**1.4 Trapped Gases:**

Definition of trapped gas

Predisposing and contributing factors

Anatomical areas affected

Signs and symptoms

Implications of trapped gases on pilot performance

Prevention and treatment

As we ascend in the atmosphere, barometric pressure falls from 760 mmHg
at sea level, to approximately 550 mmHg at 7,000 feet, and 380 mmHg at
18,000 feet. The resultant fall of barometric pressure to half that of
sea level will proportionally cause any gas to expand to double the
volume at the same temperature (Dalton's Law). This creates the
potential for expansion of trapped gas within the human body, which
maintains a constant temperature of 37 degrees Centigrade.

Virtually all areas where gas can be trapped in the human body are
normally able to ventilate, but under abnormal physiologic conditions
the ability to ventilate can become lost and create a problem in flight.
Trauma and medical intervention may introduce gas into the human body.
Trapped gas is defined as any gas within the human body that is unable
to communicate with the external world. Thus gas within the lungs is not
trapped. Gas within the stomach and gastrointestinal tract, however, has
the potential to become trapped through function of peristalsis and
external compression. The most common anatomical areas affected are the
middle ear and sinuses.

The sinuses are able under normal conditions to passively ventilate
through their ostia. Ventilation of the middle ear, connected to the
pharynx by the eustachian tube, is controlled by a flutter valve. This
valve allows expansion of air within the middle ear to spontaneously
ventilate and be expelled into the pharynx. Thus on ascent, as
barometric pressure drops, gas trapped within the middle ear should
ventilate spontaneously. However, on decent, as barometric pressure
drops, and the relative pressure within the middle ear, as opposed to
the external ear across the tympanic membrane, will be much lower, and
the tympanic membrane may bow inward. Relief of this bowing may require
active ventilation of the middle ear through contraction of the levator
palatini and tensor palatini muscles. This can be performed by yawning
or with the Valsalva maneuver, where in the nasopharyngeal pressure is
increased against a closed eustachian orifice while compressing the
nares. A modified Valsalva is performed by swallowing while holding the
nose closed. Additionally the Frenzel maneuver may be performed,
thrusting the jaw forward and opening the eustachian tube. Aeration of
the middle ear also permits passive ventilation of the communicating
mastoid air cells.

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**Middle ear anatomy**

In the event of a respiratory infection the eustachian tube may become
compromised due to edema, and gas in the middle ear becomes trapped. If
air is unable to escape, or more commonly, unable to re-ventilate the
middle ear on descent, significant pain will result as the tympanic
membrane is contracted inward. Eventually, if unrelieved, the tympanic
membrane will rupture and equalize the pressure. This rupture, while
seemingly catastrophic, will typically heal without treatment in 2 to 3
weeks. A similar condition can occur when a respiratory infection
compromises the ostia of the paranasal sinuses; the sinus will tend to
spontaneously ventilate during ascent as air escapes through the
narrowed opening. However, on descent there is insufficient ability to
ventilate the sinuses and the result is exquisite pain and occasionally
hemorrhage into the sinus. Other types of barotraumas occur, including
rupture of the round or oval windows of the inner ear, causing perilymph
fistula, infections, and in some cases, an onset of ear pain many hours
later, known as a delayed ear block.

Delayed ear block occurs after breathing 100% oxygen in flight. By the
time the aviator lands, the middle ear air containing 21% oxygen, has
been fully replaced with 100% oxygen. The resulting metabolism of the
oxygen by middle ear mucosal cells can produce a relative vacuum some
hours later. While painful, it can be relieved by ventilating the ear.
In fact, it can be prevented by ventilating the ear several times after
landing, thereby returning the gas mixture to normal.

Bacterial infections within the teeth can result in gas formation with
potential trapping. This is typically from pulpitis or a significant
abscess, although occasionally gas may be trapped under a crown or occur
acutely following dental surgery. Trapped dental gas can cause
significant pain on ascent, which is usually relieved upon descent. Gas
may become trapped in the lungs, where villi may fail to ventilate and
spontaneously rupture. This may result in a catastrophic pneumothorax.

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**Pnemothorax**

Trapped gas within the gastrointestinal tract is usually relieved by
belching or passing flatus. Failure to do so may result in significant
stretching of the intestinal contents, causing pain. Only in rare cases,
such as a weakened diverticulum of the colon, does bowel rupture occur
due to gas trapping. Surgical or medical procedures may introduce gas
into the vitreous humor of the eye or into the cerebrospinal fluid of
the brain. Such gas, when expanding on flight, may cause a fatal
vasovagal syncope. Any individual with retained gas within the eye or
central nervous system, should be strictly forbidden from flying until
it is completely reabsorbed. Post-surgical retained gas within the
abdominal cavity is a relatively minor problem and the individual can be
permitted to fly at the discretion of the treating surgeon.

The major implications of trapped gas within the body from a pilot's
point of view are interference with the pilot's ability to function
properly and concentrate on operating the

aircraft. The most common barotraumas involving pilots are an ear or
sinus block with severe pain while flying. The pain may be sufficient
enough to completely distract the pilot from operating the aircraft,
resulting in loss of control. Educating pilots not to fly with a cold is
the most effective preventative measure. Occasionally it is necessary
for a pilot to carry topical decongestant sprays, such as
Neo-Synephrine, capable of opening edematous passageways and permitting
equilibration of pressure. It must be emphasized that decongestants
should only be used in an emergency. Chronic use of such medications
results in a rebound phenomena of rhinitis medicamosa, where repeated
use of decongestant sprays are required to maintain a semblance of
normal activity in an otherwise unaffected nasal airway. In an
emergency, a myringotomy may be required to relieve pressure in the
middle ear. These can be expected to heal without complications in two
to three weeks.

Reference

Yarington CT, Hanna HH, Otolaryngology in Aerospace Medicine, in
Fundamentals of Aerospace Medicine Chapter 15, 567-92, and Fundamentals
of Aerospace Medicine, 3rd Edition, DeHart and Davis, 2002.