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Voice of the Editor
Professor Colleen E. Clancy, Editor-in-Chief of The Journal of Precision Medicine: Health and Disease
Diving, human physiology under stress and the role of precision medicine offering insights into the underlying mechanisms governing individual responses to the underwater extreme environment. Professor Colleen Clancy tells us more.
Professor Colleen E. Clancy
Fundamental principles of physiology can be revealed in extreme environments even when not visible under ordinary conditions. Diving to depth is a prime example of an extreme environment in which human physiology is under stress. During diving descent and ascent humans experience rapid changes in pressure that alter the behaviour of dissolved gases within tissues and blood. For more than a century, physiologists, have studied human responses to depth with a particular focus on the risks of gas toxicity. Interestingly, there is a strong precision medicine element to gas toxicity for divers under pressure at depth: Two divers following the same dive profile can experience very different outcomes, where one diver experiences gas toxicity and the other experiences no problem.
Under pressure
The most common concern for new and experienced divers is nitrogen toxicity, which is experienced as nitrogen narcosis during deep or long dives that may impair cognitive and motor function, affect judgment and coordination. Pressure at depth increases the amount of nitrogen that dissolves in body tissues. Excess dissolved nitrogen in the blood may come out of solution during rapid ascent, forming bubbles in tissues and blood, leading to decompression sickness (“the bends”). As a diver returns toward the surface, ambient pressure falls and dissolved gas may leave solution. If elimination through the lungs cannot keep pace with the reduction in pressure, bubbles may form within blood or tissues leading to decompression sickness that occurs when inert gas dissolved in tissues forms bubbles during ascent.
Oxygen toxicity is less common but can occur in the most extreme diving environments when oxygen is breathed at high partial pressures, potentially leading to symptoms such as visual disturbances, seizures, or lung injury. The potential for gas toxicity highlights the importance of carefully managing gas composition, pressure, and ascent rates in high-pressure environments.
Early research into gas toxicity during diving and likelihood of decompression sickness has been assessed largely through population models. Physiologists developed tables that estimate how quickly nitrogen accumulates and clears from different tissue compartments. Dive schedules were then designed to limit bubble formation for the average diver. Such models have been remarkably effective and remain the foundation of safe diving practice.
Gas toxicity
In general, such an approach has allowed safe recreational and industrial diving for most divers, but not for all divers. Evidence abounds showing that identical dive profiles do not always produce identical outcomes. Some divers complete repeated exposures without difficulty, while others develop decompression sickness under seemingly conservative conditions. The discrepancy raises a broader physiological question. To what extent can biological responses be predicted using population averages?
Variation between individuals may arise from varied sources. Gas kinetics is one such source. Nitrogen dissolves readily in lipid rich tissues, and individuals with greater adipose mass may accumulate larger inert gas loads during a dive. Circulatory dynamics also influence how quickly dissolved gases move between tissues and the lungs. Small differences in perfusion can therefore alter nitrogen uptake and elimination. Cardiovascular anatomy is also a key indicator of proclivity to diving induced gas toxicity. Approximately one quarter of adults have a patent foramen ovale (PFO), an opening between the right and left atria that persists as a congenital remnant of fetal circulation. Venous gas bubbles normally pass through the lungs, where they are filtered before reaching the arterial circulation. In the presence of a patent foramen ovale, bubbles may bypass pulmonary filtration and enter the arterial system directly and increase the probability that bubbles reach the brain or spinal cord.
Biological responses to bubble formation
Vascular biology may also influence gas toxicity and decompression risk. Experimental studies show that bubble formation is more likely to occur on microscopic nucleation sites associated with biological surfaces. Endothelial function and nitric oxide signaling appear to affect both bubble formation and vascular response. Some divers consistently produce fewer detectable bubbles after identical dives, suggesting that biological characteristics of the vascular system modify decompression dynamics. Hydration and circulating blood volume provide further sources of vascular variation. Reduced plasma volume may slow inert gas elimination and increase blood viscosity, allowing bubbles to persist longer in the circulation. Differences in hydration status between divers may therefore influence risk even when dive profiles are identical.
Finally, metabolic and inflammatory responses of individuals comprise and additional dimension of variability. Bubble formation alone does not determine the severity of decompression illness, rather the biological responses to bubbles including inflammatory activation, platelet aggregation, and endothelial injury contribute to clinical symptoms. Divergent responses among individuals suggest that decompression sickness reflects an interaction between physical gas behavior and physiological regulation.
Diving insights shaping precision human physiology
Diving physiology is an illustrative topic for precision medicine, where identical environmental exposure among individuals does not produce a uniform physiological outcome. Traditional decompression models assume that divers behave as interchangeable units whose responses can be approximated by average parameters. Increasing evidence indicates that individual biology plays a substantial role. Precision medicine seeks to understand why individuals respond differently to comparable physiological stress and diving offers a natural laboratory to study individual differences.
Environmental conditions can be clearly documented, dive profiles can be measured with high accuracy, as most dive operations require utilisation of dive computers that track dive profiles in detail. High dimensional analyses of differences between divers and profiles may be useful to predict biological factors that determine proclivity or resilience to gas toxicity. Extreme environments, including the underwater world explored by divers, is a rich source of data and resulting insight from these data are likely to offer important insights into the underlying mechanisms that govern precision human physiology.
Find out more about The Journal of Precision Medicine: Health and Disease and the journal’s latest call for papers.
