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AD 2016 Q1


RESEARCH, EDUCATION & MEDICINE EXPERT OPINIONS At each of these three stages there are factors that modify the effects and outcomes of exposure. In the absence of specific knowledge, the most common preventive strategy is to reduce supersaturation by limiting the depth/time exposure and ascent rate for all divers to avoid substantial bubbling — even though bubbles put only some divers (those with a right-toleft shunt [RLS]) at risk. Without bubbles (or with only a few), there will be no arterialization, even in divers with RLS, and there will be no DCS in those susceptible to arterial embolization. Several approaches to mitigating postdive bubble occurrence have been studied; these include predive removal of hypothetical bubble nuclei by wholebody vibration, attempts to influence oxygen radicals or nitric oxide suspected of contributing to bubble generation, stimulation of heat-shock protein production and various other methods, including predive chocolate treats. Although some of these factors may reduce the amount of bubbles, the effects vary and may be of less importance than the individual variation in response to decompression. If every diver had a consistent individual propensity for VGE production and could be classified as a “bubbler” or “nonbubbler” across a broad spectrum of reasonable dive exposures, safe exposure limits could be tailored individually. We would achieve greater precision if we could identify those divers who have persistent or occasional RLS and then customize a dive exposure for them. At present we already know that a large patent foramen ovale (PFO) enables arterialization and Figure 1. apparently increases the risk of DCS. A test for PFO is available. We can identify divers with a large PFO and close it, but this does not solve the problem for all divers because RLS may occur in lungs regardless of PFO. Risk of DCS due to pulmonary RLS may be more difficult to tackle because pulmonary RLS seems to be part of the normal physiological response to exercise and far more prevalent in the population than large PFOs. It is important to note that even if we could prevent the occurrence of VGE, DCS could still occur. VGE do not play a role in pain-only DCS. Some cases of spinal DCS may be caused by bubbles occurring locally in tissue, without bubbles in venous blood. Similarly, some cases of inner-ear DCS may be caused by local bubbles rather than by arterialized gas emboli. Cutaneous (skin) manifestations of DCS may be caused by various mechanisms, some involving embolization of arterialized VGE and others independent of VGE. As in lung cancer, the definitive cause of DCS is an external physical factor that, unlike smoking, we cannot eliminate if we want to dive. But by controlling the magnitude of exposure, we can minimize the risk of DCS. There is still a lot of room to improve the precision of DCS prediction and develop individually tailored preventive exposure restrictions. This can be achieved by advancing VGE study methods and dive population studies to identify individuals who may easily produce VGE or are prone to arterialization. Understanding why some divers easily produce VGE and why some are more prone to DCS in the case of 52 | WINTER 2016

Below: Studies have identified changes in gene expression and transcription following breath-hold and compressed-gas dives. Opposite, from top: Venous gas emboli (VGE) may enter arterial circulation through a right-to-left shunt such as a patent foramen ovale (PFO). VGE do not occur after every dive, not all VGE enter the arterial circulation, and arterialized VGE do not always lead to DCS. VGE arterialization, however, will need a novel approach. One promising direction is the study of the innate immune system and inflammation, both of which are involved in the human body’s response to stress. Diving involves exposure to environmental changes that challenge the entire body and elicit immune responses and inflammation controlled by genetic mechanisms. However, measurable changes are rarely present in asymptomatic divers, making it difficult to study the transition of physiological adaptive stress responses into maladaptive or pathological reactions that lead to impaired organ function. SHUNYU FAN/ISTOCKPHOTO.COM GENETIC STUDIES The idea that DCS, like breast cancer, may be promoted by a particular genetic or epigenetic variation attracts more and more researchers. Genes are the basic physical units of inheritance passed from parents to offspring; they contain the information needed to specify traits. Humans have approximately 20,000 genes, an impressive collection that constitutes an individual’s genotype. Genes are grouped into 23 pairs of chromosomes, each pair containing approximately 3.1 billion distinct code elements. The information encoded in genes is expressed through Galapagos N Turks & Caicos Maldives Unique liveaboard experiences. Exceptional value. Dive with us! N Saba/St. Kitts Silver Bank USA/Canada: 1.800.322.3577 +1.307.235.0683 ALERTDIVER.COM | 53

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