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Decompression: Revisiting Old Assumptions Equipment & Training Decompression: Revisiting Old Assumptions By Sergio Rhein Schirato D. Remmers D. Decmompression sickness is a complex condition that is still not entirely understood. he first comprehensive attempt to understand the (or “fit-to-reality adjustments”), the differential equations illness related to the exposure to hyperbaric environ- used by Haldane are the same ones used in almost every Tments was led by John Scott Haldane in collaboration computer or software available on the market today. with Arthur Edwin Boycott and Guybon Chesney Castell Given the information that was available at the time, it is Damant, and published early in the 20th century as “The understandable that Haldane and his coworkers treated 1 Prevention of Compressed-air Illness.” In retrospect, the the matter as a physical (or mechanical) problem caused by methodology used in the study, the assumptions that differ- bubbles forming during decompression. Having said that, ent tissues would absorb and eliminate gas at different rates it is worthwhile to note that in this study they specifically and how he modeled it, and the arguments used against the recognized that many of the animals that died did not reveal linear decompression (a method widely used at the time) signs of bubbles during necropsy and Haldane speculated are remarkable, especially if the knowledge and resources that bubbles may have formed in parts of the body they did available at the time are taken into consideration. In many not study. respects, most of Haldane’s conclusions remain the basis for many procedures still in use today. With a few improve- ments to supersaturation values, and other refinements 4 | Quest, Vol. 20, No. 1 D. Remmers D. It's long been thought that a lack of bubbles indicated a successful decompression. Nevertheless, he laid the foundation for an idea that is still from the venous to the arterial (i.e., systemic) circulation, very much accepted: decompression sickness (DCS) is a bypassing the filtering effect of the lungs. mechanical problem caused by bubbles. While this statement might hold true for large venous gas The purpose of this article is to discuss this and some other emboli most of the time, there are other facts that must be assumptions widely accepted as true by the diving commu- considered: (1) Patent foramen ovale (PFO), a remnant of our nity in light of recently published studies. fetal circulation, is found in approximately one-third of the Assumption 1: Venous gas emboli are formed during decom- population; (2) pulmonary shunts are, among other things, a pression and filtered by the lungs, while bubbles formed in physiological response to handle the cardiac afterload, and or transported to the tissues are the cause of decompres- different studies with high-performance athletes have shown sion sickness. that all subjects studied presented some level of pulmonary For many years it was believed that bubbles were related shunting as the physical effort to which they were submitted to decompression sickness and that their absence would increased; (3) the central nervous system has fast inert gas mean a successful decompression. However, with the kinetics,2 meaning that bubbles eventually shunted through development of Doppler ultrasound technology late in the the heart to these tissues tend to lose gas to the media, 1970s, it became clear that even mild exposures to hyper- being reduced in size and quickly collapsing. This assump- baric environments and subsequent decompression would tion can be supported by the fact that the gold standard lead to bubble formation in the venous circulation. Though for PFO detection is the transesophageal echocardiogram bubbles were commonly found in the right chambers of the coupled with the injection of agitated (full of bubbles) saline heart, Doppler echocardiograms showed that most of them solution, in which gas serves as a contrasting media to the were filtered by the lungs and were not observed in the left ultrasound. There are no known cases of decompression chambers of the heart. In theory, bubbles would be pumped sickness-like symptoms related to the use of such contrast, from the left chambers into the systemic circulation, which even when bubbles are clearly shunted to the left atrium. would send them to the central nervous system, causing Additionally, post-dive bubbles detected by Doppler have the neurological symptoms of decompression sickness. This diameters larger than 30 µm. A recent study using contrast- finding led to the endless discussion about the role of cardiac enhanced imaging techniques capable of detecting bubbles or pulmonary shunts in decompression sickness since the with diameters smaller than 10 µm indicated the presence of existence of a