Topics of Interest


Trapped Gas
By Roger A. Storey
As an instructor with CAMI's physiological training program, we cover a variety of subjects that deal with the problems of pressure change at altitude, and its effects on the human body. One of these areas is the effects of pressure change on the middle ear, parasinuses, gastrointestinal tract, and the teeth. These areas can withstand enormous changes in barometric pressure as long as the air pressures within these body cavities are equalized with the pressure surrounding them.

The mechanical responses to changes in pressure are in accordance with Boyle's Law, which states that a volume of gas is inversely proportional to the pressure to which it is subjected, temperature remaining constant. When the gases in these cavities can't equalize with the ambient environment, the gas is considered to be "trapped."

Of the areas of trapped gas, the most commonly dealt with is that of the middle ear. There is seldom any difficulty upon ascent; most often difficulty is experienced on descent in the form of an ear block (barotitis media). An ear block is usually preceded by a fullness in the ear, gradual loss of hearing and eventually pain. From my experiences in the chamber, I have drawn the conclusion that most ear blocks are a result of not knowing how to properly equalize the pressure in the middle ear, or trying to fly with a cold.

Normally, there is little difficulty equalizing pressure during descent, by occasionally swallowing, yawning, or tensing the muscles of the throat; this will allow the pressure to equalize. During sleep, the rate of swallowing slows down. For this reason, it is advisable to awaken sleeping passengers prior to descent for the purpose of permitting them to ventilate their ears. Infants should be given a bottle or pacifier to aid in equalization. Small children can avoid difficulty by chewing gum.

If these actions fail to equalize the pressure, a Valsalva maneuver should be performed. The Valsalva maneuver is performed by closing the mouth, pinching the nostrils closed and blowing air through the nose. This will force air up the eustachian tube and into the middle ear. This is not a dangerous procedure and should not be delayed until the pressure in the ears becomes painful, otherwise it may be extremely difficult to open the eustachian tube. Painful ear blocks generally occur when the descent rate is too rapid. To relieve this pain, a level off and ascent to a higher altitude is recommended. This should be followed by a slower descent, if possible. During the second descent, close attention must be given to the prompt use of equalization techniques. Prudent use of antihistamines and/or decongestants may also prove to be very helpful but should be used sparingly due to their compounding effect with hypoxia.*

Along with the lack improper equalization maneuvers, flying with a cold can be just as much a problem, if not more so. Equalization of the middle ear can be impaired when the eustachian tube, or its opening, becomes restricted as the result of inflammation, upper respiratory infection, sore throat, infection of the middle ear, or sinusitis. It may be possible to equalize the middle ear by a forceful Valsalva, but this may result in the infected material being carried into the eustachian tube, along with the air causing infection of the middle ear. Since the resulting infection may result in a longer grounding than the cold, it may be advisable not to fly if you suspect you have a cold.

After a flight in which you use 100 percent oxygen, the Valsalva procedure should be accomplished several times to ventilate the middle ear. This is recommended because the middle ear will be filled with pure oxygen, which is then gradually absorbed by the tissue of the middle ear. This, in turn, will cause a reduction of pressure, which may become painful later in the day, or night, if left unequalized.

For those who have experienced a sinus block (barosinusitis), to some degree it is not a pleasant experience. My personal experience occurred within an altitude chamber on a rapid descent (12,000 feet per minute) from 43,000 feet (unpressurized). At approximately 22,000 feet, it felt as though someone or something had softly touched me above my right eyebrow, and within seconds, it felt as though a sharp object was being forced into the same area. We immediately leveled off and then ascended 2,000 feet at approximately 12,000 feet per minute. The pain immediately ceased and Afrin was administered. We slowed the descent rate down and I had no further complications.

