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.

Arterial Gas Embolism

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.

Mediastinal and Subcutaneous Emphysema

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.


figure3-13 Subcutaneous Emphysema.


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.




figure3-15Tension Pneumothorax.

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