| Diving, which is an exposure to the underwater hyperbaric environment for recreational or occupational purposes, is growing in popularity every year. Personnel into the water, they must breathe with gas which has the same pressure with the surrounding water pressure. During a diving, inert gas (nitrogen is the most common one) becomes dissolved in tissues as a function of time and depth. This requires the following decompression must follow certain rules. As the partial pressure of inert gas in the blood and tissues exceeds ambient pressure, bubbles form in the tissues and blood vessels, which may result in the clincal syndrome of decompression sickness (DCS). It may present not only as minor symptoms such as skin itching and pain in joint (often termed "bends"), but also as serious neurological symptoms that can even lead to death. Prevention of DCS is becoming the core of the underwater security operations.Hyperbaric oxygen (HBO) is a clinical therapy method for many diseases, such as carbon monoxide poisoning. HBO therapy is the most effective treatment for DCS. Furthermore, HBO preconditioning has been used to prevent various ischemia and anoxemic injuries. DCS also experiences the pathological process of ischemia and anoxemic. Does HBO preconditioning have the protective effect on DCS?The objective of our study is to investigate the prophylactic effect of HBO preconditioning on DCS. The study is divided into two parts.The first part is to establish the rat model of air diving DCS. Adult SD rats were exposed with appropriate "pressure-duration-decompression rate" profiles of hyperbaric air, and were subjected to run in the cylindrical cage rotating at 3 m/min for 30 min following exposure. Evaluating parameters were selected from the ethology, macropathology, micropathology and inflammatory indices. Our results suggested that controlling the pressure-duration of hyperbaric exposure and the rate of decompression, and making the animals exercises in a rotating cage at a constant speed, can produce a model of rat decompression sickness with a stable incidence, which could be observed efficiently. The evaluating parameters of ethology, macropathology, micropathology and inflammation were established.The second part is to investigate the prophylactic effect of HBO preconditioning on DCS. The study is divided into two steps.â‘ HBO preconditioning effect observing. Adult SD rats were pretreated with HBO (250 kPa-60 min) (all the pressures described here are absolute pressure) 18 h before a simulated air dive (700 kPa-100 min) with fast decompression to the surface at the rate of 200 kPa/min (n=33). During the following 30 min, the rats walked in a 3 m/min rotating cage and were monitored for signs of DCS. The control rats were pretreated with normobaric air (n=30) or normoxic hyperbaric nitrox (250 kPa,8.4% O2) (n=13). The results showed that the incidence of DCS in rats pretreated with HBO was 30.3%, which was significantly lower than those treated with normobaric air (63.3%) (p<0.05); the hyperbaric nitrox preconditioning had no significantly effect on the incidence of DCS (61.5%).â‘¡Study of mechanism is divided into two steps. First is the role of nitric oxide (NO) in the effect of HBO preconditioning on DCS. NO levels were recorded immediately and 18 h after HBO exposure in the brain, spinal cord and lung; the effect of the nitric oxygen synthetase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the prophylactic effect of HBO was observed. HBO increased NO level significantly in the rat brain, spinal cord and lung right after exposure; this effect was inhibited by L-NAME. L-NAME nullified the prophylactic effect of HBO on DCS, and dragged the morbidity back to 71.4%.Second is the role of the heat shock protein (HSP) 70 in the prophylactic effect of HBO preconditioning on DCS via NO. HSP70 levels were detected 18 h after HBO exposure in the brain, spinal cord and lung; the effects of L-NAME and quercetin (Q) (the inhibitor of HSP) on the expression of HSP70 induced by HBO exposure were observed; the effect of Q on the prophylactic effect of HBO was also observed. It was showed that HBO increased HSP70 level in the rat brain, spinal cord and lung 18 h after HBO exposure, which could be inhibited by both L-NAME and Q. Moreover Q could nullify the prophylactic effect of HBO on DCS, and dragged the morbidity back to 69.2%.The main conclusions are as follows:1. HBO preconditioning 18 h ahead of diving could effectively prevent the DCS;2. The prophylactic effect of the HBO preconditioning might be through the induced expression of HSP70, in which NO might be involved...
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