**1.5 Altitude-Induced Decompression Sickness:**

Definition

Predisposing and contributing factors

Anatomical areas affected

Symptoms and signs

Performance effects

Prevention and treatment

100% oxygen pre-breathing

Flying after diving (sea level and high elevation scuba diving)

Ebullism

Decompression sickness is the evolution of gas bubbles within the human
body caused by exposure to decreased barometric pressure. The primary
gas within the human body under normal atmospheric conditions is the
inert gas nitrogen. During reduction in barometric pressure, some
quantity of inert gas in the tissues diffuses into the blood, travels to
the lungs and eventually leaves the body in expired air. Henry's Law
describes the actual amount of gas dissolved in a fluid as a function of
the partial pressure and saturation constants. Some liquids rapidly
equilibrate the dissolved gas with the gas above it. Other liquids, such
as fat, equilibrate slowly, but eventually an equilibrium is achieved.
If the gas is consumed in the fluid, such as oxygen being metabolized, a
continuous diffusion process occurs. Inert gases achieve equilibrium.
However, it is possible for this inert gas within the body tissues to
become supersaturated.

From a practical point, Henry's Law is difficult to apply. A rule of
thumb, described by Haldane, is that any time the barometric pressure is
reduced to half of its previous pressure, supersaturation and bubble
nuclei formation becomes a risk. In addition, besides the evolution of
bubbles, acute rupture of gas trapped within the lungs may cause air to
enter the pulmonary veins, causing an air embolism. The basic
predisposing factors involving decompression sickness must first include
reduction of gas pressure to half of the previous exposure. For a pilot
taking off from sea level, where barometric pressure is 760 mmHg, they
must ascend to at least 18,000 feet, where the barometric pressure falls
to 380 mmHg. At any altitude above this, gas bubbles can form within the
body in any tissue. Below this altitude, decompression sickness should
never occur. However, if the individual has started from below sea
level, then gas bubbles may form at a much lower altitude, such as can
occur in individuals who have been SCUBA diving, breathing compressed
air at 2 to 6 times that of atmospheric pressure. Thus, when a SCUBA
diver returns to sea level, they may have already reduced the barometric
pressure by half and the potential of circulating bubbles already
exists. In fact, most SCUBA dive tables are designed to have the diver
safely return to the surface, but not go to any higher altitude. It is a
strict rule that after a single dive using compressed air, without any
decompression time required, that at least 12 hours elapse before the
individual attempts to fly. If any dive is deeper or longer or multiple,
then at least 24 hours must elapse before the individual flies.
Otherwise the moment they take off they will already have bubbles in the
circulatory system, which can only expand, coalesce and embolize.

![](media/image1.wmf){width="4.977083333333334in" height="3.74375in"}

Decompression risk in a diver ascending to above surface level.

Physiologically, gas bubbles, primarily nitrogen bubbles, evolve in the
lower pressure areas of the body. This may include the skin, producing
an itching, pruritic sensation, as well as a morbilliform rash.
Classically, in SCUBA and hard-hat divers and caisson workers, synovial
fluids in the joints are affected, creating severe pain, known as the
bends. The bends are less common, however, in aviators. More commonly,
in aviators, bubbles evolve within the lungs and the lung vasculature,
causing a dry, hacking cough, known as the chokes. Bubbles may evolve
anywhere in the venous system and embolize, or evolve directly in the
central nervous system, causing a neurologic type syndrome of protean
appearance, including evolving in the inner ear and causing acute
vertigo, a condition known as the staggers. Central nervous system
involvement in aviators most commonly to involve the cerebrum, whereas
spinal decompression sickness is more commonly seen in divers.

Bubbles may also occur in the heart, causing a myocardial infarction or
may be whipped to a froth within the ventricles and create a syndrome of
pump failure. Some individuals with a latent but persistent patent
foramen ovale are subject to paradoxical air embolism. Other factors
predispose individuals to evolved gas, including exercise, prolonged
duration of the exposure, and repeat exposures to depressurization.
There is a significantly higher risk in those over 40 years of age,
although some researchers feel this is attributable to body fat
composition, where obesity is the predisposing factor.