The sinuses most often affected by pressure change are the frontal and the maxillary sinuses. These air-filled, rigid, bony cavities lined with mucous membrane are connected with the nasal cavity by means of one or more small openings. When these openings into the sinuses are normal; air passes through these cavities without difficulty, and can accommodate any moderate rate of ascent or descent. If the openings of the sinuses are obstructed by the swelling of the mucous membrane lining, ready equalization of pressure becomes difficult and the possibility of a sinus block will increase. This is another example of what could happen as a result of flying, or diving with a cold. Keep in mind that most sinus blocks occur on descent and will give little or no warning.

When the maxillary sinuses are affected, the pain will probably be felt on either side of the nose, under the cheek bones. Maxillary sinusitis may produce pain referred to the teeth of the upper jaw and may be mistaken for a toothache.

When the frontal sinuses are affected, the pain will be located above the eyes and usually is quite severe. This type of sinus problem is the most common.

Equalization of pressure to relieve pain in the sinuses is best accomplished by use of the Valsalva procedure, and/or inhalants, previously mentioned in conjunction with ear blocks. Again, you should be very cautious in your use of any over the counter medication.* Reversing the direction of pressure change as rapidly as possible may be necessary to clear severe sinus blocks.

As mentioned before, the middle ear is the most common generator of discomfort upon descent. The gastrointestinal tract, however, is the area most commonly associated with pain or discomfort upon ascent. This discomfort is caused by the expansion of gas within the digestive tract during ascent. Fortunately, the symptoms are not serious in most individuals, although in flights above 25,000 feet, enough distention may occur to produce severe pain.

The gastrointestinal tract normally contains variable amounts of gas with pressure approximately equivalent to that of the ambient atmosphere. The chief sources of this gas are swallowed atmospheric air and, to a lesser extent, gas formed as a result of the digestive processes. As gases in the stomach and intestines expand during ascent, extreme discomfort can occur unless there is relief, ordinarily obtained by belching or by passing flatus.

Gas pains of even moderate severity may result in lowered blood pressure. Shock, or syncope, will be the eventual result if relief from distention is not obtained. Immediate descent from altitude should be made to obtain relief.

It can be beneficial to you as a pilot, crew member, or passenger to be aware of certain things before you fly. For instance, if you have a cold, maybe you should pass on that day's flight.

Another area to plan ahead for is to watch what you eat before you fly. Staying away from foods you know cause you problems could help you avoid, or lessen, the discomfort or pain in the gastrointestinal tract. Some of the foods that more commonly disagree with individuals are: onions, cabbage, raw apples, radishes, dried beans, cucumbers, melons-or any food that you know causes you problems.

It is probably wise to avoid carbonated beverages in large quantities, as well as anything else bubbly immediately before flight. One thing that we stress in our course, is that, every individual is different and has different tolerances. Because of this, it is important to know what affects you, not what affects someone else.

Of all the areas of possible trapped gas problems, tooth pain (barondontalgia) is the least common. A toothache may occur at altitude during flight. The pain may, or may not, become more severe as altitude is increased, but descent almost invariably brings relief. The toothache often disappears at the same altitude at which it was first observed on ascent.

Common sources of this difficulty are abscesses, mechanically imperfect fillings (very rare in occurrence), inadequately filled root canals, and pulpits. Anyone who experiences a toothache at altitude should see a dentist without delay for examination and treatment. As mentioned before, maxillary sinus discomfort may be misinterpreted as a toothache.

The Physiological Training course provides comprehensive instruction on trapped gas problems during flight. The course can be attended at a military installation in your area or at the Civil Aerospace Medical Institute in Oklahoma City. For more information, contact the Airman Education Programs Branch at 405-954-4837.

* Medical Facts for Pilots Publication AM-400-92/1, Prepared by: FAA Civil Aerospace Medical Institute, Aeromedical Education Division, AAM-400, P.O. Box 25082, Oklahoma City, Oklahoma 73125

Mr. Storey is a physiological training instructor at the Civil Aerospace Medical Institute's Airman Education Programs Branch.