Ebullism is the formation of water vapour bubbles in bodily fluids due to reduced environmental pressure, usually at extreme high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lowers; for example, liquids reach their boiling points at lower temperatures when the pressure on them is lowered. The injuries and disorder caused by ebullism is also known as ebullism syndrome. Ebullism will expand the volume of the tissues, but the vapour pressure of water at temperatures in which a human can survive is not sufficient to rupture skin or most other tissues encased in skin. Ebullism produces predictable injuries, which may be survivable if treated soon enough, and is often accompanied by complications caused by rapid decompression, such as decompression sickness and a variety of barotrauma injuries. Persons at risk are astronauts and high altitude aviators, for whom it is an occupational hazard.
Symptoms
Symptoms of ebullism include bubbles in the membranes of the mouth and eyes, swelling of the soft tissues with possible bruising, and bubbles in the blood. Blood circulation and breathing may be impaired or stopped by cardiac vapourlock. The brain tissue may be starved of oxygen because of blockage of arteries resulting in rapid loss of consciousness, and the lungs may swell and hemorrhage. Death results unless recompression is rapid enough to restore oxygenation and reduce the bubbles before excessive tissue damage occurs. Head exposure may result in freezing of the corneal surface of the eye, impairing vision. Other signs and symptoms of rapid decompression injury may also be present.
Complications
A decompression event leading to ebullism will cause acute anoxemia and is likely to cause other decompression injuries such as decompression sickness and possibly one or more forms of decompression barotrauma.
Causes and mechanism
In the atmospheric pressure present at sea level, water boils at . At an altitude of , it boils at only , the normal body temperature of humans. This altitude is known as Armstrong's Line. Ebullism occurs when unprotected humans are exposed to altitudes above the Armstrong limit where the vapor pressure of tissues is less than the ambient pressure. In practice bodily fluids do not boil off continuously at this altitude because the skin and outer organs have enough strength to withstand the internal pressure, so the pressure inside the tissues would increase to match vapour pressure. Nitrogen dissolved in the tissues may also accumulate in the vapour bubbles causing altitude decompression sickness.
Pathophysiology of ebullism has mostly been studied in animals, including large primates, but no reasons have been found to suggest that the results should not be reasonably extrapolated to predict effects on humans. The experiments show that ebullism occurs non-uniformly at sites where conditions are most conducive to vapourisation of water and outgassing of nitrogen. Factors include temperature, hydrostatic pressure, tissue elasticity, solute concentration, and the presence of gas bubble nuclei, and which can cause vapour bubbles to form at pressures slightly above the theoretical ambient pressure of in some places such as the pleural cavity, where the pressure can drop below ambient, and large central veins where hydrostatic pressure is minimum and blood temperature is at a maximum.
Diagnosis
Ebullism occurs as a consequence of exposure to ambient pressures below about . At higher pressures similar symptoms are likely to be caused by decompression sickness and some forms of barotrauma.
Prevention
To prevent ebullism, the tissues must be kept under sufficient pressure that vaporisation of the aqueous constituents is not possible in the range of temperature those tissues may experience.