Pulmonary overinflation syndromes are a group of barotrauma-related diseases
caused by the expansion of gas trapped in the lung during ascent (reverse squeeze)
or overpressurization of the lung with subsequent overexpansion and rupture of
the alveolar air sacs. Excess pressure inside the lung can also occur when a diver
presses the purge button on a single-hose regulator while taking a breath. The two
main causes of alveolar rupture are:
- Excessive pressure inside the lung caused by positive pressure
- Failure of expanding gas to escape from the lung during ascent
Pulmonary overinflation from expanding gas failing to escape from the lung
during ascent can occur when a diver voluntarily or involuntarily holds his breath
during ascent. Localized pulmonary obstructions that can cause air trapping, such
as asthma or thick secretions from pneumonia or a severe cold, are other causes.
The conditions that bring about these incidents are different from those that
produce lung squeeze and they most frequently occur during free and buoyant
ascent training or emergency ascent from dives made with lightweight diving
equipment or scuba.
The clinical manifestations of pulmonary overinflation depend on the location
where the free air collects. In all cases, the first step is rupture of the alveolus with
a collection of air in the lung tissues, a condition known as interstitial emphysema.
Interstitial emphysema causes no symptoms unless further distribution of the air
occurs. Gas may find its way into the chest cavity or arterial circulation. These
conditions are depicted in Figure 3-10.
figure3-10 Pulmonary Overinflation Consequences. Leaking of gas into the pulmonary
interstital tissue causes no symptoms unless further leaking occurs. If gas enters the arterial
circulation, potentially fatal arterial gas embolism may occur. Pneumothorax occurs if gas
accumulates between the lung and chest wall and if accumulation continues without
venting, then tension pneumothorax may result.
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Arterial gas embolism is the most serious potential
complication of diving and is caused by an excess pressure inside the lungs that
fails to vent during ascent (Figure 3-11). For example, if a diver ascends to the
surface from a depth of 100 fsw, the air within his lungs expands to four times its
original volume. If this expanding air is not allowed to escape, pressure builds up
within the lungs, overexpanding them and rupturing their air sacs and blood
vessels. Air is then forced into the pulmonary capillary bed and bubbles are carried
to the left chambers of the heart, where they are then pumped out into the arteries.
Any bubble that is too large to go through an artery lodges and forms a plug
(embolus). The tissues beyond the plug are then deprived of their blood supply and
their oxygen. The consequences depend upon the area or organ where the
blockage occurs. When the brain is involved, the symptoms are usually extremely
serious. Unless the victim is recompressed promptly to reduce the size of the
bubble and permit blood to flow again, death may follow. The symptoms and
treatment of arterial gas embolism are discussed more fully in volume 5.
A diver shall never hold his breath on ascent. A diver who does may feel a sensation
of discomfort behind the breast bone (sternum) and a stretching of the lungs.
Fear and inhalation of water can also trigger a spasm of the laryngeal muscles
(laryngospasm) that seals the main lung passageway and thus brings about the overexpansion of the lungs. Under these circumstances, death has occurred during
ascent from depths of only a few feet. Every diver shall make it an absolute rule to
breathe normally and continually during ascent. However, a diver who cannot
breathe because he is out of air or because his gear is not working must exhale
during ascent.
figure3-11 Arterial Gas Embolism.
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Mediastinal emphysema (Figure
3-12) occurs when gas has been forced through torn lung tissue into the loose
mediastinal tissues in the middle of the chest, around the heart, the trachea, and the
major blood vessels. Subcutaneous emphysema (Figure 3-13) results from the
expansion of gas that has leaked from the mediastinum into the subcutaneous
tissues of the neck. These types of emphysema, including interstitial emphysema,
should not be confused with the emphysema brought on by the aging process or by
smoking.
figure3-12 Mediastinal Emphysema.
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figure3-13 Subcutaneous Emphysema.
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Pneumothorax is the result of air entering the potential space
between the lung covering and the lining of the chest wall (Figure 3-14). In its
usual manifestation, called a simple pneumothorax, a one-time leakage of air from
the lung into the chest partially collapses the lung, causing varying degrees of
respiratory distress. This condition normally improves with time as the air is reab-
sorbed. In severe cases of collapse, the air must be removed with the aid of a tube
or catheter. The onset of pneumothorax is accompanied by a sudden, sharp chest
pain, followed by difficult, rapid breathing, cessation of normal chest movements
on the affected side, tachycardia, a weak pulse, and anxiety. A diver believed to be
suffering from pneumothorax shall be thoroughly examined for the presence of
arterial gas embolism. This is covered more fully in volume 5.
In certain instances, the damaged lung may allow air to enter but not exit the
pleural space. Successive breathing gradually enlarges the air pocket. This is
called a tension pneumothorax (Figure 3-15) due to the progressively increasing
tension or pressure exerted on the lung and heart by the expanding gas. If uncorrected,
this force presses on the involved lung, causing it to completely collapse.
The lung, and then the heart, are pushed toward the opposite side of the chest,
which impairs both respiration and circulation. The symptoms become progressively
more serious, beginning with rapid breathing and ending in cyanosis (a
bluish skin color), hypotension (low blood pressure), shock and, unless corrected,
death.
If a simple pneumothorax occurs in a diver under pressure, the air will expand
during ascent, according to Boyle’s law, creating a tension pneumothorax. Thevolume of air initially leaked into the pleural cavity and the remaining ascent
distance will determine the diver’s condition upon surfacing.
All cases of pneumothorax must be treated. This is sometimes done by removing
the air with a catheter or tube inserted into the chest cavity. In cases of tension
pneumothorax, this procedure may be lifesaving. Volume 5 fully discusses the
treatment of simple and tension pneumothorax.
figure3-14Pneumothorax.
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figure3-15Tension Pneumothorax.
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