Blood biochemical factors in humans resistant and susceptible to formation of venous gas emboli during decompression

Plasma Proteomics-Based Discovery of Mechanistic Biomarkers of Hyperbaric Stress and Pulmonary Oxygen Toxicity

Our aim was to identify proteins that reflect an acute systemic response to prolonged hyperbaric stress and discover potential biomarker pathways for pulmonary O2 toxicity. The study was a double-blind, randomized, crossover design in trained male Navy diver subjects. Each subject completed two dry resting hyperbaric chamber dives separated by a minimum of one week. One dive exposed the subject to 6.5 h of 100% oxygen (O2) at 2ATA. The alternate dive exposed the subjects to an enhanced air nitrox mixture (EAN) containing 30.6% O2 at the same depth for the same duration. Venous blood samples collected before (PRE) and after (POST) each dive were prepared and submitted to LC-MS/MS analysis (2 h runs). A total of 346 total proteins were detected and analyzed. A total of 12 proteins were significantly increased at EANPOST (vs. EANPRE), including proteins in hemostasis and immune signaling and activation. Significantly increased proteins at O2PRE (vs. O2POST) included neural cell adhesion molecule 1, glycoprotein Ib, catalase, hemoglobin subunit beta, fibulin-like proteins, and complement proteins. EANPOST and O2POST differed in biomarkers related to coagulation, immune signaling and activation, and metabolism. Of particular interest is (EANPOST vs. O2POST), which is protective against oxidative stress.

Exhaled breath condensate profiles of U.S. Navy divers following prolonged hyperbaric oxygen (HBO) and nitrogen-oxygen (Nitrox) chamber exposures

Prolonged exposure to hyperbaric hyperoxia can lead to pulmonary oxygen toxicity (PO2tox). PO2tox is a mission limiting factor for special operations forces divers using closed-circuit rebreathing apparatus and a potential side effect for patients undergoing hyperbaric oxygen (HBO) treatment. In this study, we aim to determine if there is a specific breath profile of compounds in exhaled breath condensate (EBC) that is indicative of the early stages of pulmonary hyperoxic stress/PO2tox. Using a double-blind, randomized 'sham' controlled, cross-over design 14 U.S. Navy trained diver volunteers breathed two different gas mixtures at an ambient pressure of 2 ATA (33 fsw, 10 msw) for 6.5 h. One test gas consisted of 100% O2(HBO) and the other was a gas mixture containing 30.6% O2with the balance N2(Nitrox). The high O2stress dive (HBO) and low O2stress dive (Nitrox) were separated by at least seven days and were conducted dry and at rest inside a hyperbaric chamber. EBC samples were taken immediately before and after each dive and subsequently underwent a targeted and untargeted metabolomics analysis using liquid chromatography coupled to mass spectrometry (LC-MS). Following the HBO dive, 10 out of 14 subjects reported symptoms of the early stages of PO2tox and one subject terminated the dive early due to severe symptoms of PO2tox. No symptoms of PO2tox were reported following the nitrox dive. A partial least-squares discriminant analysis of the normalized (relative to pre-dive) untargeted data gave good classification abilities between the HBO and nitrox EBC with an AUC of 0.99 (±2%) and sensitivity and specificity of 0.93 (±10%) and 0.94 (±10%), respectively. The resulting classifications identified specific biomarkers that included human metabolites and lipids and their derivatives from different metabolic pathways that may explain metabolomic changes resulting from prolonged HBO exposure.

Post-dive ultrasound detection of gas in the liver of rats and scuba divers

In a previous study, we obtained histologic documentation of liver gas embolism in the rat model of rapid decompression. The aim of the study was to assess in the same model occurrence and time course of liver embolism using 2-D ultrasound imaging, and to explore by this means putative liver gas embolism in recreational scuba divers. Following 42 min compression at 7 ATA breathing air and 12 min decompression, eight surviving female rats were anesthetized and the liver imaged by ultrasound at 20 min intervals up to 120 min. A significant enhancement of echo signal was recorded from 60 to 120 min as compared to earlier post-decompression times. Enzymatic markers of liver damage (AST, ALT, and GGT) increased significantly at 24 h upon decompression. Twelve healthy experienced divers were studied basally and at 15-min intervals up to 60 min following a 30-min scuba dive at 30 msw depth. At 30 min upon surfacing echo images showed significant signal enhancement that progressed and reached plateau at 45 and 60 min. Total bilirubin at 24 h increased significantly (p = 0.02) with respect to basal values although within the reference range. In conclusion, 2-D ultrasound liver imaging allowed detection of gas embolism in the rat and defined the time course of gas accumulation. Its application to scuba divers revealed liver gas accumulation in all subjects in the absence of clear-cut evidence of liver damage or of any symptom. The clinical significance of our findings remains to be investigated.

Gas embolization of the liver in a rat model of rapid decompression

Occurrence of liver gas embolism after rapid decompression was assessed in 31 female rats that were decompressed in 12 min after 42 min of compression at 7 ATA (protocol A). Sixteen rats died after decompression (group I). Of the surviving rats, seven were killed at 3 h (group II), and eight at 24 h (group III). In group I, bubbles were visible in the right heart, aortic arch, liver, and mesenteric veins and on the intestinal surface. Histology showed perilobular microcavities in sinusoids, interstitial spaces, and hepatocytes. In group II, liver gas was visible in two rats. Perilobular vacuolization and significant plasma aminotransferase increase were present. In group III, liver edema was evident at gross examination in all cases. Histology showed perilobular cell swelling, vacuolization, or hydropic degeneration. Compared with basal, enzymatic markers of liver damage increased significantly. An additional 14 rats were decompressed twice (protocol B). Overall mortality was 93%. In addition to diffuse hydropic degeneration, centrilobular necrosis was frequently observed after the second decompression. Additionally, 10 rats were exposed to three decompression sessions (protocol C) with doubled decompression time. Their mortality rate decreased to 20%, but enzymatic markers still increased in surviving rats compared with predecompression, and perilobular cell swelling and vacuolization were present in five rats. Study challenges were 1) liver is not part of the pathophysiology of decompression in the existing paradigm, and 2) although significant cellular necrosis was observed in few animals, zonal or diffuse hepatocellular damage associated with liver dysfunction was frequently demonstrated. Liver participation in human decompression sickness should be looked for and clinically evaluated.