Prevention of decompression sickness is primarily through avoidance of
high altitude exposure, by pressurizing the container. Pressurized
cabins are routine in commercial airline traffic, and Part 121
commercial aircraft are not permitted to operate over 8,000 feet
altitude equivalent, or 525 mmHg, although the flight may cruise at
altitudes of up to 60,000 feet. However, decompression does occasionally
occur due to leaks and mechanical failures. Other aircraft may operate
unpressurized and some pilots will fly at significantly higher altitudes
using supplemental oxygen, sufficient for oxygenation but creating a
risk of decompression sickness. Specific training is included in most
aviation altitude chamber training programs.

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Altitude chamber students performing denitrogentation pre-breathe before
ascending.

When exposure to higher altitudes is intended, removing the nitrogen
from the body tissues can be performed by a denitrogenation pre-breathe.
Under these circumstances, the aviator will breathe 100% oxygen for at
least half an hour prior to entering the low-pressure environment. An
example of this is astronauts during extra-vehicular activity. The cabin
of the space shuttle is sea level (760 mmHg), while the space suits are
at 230 mmHg. Eliminating nitrogen from the tissues by breathing 100%
oxygen prior to the hypobaric exposure, significantly reduces the risk
of decompression sickness. In addition, a brief period of exercise while
performing the oxygen pre-breathe also opens up additional vascular beds
and speeds the elimination of nitrogen from body tissues. Substitution
of other inert gasses, such as argon or helium, has not proved to be
successful for spaceflight, and some of these are even slower to
eliminate from the body tissues than nitrogen. However in SCUBA diving,
helium is often added to air to form "trimix" used in dives deeper that
160 feet of sea water.

Treatment of decompression sickness in aviation is by rapid return to
sea level and initiating 100% oxygen treatment. This is effective in
minor forms of decompression such as mild bends and skin manifestations.
However, anything more than complete resolution of symptoms upon return
to sea level should be treated by hyperbaric compression, taking the
individual to as much as 3 atmospheres of barometric pressure,
functionally squeezing the evolved bubbles smaller and smaller, until
they are hopefully of insignificant size. During hyperbaric treatment
the individual also breathes 100% oxygen, which speeds the elimination
of nitrogen. Specific dive tables are available at treatment centers
which outline time and pressure of treatment protocols. Application of
these tables can be complicated by activities such as high-altitude
SCUBA diving in mountain lakes. Under such an environment, with a lake
surface at 8,000 feet and a barometric pressure of 500 mmHg, will result
in SCUBA tables being significantly affected. These tables are in
general designed for ocean salt water, and planned for a doubling of
barometric pressure every 32 feet. However, in a high mountain lake,
pressure may double every 22-23 feet, resulting in a significant
alteration of decompression tables. Again, education, training and newer
technology takes these changes into account.

![](media/image3.jpeg){width="5.840277777777778in"
height="3.8930555555555557in"}

Navy Table 6 Protocol for Decompression Sickness Treatment

Air embolism, where gas enters the pulmonary vein through a rupture
within the lungs, results in gas bubbles flowing through the vascular
system. This classically occurs when the individual attempts to hold
their breath during sudden decompression, either during loss of cabin
pressure or, more commonly, when a diver holds his/her breath while
ascending to the surface. The result is a rise in interalveolar pressure
of 50-100 mm above ambient pressure, forcing gas into the pulmonary
vein, and thence into the general circulation. More than 100 mm of air
entering the system can cause death. Prevention through education and
training is a vital key.

References:

Heimbach RD and Sheffield PJ, Decompression Sickness and Pulmonary
Overpressure Accdients, In: RL DeHart, editor. *Fundamentals of
Aerospace Medicine*, 2^nd^ edition, Williams and Wilkins, Baltimore,
1996.

Davis JC, Kizer KW. Diving Medicine. In: Auerbach PS, Geehr EC, editors.
*Management of Wilderness and Environmental Emergencies*, 2nd edition.
The C.V. Mosby Co., St. Louis, 1989.