The increasing popularity of SCUBA diving and growth of commercial diving has increased the incidence of decompression illness (DCI).
As more people of varying ages and fitness dive more often, helped
by developments in technology to go deeper and for longer, then doctors will see more cases of this condition.
At our Hyperbaric Chamber in London we see many cases of DCI in divers who have observed all the rules and stayed within their
tables or computer algorithms, but still develop DCI. No diver, diving school or independent instructor should think that they are immune to DCI.
Here we explain how it can develop, how it is diagnosed and how it is treated.
Direct effects of increasing pressure occur only on the gas filled spaces in the body.
The human body is primarily made of water, which is non-compressible and transmits pressure evenly. However, the gases in hollow organs - lungs, middle ear, sinuses, poorly filled teeth, bowels, and those dissolved in the blood - are at the mercy of pressure changes.
The physical behaviour of gases is governed by the following 3 gas laws. They define the physics and problems involved in descending and ascending in water.
To understand how DCI can occur and how it is treated, a diver needs to understand these 3 laws.
The volume of a given mass of gas is inversely proportional to the pressure being exerted on it (temperature remaining steady).
For every 10 metres of descent the pressure increases by one atmosphere (atm). Therefore, total lung volume during a breath-hold dive at 10 metres is one-half that at the surface.
At 20 metres it is 1/3, at 30 metres it is 1/4 and at 40 metres it is 1/5. On surfacing these figures are reversed. However when breathing compressed gases as in diving the mass of gas in the lungs is increased to fill the normal volume. An ascent from 30 metres to the surface without venting (exhaling) would cause the gas, in already full lungs, with minimal ability,
to expand further to increase its volume to three times normal with the greatest change
occurring in the last 10 metres where it would double. This is the key law to explain
pressurisation and depressurisation issues and injuries.
As a diver descends the total pressure of breathing air increases in accordance with
Boyle's Law; therefore, the partial pressures of the individual components of the breathing
air are increased proportionally. As the individual descends deeper under water, nitrogen dissolves
in the blood and is carried to all body tissues until a new equilibrium is reached. Long before this
however Nitrogen at the higher partial pressures in blood alters the electrical properties of
cerebral nerve cell membranes, causing an anaesthetic effect termed nitrogen narcosis. For every
15 metres of depth this is roughly equivalent to one alcoholic drink. At 50 metres divers may
experience alterations in reasoning, memory, response time, and other problems such as idea fixation,
overconfidence, and calculation errors.
During descent the partial pressure and hence the amount of dissolved oxygen increases.
Breathing 100% oxygen at 2.8 atmospheres absolute (1.8 atm or 18 metres) may cause oxygen
toxicity in as little as 30-60 minutes. At 100 metres, the normal 21% oxygen in compressed air
can become toxic, because the partial pressure of oxygen is approximately equal to 100% at 10 metres.
For these reasons deep divers (usually professional or military, but increasingly sport divers as well)
use specialised mixtures that replace nitrogen with helium and allow for varying percentages of oxygen
depending on depth. The percentage is small and provides a partial pressure which supports life and
strenuous activity without inducing oxygen toxicity
With increasing depth, nitrogen in compressed air equilibrates through the alveoli of the lungs
into the blood and thence into the tissues. Over time nitrogen dissolves and accumulates initially
in the mainly aqueous tissues or those with a high rate of blood flow e.g. the brain, and progressively
in the lipid or fatty component of tissues. On longer dives some or all tissues become 'saturated' and
will not take up any more nitrogen. As an individual ascends, there is a lag before saturated tissues
start to release nitrogen back into the blood. It is this delay that creates problems.
When a critical amount of nitrogen is dissolved in the tissues, reduction of pressure caused by ascending
induces the dissolved gases to 'outgas' and form small but myriad bubbles in tissue cells, tissue spaces
and blood. Ascending too quickly causes the dissolved gases - nitrogen - to return to gas form more quickly
increasing the number and size of the bubbles and while still in the blood or tissues causing local damage
which may be felt as symptoms of DCI Further reductions in pressure through flying or ascending to a higher
altitude also contribute to bubble formation. The average airline cabin is pressurised only to 8000 feet or 0.8 atm.
If a person flies too soon after diving, this additional decrease in pressure may be enough to precipitate
bubbling or enlarge any bubbles already in existence. With or without the effects of flying If the bubbles
are in the blood in some divers paradoxical embolisation may occur through a PFO.