Barotrauma related to diving

barotrauma in diving

• 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

Reference:

• 1) Edge CJ. Recreational diving medicine.Current Anaesthesia Critical Care 2008; 19 (4): 235-246.
• 2) Knauth, M. et al. Cohort study of multiple brain lesions in sport divers: role of a patent foramen ovale. BMJ 1997; 314: 701-5.