Diving in Water Recompression

As any person ventures into an increased pressure environment, they begin to absorb the inert gasses in their breathing media in proportion to the percentage of each gas inspired (Dalton’s law). In the air, the bulk of the inert gas is nitrogen (approximately 78%), and the gas is absorbed based on the amount of increased pressure and the time the individual inspires the gas. Functionally, this can be seen underwater, in deep tunnel drilling, in caissons, or in other exotic increased pressure environments. It is most easily conceptualized in a diver descending underwater, so that model will be used for this explanation. Diving using an underwater breathing apparatus (UBA) of any type involves the inspiration of gas by the diver at ambient pressure, which is increased from normal surface pressure. Seawater is sufficiently denser than air that 1 atmosphere (atm) of air is equivalent to 33 feet of seawater (FSW), meaning the diver can double their ambient pressure by descending only 33 feet in the water column. As pressure increases, the amount of inert gas dissolved in the diver’s bloodstream and tissues also increases, which exponentially decreases the time the diver can remain at depth without taking time to off-gas (decompress) prior to returning to the surface. Divers who remain at depth past the no-decompression time limit for that specific depth are at increased risk of the dissolved inert gas in their bodies reaching a critical supersaturation point during their ascent and having gas bubbles form  either in the bloodstream or in tissues. These bubbles are often asymptomatic but can result in decompression sickness when the gas bubbles themselves do damage to the surrounding tissues. Because the amount of dissolved gas is related to the time as well as the ambient pressure increase, it is necessary to address both components. As depth increases, the probability of decompression sickness increases exponentially. A diver at 15 feet of saltwater, typically, can stay indefinitely with almost no chance of decompression sickness while a diver at 60 feet can only remain for about an hour before needing to stop and decompress prior to direct return to the surface. The same diver at 100 feet of saltwater would only have 25 minutes and only about 5 minutes at 150 feet. Once a diver remains beyond these limits, direct return to the surface would mean a statistically significant increase in the probability of decompression sickness via bubble formation in the bloodstream or tissues and must be mitigated with stops on the way back to the surface to provide the diver with the opportunity to breath out inert gas and thereby prevent bubble formation. Once bubbles have formed and then become symptomatic, recompression is required for resolution. Recompression in a chamber is safest, but if recompression in a chamber is unavailable or will be delayed many hours due to travel time, recompression in the water can be considered. Divers experiencing decompression sickness (DCS) require recompression on pure oxygen to dissolve the gas bubbles in their blood and tissues, allow excess nitrogen to diffuse out, and oxygenate ischemic tissues, thereby treating the disease process. Recompression has classically been achieved with a special chamber allowing a controlled increase in ambient pressure as well as a treating physician or technician to care for the stricken diver. There has been an alarming decrease in the number of recompression chambers available for 24-hour emergency care across the nation, resulting in divers with DCS facing long transit times to available chambers. This delay in care negatively affects the diver’s probability of complete recovery, resulting in many divers considering the alternative known as in-water recompression (IWR). IWR involves intentionally placing the stricken diver back in the water on pure oxygen, typically with a prolonged oxygen-breathing period at 30 FSW or less, with a gradual ascent to the surface. In water, recompression has been used for decades by several navies throughout the world as well as globally in remote areas where local recompression chambers are simply not available. The Australians developed and have used a relatively standardized protocol for several years, as has the United States Navy for use in extraordinary circumstances, as it is normal navy policy to have a chamber on-site for diving operations.