Decompression is a real threat to scuba diversBy BILL MCKELL,
Time is nigh for the wet suits and SCUBA tanks to come out of the closet, and although the odds are in the diver's favor (decompression sickness occurs in about 2-4/10,000 dives, affecting approximately 1,000 U.S. divers per year), it's also time to think about decompression sickness (aka the bends, caisson disease). These terms describe a condition arising from dissolved gasses coming out of solution into bubbles inside the body on decompression. The longer a diver stays down and the deeper the dive, the more compressed gas is absorbed by the body. When the diver ascends, time must be allowed for the additional gases to be expelled slowly or they will form bubbles in the tissues. The major component of air that causes decompression maladies is nitrogen.
I feel sure that everyone remembers Henry's law from high school physics, right? Just in case it has momentarily slipped your mind, it states that the solubility of a gas in a liquid is directly proportional to the pressure exerted on the gas and liquid.
Gas is dissolved in all tissues, but decompression sickness is only clinically recognized in the nervous system, bone, ears, teeth, skin and lungs. Generally, there are two types of decompression sickness. Type I, the more common type, involving skin, muscles and lymphatics, is milder. The term “the bends” refers to joint or muscle pain derived from this affliction. The joints most frequently involved are the shoulders, elbows, knees and ankles. Type II is more serious and may be life-threatening. Nitrogen is much more soluble in fatty tissue than in other types; therefore, tissues with a high fat (lipid) content tend to absorb more nitrogen than do other tissues. The nervous system (brain, spinal cord and peripheral nerves) is composed of about 60 percent lipids. Bubbles forming in the nervous system may cause paralysis, convulsions (divers' palsy), muscle coordination and sensory problems (divers' staggers), personality changes, coma and death. Small nitrogen bubbles trapped under the skin may cause a red rash and an itching sensation (divers' itches). Excessive coughing and difficulty breathing (the chokes) indicating bubbles in the respiratory system is rare. Although the onset of decompression sickness may occur immediately after the dive, in more of half of all cases symptoms do not begin for at least an hour or longer.
Risk factors include cold temperature dives, dehydration, exercise after diving, fatigue, flying after diving, obesity, older age, prolonged or deep dives and rapid assents.
Significant bubble formation can usually be avoided by limiting the depth and duration of dives to a range that does not need decompression stops (no-stop limits) during assent or by ascending with decompression stops as specified in published guidelines (US Navy Diving Manual). Many professional divers wear a portable dive computer that tracks depth, time at depth and calculates a decompression schedule; and many divers may make a safety stop for a few minutes at 15 feet below the surface.
Diagnosis is clinical and treatment should begin based on clinical suspicion. Treatment consists of 100 percent oxygen and recompression therapy (Hyperbaric Oxygen Chamber). Approximately 80 percent of patients recover completely. Initially, high-flow 100 percent oxygen enhances nitrogen washout. Recompression therapy is indicated for all patients except perhaps those whose symptoms are limited to itching, skin mottling and fatigue. Other patients should be transported to a recompression facility immediately for time to treatment is a main determination of outcome. If air evacuation is utilized, an aircraft capable of one atmosphere internal pressure is preferred. In unpressurized planes, low altitude (less than 2,000 feet) must be maintained. The diver should have information regarding the location of the nearest recompression chamber, the quickest way to get there and the most appropriate source to contact via telephone.
Dr. William McKell is a Northsider. His email is firstname.lastname@example.org.