shunt would allow the migration of bubbles smaller emboli in both sides of the heart, demonstrating that: 5 | Quest, Vol. 20, No. 1 (1) there are small bubbles in humans that are not filtered by the lungs; (2) there are small bubbles even in the absence of larger venous gas emboli; and (3) smaller bubbles follow a different timeline than larger venous gas emboli.3 Bubbles forming in the arterial circulation have also been identified in previ- ous studies though their role in decompression sickness, especially in the presence of neurological symptoms, is yet to be understood. Vascular bubble models, designed to study nucleation on a flat hydrophobic surface and how D. Remmers D. they expand to form bubbles after decompression, hold Decompression sickness is not just a physical or mechanical issue. great promise for the improve- ment of decompression proce- dures in the future.4 Assumption 2: Mechanical damage caused by bubbles is due gas and follows the rank: argon ~ nitrogen > helium.7 This to decompression sickness. ranking might explain the reduced endothelial dysfunction Several studies over the past two decades have shown that identified after hyperbaric exposures where helium was part decompression has many physiological implications, ranging of the breathing mix.8 from reduction in endothelial function to activation of the The mechanism behind decompression sickness appears immune system. As discussed above, formation of bubbles to be more complicated than the simple growth of bubbles, is a common finding in subjects exposed to hyperbaric and a lot remains to be understood. environments and subsequent decompression. The causal Assumption 3: Decompression profiles with deep stops are relationship between bubbles and physiological alterations, safer. however, is yet to be proven. In recent years, the endothelial dysfunction hypothesis, which postulates that micropar- With divers pushing the boundaries of deeper diving ticles associated with endothelial damage act as nucleation beyond military and commercial diving, and the introduc- sites for bubble formation, has drawn attention and gained tion of helium in the breathing mixes in the 1990s, different support. This has resulted in decompression sickness being decompression techniques for bounce (non-saturation) dives seen not as merely a physical or mechanical problem, but started to be tested. Richard Pyle, an American ichthyolo- instead as a result of a complex biochemical process. gist from Hawaii, was probably one of the first to publicly advocate for decompression stops deeper than those calcu- Recent studies have shown that the exposure to high- lated by algorithms derived from Haldane´s theory. On dives pressure environments is sufficient to increase the produc- ranging in depth from 40 to 70 m, he correlated catching tion of IL-1β, an interleukin that belongs to cytokines, which fishes with his overall feeling after diving, and attributed is an important mediator in inflammatory responses.5 The feeling better to the fact that when a fish was caught, he mechanism behind the formation of such microparticles is had to stop much deeper than determined by decompres- related to high inert gas pressure through a mechanism that sion algorithms to release gas out the fish´s swim bladder. causes singlet oxygen formation, a potentially toxic free radi- Decompression algorithms based on the control of bubble cal initiated by a cycle of actin S-nitrosylation, nitric oxide formation and growth including the Varying Permeability synthase-2, and NADPH oxidase activation ultimately lead- 6 Model developed by David Yount, which is the most well- ing to microparticle formation. Despite their harmful effects known algorithm based on this strategy (probably because to the host, the production of reactive oxygen species it is open code software), require decompression stops at (ROS) is part of an orchestrated physiological response of greater depths, corroborating Richard Pyle´s conclusions. the immune system to stop bacteria and fungus. Exposure At some point, it became well-established within the diving to high inert gas pressures, even in the absence of decom- community that deeper stops were mandatory and even pression, is apparently linked to an increased production of Albert Bühlmann´s ZHL 16 algorithm was adjusted; gradient ROS. The potential to trigger this reaction depends on the factors were implemented to calculate deeper stops. 6 | Quest, Vol. 20, No. 1 There is, however, no scientific data available to support the Both profiles were calculated to provide similar decompres- belief that the modification of the decompression schedule sion times for a dive to 51 m of depth and a bottom time of with the inclusion of deeper stops reduces the expected 30 minutes. The profile with deeper
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