Although scuba divers can experience a myriad of injuries from wildlife and trauma, the two most serious forms of diving injuries are decompression sickness (DCS) and arterial gas embolism (AGE), both of which are directly related to the behaviour of pressurized gases. In many cases, the signs and symptoms of decompression sickness and gas emboli overlap significantly; it is not important to differentiate between the two in the out-of-hospital environment with treatment for both being essentially identical.
Scuba divers breathe compressed air. At depth, the nitrogen in this air dissolves into the bloodstream and diffuses into body tissues at variable rates. The water pressure around the diver keeps this gas dissolved in the blood and tissues, but as a diver ascends, water pressure decreases, allowing the dissolved gases to withdraw. (This is similar to opening a pop can – the carbon dioxide remains in the liquid because of the pressure inside the can – and the behavior of gases under pressure is described by Henry’s Law.) Normally, during an ascent, divers change depths slowly and breathe constantly ensuring that the nitrogen is released from their lungs. Under some circumstances – a rapid ascent from too deep a dive, for instance – the dissolved gas may not diffuse into the lungs and may instead accumulate in the blood, musculoskeletal system, or other body tissues, as bubbles.
Type 1 DCS is limited to the capillaries of the skin, lymphatic vessels, and the musculoskeletal system. It generally includes skin rashes or urticarial and joint pain. In its milder form, the symptoms can be fleeting and last only a few minutes as the bubbles break down and the diver off-gases; these do not generally require treatment. Pain at or around joints is rarely symmetrical. In more severe cases, the pain can increase over 12 to 24 hours after surfacing, and if untreated, will resolve slowly over the next three to seven days to a dull ache.
Type 2 DCS is more serious. It involves the central nervous, cardiovascular, and respiratory systems; common symptoms include headache, blurred vision, nausea, dizziness, and ataxia. Shortness of breath, hypotension, and weakness can occur. In many cases, Type 1 symptoms are also present.
The pressurized gas breathed by a diver at depth expands as they ascend, following the relationship described by Boyle’s Law. If the expansion is not accommodated or controlled, the expansion can be fatal. In the lungs, gas can expand and rupture alveoli, introducing air into the bloodstream. Once in the blood, the bolus of air is carried into the heart, and then into the arterial circulation. Air can also be forced into the pleural space between the lungs and chest wall; in some cases, this is the result of a congenital weakness. Pleural air expansion can lead to either mediastinal emphysema (a collection of air in the mediastinum) or subcutaneous emphysema in the neck or upper chest.
AGE is the most common cause of death in scuba diving.
Signs of DCE can be subtle, and may take time to develop. An emergency physician should always see patients suspected of having suffered a dive injury. Consultation with a specialist in hyperbaric medicine is highly recommended.