Advances in Diving Medicine
By Fred Bove, M.D., Ph.D.
We attended the annual meeting of the Undersea and Hyperbaric Medical Society this June. The UHMS is the scientific and professional organization that is devoted to the science of diving and hyperbaric medicine. The annual meeting consists of presentations of original research, clinical reports and workshops. This meeting also gives us the opportunity to talk to most of the world's experts in diving and hyperbaric medicine.
Several papers provided new information on the causes of tissue injury in decompression sickness (DCS). They showed changes in blood cells (platelets and leukocytes) with DCS in animal models, and some mild changes in lung function of healthy divers after air dives to 120 feet in a wet pressure chamber. The changes are small and likely to return to normal after six to eight hours.
A paper from Italy on breath-hold diving reminded us that DCS can occur from breath-hold diving if multiple deep freedives are done over a few hours. Symptoms ranged from paralysis to vision changes. If DCS occurs from breath-hold diving, the treatment is still recompression, just as it is with scuba diving. In this study, all divers recovered completely after treatment.
There is still much interest in lung and heart function with deep breath-hold diving. The current record for a freedive is over 400 feet. Based only on lung volume considerations, the lungs should be injured when a freediver descends below about 165 feet.
Dr. Claes Lundgren, a physiologist from Buffalo, New York, has done many years of research to understand how freedivers avoid injury to the lungs when diving to deep depths. His research shows that lung squeeze does not occur at the deeper depths because blood from the legs and abdomen is forced into the blood vessels of the lungs, allowing further compression without tissue injury.
The staff of the Divers Alert Network (DAN) presented several papers about sport diving injuries. They continue to collect information on diving accidents and report causes and circumstances that are associated with DCS and arterial gas embolism.
Understanding the factors that cause DCS and adhering to table or computer schedules will keep the incidence of DCS low and will maintain the safety record that sport diving currently enjoys. The recommendation for sport diving is to dive within the no-D limits. DAN presented information on a computer-based reporting system that will allow easier accident reporting and quicker analysis of their data.
Another important observation presented by DAN involved fatalities classified as technical dives. These included cave, wreck or ice diving; diving with mixed gas other than 32 percent nitrox; deep dives requiring staged decompression; and dives with semi-closed equipment (rebreather). From 1989 to 1997, 15.4 percent of fatalities were technical divers. Of those fatalities, only 24.2 percent of the divers had formal training in technical diving. This information points out the need for advanced training before undertaking this demanding sport.
Warm Water and Slow Ascents
Interesting data continue to come from the Navy diving experience during the recovery of the wreckage of TWA Flight 800. This recovery required over 4,400 dives to a depth of 115 to 120 feet in Long Island Sound. An increase in DCS cases was noted in divers using heated diving suits. Similar data were analyzed from experimental dives to determine the risk of diving in warm water. The risk of DCS was doubled for an 18 Degrees F (-22 Degrees C) increase in water temperature. Increase of blood flow to skin and muscles in warm water causes a more rapid uptake of nitrogen that is not accounted for in the diving tables or computers. Although most sport diving is done in waters where the temperature is below 80 Degrees F, if water temperature is high, shorter bottom time will add a safety factor.
Another important factor in avoiding DCS is the use of slow ascent rates. One paper from Japan indicated that slow ascent rates are even useful for workers employed in construction of tunnels where work is done under pressure.
The same principles can be applied to sport divers. If there is an opportunity to conduct your dive at progressively shallower depths, making a conscious effort to ascend throughout the dive will reduce the risk of DCS.
What's in a Name?
There is an effort underway to reclassify the diving disorders by removing the distinction between arterial gas embolism and decompression sickness. This effort is based on the overlapping symptoms of the two disorders and the frequent use of the same recompression procedures for both.
The mildest form of decompression injury (decompression illness) is joint pain, itchy skin and numbness. I take issue with numbness being included in mild symptoms because it indicates that there is injury of the spinal cord. The consequences of spinal cord injury are greater than those of a local joint injury.
You can access more diving medicine articles at www.scubamed.com, where you will also find links to the Undersea and Hyperbaric Medical Society and DAN websites.