, see this post.][caption id="" align="alignnone" width="298"]

Trapped Gas
Boyles Law

Although shampoo on one's garments is undoubtedly an annoyance to the unlucky traveler, the effects of Boyle's Law is similarly implicated in more significant medical conditions affecting aircrew and passengers. There are several locations in the human body where gas can accumulate and possibly become trapped. These medical conditions, known collectively as 'Trapped Gases' often lead to discomfort, pain, and possibly destruction of surrounding tissues. Much like the condition of decompression illness, trapped gases commonly affect aircrew and scuba divers, due to the frequent exposure to significant pressure changes. The most common locations where this medical condition will develop are the middle ear, the sinuses, the lungs, the gastrointestinal tract, and the teeth. Let's look at each manifestation individually.

Ear Block

Known to medical professionals as the condition called Barotitis Media, ear block occurs in the presence of a pressure gradient (either increased or decreased) in the middle ear and an ineffective eustachian tube. When these two conditions arise, gas will attempt to either expand or retract in the middle ear (in accordance with Boyle's Law). Due to the fixed space of a middle ear or sinus (see below), pressure in the ear will either be exerted out against the mucosal walls on ascent or pulling against the walls as a relative vacuum forms on descent. Normally during ascent, as pressure decreases and the volume of gas in the ear expands, air is transmitted down through the eustachian tube to equilibrate the pressures. On descent, however, when ambient pressure increases, a relative vacuum forms in the middle ear and the eustachian tube will often not open without conscious assistance. For this reason, ear blocks are more common on descent in aviators. This phenomenon is worsened in the presence of inflammation or congestion, which often accompany allergies or an upper respiratory infection, aka the common cold.

Anatomy of Barotitis Media

When the eustachian tube does not function properly due to inflammation and swelling from an acute upper respiratory infection or allergies, pressures do not equilibrate and as a result this relative vacuum will pull fluid from the surrounding tissue into the middle ear, resulting in either a serous or bloody effusion. This can be seen on otoscopic (inner ear) exam by a physician. In extreme cases, the eardrum (aka tympanic membrane) can rupture and a small tear or hole will be visualized on exam. An example is seen below.

Inflamed eardrum with obvious perforation
Ruptured Eardrum

The two scenarios described above can occur as either a pilot or scuba diver descends, in both cases going from a lower pressure environment to a higher pressure one. In order to avoid barotitis media, one needs to force the eustachian tube to open, allowing the lower pressure vacuum developing in the middle ear to equilibrate with the higher pressure of the external environment. This can be accomplished by yawning, drinking, learned (conscious) control of the muscles responsible for opening the tube, and several other maneuvers. Most of the inventors of these maneuvers have solidified their place in medical history as demonstrated by the various names- the Valsalva maneuver (see above photo of Antonio Valsalva), the Toynbee maneuver, the Frenzel maneuver, or the Politzer device. These various maneuvers and device will be discussed individually in a separate post.

Sinus Block

The underlying pathophysiology of barosinusitis is very similar to barotitis media (ear block) as described above. As the name indicates, the main difference between the two conditions is the location of the ailment. Human beings have 4 pairs of air-filled paranasal sinuses. These can be seen in the accompanying diagram. Each sinus is lined with mucosa and has a small opening to the external environment to ensure air and pressure exchange.

Paranasal Sinus Anatomy

The medical condition most commonly occurs when moving from a relatively hypobaric environment into one of greater pressure. Again, a negative pressure gradient in a closed space (this time the sinuses) causing mucosal congestion and possibly a hematoma (collection of blood) as these fluids are pulled from deeper tissues to the surface, sometimes culminating in actual tissue damage. This results in considerable pain experienced by the aviator or diver, which can be incapacitating, thus interfering with ability to safely operate an aircraft or make rational decisions. In most cases, sinus block is experienced due to an underlying cold/upper respiratory infection or allergic rhinitis. In cases in which sinus block develops in the absence of these conditions or becomes recurrent, further workup is mandatory to determine a possible underlying medical condition or anatomic abnormality. In some cases the victim of recurrent sinus blocks has an abnormality of the sinuses or openings to the sinuses described above. In these cases, surgery by an otolaryngologist (ear, nose & throat surgeon) may be corrective.

Tooth Squeeze

Known also as Dental Barotrauma or Barodontalgia, Boyle's Law is again implicated as trapped gas expands in a closed space provoking pain and/or tissue destruction. Gas can become trapped in teeth during a number of dental procedures such as when placing a filling. It is very difficult for a dentist to ensure there is no void between tooth mineral and filling. In this void will reside ambient air. There are also certain types of bacteria that produce gas during infectious processes such as tooth abscesses.

Dental Barotrauma

In both cases, these small pockets of air may be inconsequential in a typical person, but in a pilot or SCUBA diver the result can be incredible pain or even fracture of the tooth with pressure changes. Dental barotrauma can occur during either gas expansion or contraction (ascent or descent). SCUBA divers experience a greater prevalence of tooth squeeze due to the fact that the absolute pressure changes experienced by divers is much greater than aviators. That said, military aviators at higher risk for a rapid decompression remain at risk for dental barotrauma. For this reason, a variety of dental procedures require that a pilot not fly for a specific duration of time. A pilot experiencing barodontalgia should return to their dentist in order to have a second procedure with the end goal of removing the void space, or treating the underlying condition.

Gas Expansion in the GI Tract

The Gastrointestinal (GI) tract is a continuous space from the mouth & esophagus through the stomach, small intestine, large intestine and finally out of the rectum and anus. Obviously, this space has two openings to allow gas to pass (either as a belch or flatus depending on the direction of interest), but decreasing pressure with ascent will still provoke symptoms as the volume of gastrointestinal gas expands.

The Digestive Tract

As gas expands throughout the GI tract, pain receptors sensitive to stretch often will provoke abdominal discomfort. In certain cases this expansion of gas can be so profound that the diaphragm is prevented from fully descending during inhalation, disrupting respiration. Abdominal pain can be avoided by allowing early belching and passing of flatus in addition to staying away from foods that produce gas before flight or SCUBA. Some of the most notorious offenders are beans, broccoli, cauliflower, brussels sprouts, cabbage, and carbonated drinks. If you can't say no to the baked beans, let's just hope you're in a single-seater aircraft!

Lungs & Medical Equpiment

In addition to the conditions listed above, there are several other medical conditions and implications that the flight medicine professional need be aware. In the lungs, air also expands with ascent. Usually this air is passively expired, but in the case where expansion is too rapid, lung tissue can tear allowing a direct communication between gas and either tissue or vasculature. This can ultimately result in pneumothorax, pneumomediastinum or arterial gas embolism (AGE). Lastly, the consequences of Boyle's Law will not spare medical equipment that has areas of trapped gas by design. Some common devices and instruments affected are blood pressure cuffs, chest tube pumps, and tracheal cuffs. It is critical that flight medicine physicians administering care in mobile flying medical units consider the effects of expanding or trapped gases during flight.


1. Davis, Jeffrey R. Fundamentals of Aerospace Medicine. 4th ed. Lippincott Williams & Wilkins. 2008.

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