many tissues in the body may contain natural or unnatural collections of free gas
extremely rigid containers e.g. blocked paranasal sinuses
very compliant containers of free gas e.g. unobstructed lungs or patent sinuses.
intermediately compliant containers of free gas e.g. the gastrointestinal tract and the cavity of the middle ear
the unobstructed lung has a time constant of emptying of less than 1 second and so it normally vents and fills with gas without difficulty during ascent and descent
if the lung is obstructed, as for example during descent on a breath-hold dive, it compresses according to Boyle's Law
the air in the lungs taken in by a breath-holding diver at the surface will shrink to one-half the original volume at a depth of 10 m, to one-third at a depth of 20 m and so on
scuba divers breathe gas at the ambient pressure of the surrounding water and this enables the diver to breathe with ease
however, as the diver goes deeper, the supplied gas will become denser, and the work of breathing will become harder
in the condition where the volume of gas diminishes and further gas cannot be drawn in to occupy this space, it will either be filled by body tissues or by clothing - this situation is termed a "squeeze"
further descent causes blood to be drawn into the chest from the limbs and abdomen to compensate for further diminution of lung volume
if the lung is obstructed on ascent, e.g. by a diver unwisely holding his/her breath, the gas in the lung expands until the lung reaches its bursting pressure which is roughly 70 mmHg at about 115% of voluntary total lung capacity when it ruptures
gas may escape from the lungs into other tissues in four ways:
1. pneumothorax - into the virtual space between the visceral and parietal pleura to give a pneumothorax - tension pneumothorax might occur
2. into the pulmonary venous blood from where the gas passes into the arterial circulation (arterial gas embolism, AGE) giving rise to the manifestations of DCI
3. pneumomediastinum - into the mediastinum (pneumomediastinum) and thence into the soft tissues of the neck - causes surgical emphysema.
4. pneumoperitoneaum - into the peritoneum
for unknown reasons (but may be to do with posture whilst working), aviators and tunnel workers experience pneumothoraces more commonly than AGE, while the situation is reversed in divers
obstruction of the Eustachian tube
if the Eustachian tube is obstructed
when the diver descends a painful bowing of the tympanic membrane into the middle ear occurs
if the diver descends further then will cause rupture of the tympanic membrane with entry of potentially infected water into the middle ear cavity
can result in vertigo underwater which may cause the diver to panic and ascend rapidly to the surface
obstruction of the external meatus
obstructing the external meatus (which most commonly occurs by means of a tightly fitting hood) causes an outward bowing of the tympanic membrane, known as "reversed ear"
odontocharexis
gas may become trapped in a tooth cavity at depth, commonly as a result of poor dental hygiene
when the diver ascends, the expanding gas can cause severe pain in the tooth (odontalgia); in some cases the tooth has been known to shatter (odontocharexis)
mask barotrauma is described in the linked item
Notes:
Boyle's Law
at constant temperature the pressure and the volume of the gas are inversely related:
PV=k, where P is the total pressure of the gas and V is the volume of the gas. k is a constant for a given gas or mixture of gases o cavities in the body that contain gas will be affected by pressure as indicated by Boyle's Law
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