7 Barotrauma
Barotrauma implies damage to the body structures due to changes in atmospheric pressure. A number of air-containing body structures can be affected, such as the gut, lungs and sinuses, but it is the ear that most frequently suffers pathological consequences. Barotrauma commonly occurs in flying and diving and also less frequently during hyperbaric oxygen treatment.
7.1 Physics
To understand barotrauma, it is important to remember Boyle’s law, which states that as the ambient pressure increases, the volume of a gas decreases. It is given by the formula:
PV = k
where P is pressure, V is volume and k is a constant for the gas in question.
From a diving perspective, this is illustrated in the 
Table 7.1. Thus, it can be seen that the greatest change in volume occurs within the first 10 m of a dive.
This pressure and volume relationship accounts for most of the problems of barotrauma. However, with diving there is an additional problem. Nitrogen, which makes up approximately 80% of the volume of inspired air, and is metabolically inert, is absorbed by the tissues in a manner that is proportional to depth and duration of the dive. As the diver ascends, nitrogen finds its way out of the tissues and back into the bloodstream, before being exhaled through the lungs. However, if the rate of ascent is too quick, the nitrogen will come out of solution as gas bubbles in the tissues. This condition is decompression sickness, or colloquially ‘the bends’, from the characteristic stooping posture of affected caisson workers in whom it was originally identified.
7.2 Clinical Effects
7.2.1 External Ear
The external ear is rarely affected by pressure change. However, there is the rare condition of ‘reverse- or external ear squeeze’ which occurs, usually only when diving, when a pocket of air becomes trapped in the external ear canal, usually due to wax build up or a tight-fitting diving hood. If the eustachian tube is functioning normally, then middle ear pressure will increase with descent while the pressure in the ear canal will fall, leading to an outward bulging of the tympanic membrane. The main symptom is pain, but if the pressure gradient is extreme, then perforation can result.
7.2.2 Middle Ear
The middle ear is by far the commonest site affected by barotrauma. Problems tend to occur more frequently on descent for divers and almost exclusively so for flyers.
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