Investigating the potential of statin medications as a nitric oxide (NO) release agent to decrease decompression sickness: A review article

Understanding the biochemical mechanisms influencing bubble pathophysiology may foster novel pharmacologic non-recompressive strategies that may prevent, ameliorate, and treat decompression sickness (DCS), and the injury sustained from arterial gas emboli (AGE) encountered in hyperbaric and hypobaric exposures, as well as in surgery and trauma. This review explores the biochemical effects of nitric oxide (NO) release agents, their potential impact on bubble pathophysiology, and possible use as a pharmacological intervention to reduce DCS risk and AGE injury. The hypotheses discussed contend that exogenous NO administration or mediators of endogenous NO up-regulation may reduce DCS risk and severity by mediating; (1) decreased populations of gaseous nuclei, (2) decreased bubble nuclei adherence, (3) depression of the deleterious bubble-mediated inflammatory and coagulation cascades and (4) preservation of endothelial integrity, which may defend against bubble-mediated injury. Statin medications alter numerous biochemical, and biophysical processes, which may influence bubble formation. Statins preserve endothelial integrity, reduce ischemia/reperfusion injury, and depress the interdependent inflammatory and coagulation cascades via pleiotropic properties involving up-regulation of endothelial nitric oxide synthase (eNOS) and NO. Numerous studies are researching statins, for their potential efficacy in reducing primary and secondary morbidity and mortality from cardiocerebrovascular, inflammatory (autoimmune), and infectious (sepsis) disease. Additionally, statin-mediated lipid reduction may reduce bubble generation via alterations in plasma "rheology", and surface tension. The statins are attractive potential NO release with minimal adverse side effects, and proven long-term safety, that may potentially mitigate the risk and severity of DCS. We will elaborate on the insight gained into the mechanisms proven and hypothesized for NO-mediated reductions in bubble formation, and DCS incidence and severity, with a focus on the potential for statin medications, in addition to the direct NO-donor medications such as isosorbide mononitrate and nitroglycerine.

The physiological kinetics of nitrogen and the prevention of decompression sickness

Decompression sickness (DCS) is a potentially crippling disease caused by intracorporeal bubble formation during or after decompression from a compressed gas underwater dive. Bubbles most commonly evolve from dissolved inert gas accumulated during the exposure to increased ambient pressure. Most diving is performed breathing air, and the inert gas of interest is nitrogen. Divers use algorithms based on nitrogen kinetic models to plan the duration and degree of exposure to increased ambient pressure and to control their ascent rate. However, even correct execution of dives planned using such algorithms often results in bubble formation and may result in DCS. This reflects the importance of idiosyncratic host factors that are difficult to model, and deficiencies in current nitrogen kinetic models. Models describing the exchange of nitrogen between tissues and blood may be based on distributed capillary units or lumped compartments, either of which may be perfusion- or diffusion-limited. However, such simplistic models are usually poor predictors of experimental nitrogen kinetics at the organ or tissue level, probably because they fail to account for factors such as heterogeneity in both tissue composition and blood perfusion and non-capillary exchange mechanisms. The modelling of safe decompression procedures is further complicated by incomplete understanding of the processes that determine bubble formation. Moreover, any formation of bubbles during decompression alters subsequent nitrogen kinetics. Although these factors mandate complex resolutions to account for the interaction between dissolved nitrogen kinetics and bubble formation and growth, most decompression schedules are based on relatively simple perfusion-limited lumped compartment models of blood: tissue nitrogen exchange. Not surprisingly, all models inevitably require empirical adjustment based on outcomes in the field. Improvements in the predictive power of decompression calculations are being achieved using probabilistic bubble models, but divers will always be subject to the possibility of developing DCS despite adherence to prescribed limits.

Fatty acid composition of plasma lipids and erythrocyte membranes during simulated extravehicular activity

Ten subjects (from 27 to 41 years) have been participated in 32 experiments. They were decompressed from ground level to 40-35 kPa in altitude chamber when breathed 100% oxygen by mask and performed repeated cycles of exercises (3.0 Kcal/min). The intervals between decompressions were 3-5 days. Plasma lipid and erythrocyte membrane fatty acid composition was evaluated in the fasting venous blood before and immediately after hypobaric exposure. There were 7 cases decompression sickness (DCS). Venous gas bubbles (GB) were detected in 27 cases (84.4%). Any significant changes in the fatty acid composition of erythrocyte membranes and plasma didn't practically induce after the first decompression. However, by the beginning of the second decompression the total lipid level in erythrocyte membranes decreased from 54.6 mg% to 40.4 mg% in group with DCS symptoms and from 51.2 mg% to 35.2 mg% (p<0.05) without DCS symptoms. In group with DCS symptoms a tendency to increased level of saturated fatty acids in erythrocyte membranes (16:0, 18:0), the level of the polyunsaturated linoleic fatty acid (18:2) and arachidonic acid (20:4) tended to be decreased by the beginning of the second decompression. Insignificant changes in blood plasma fatty acid composition was observed in both groups. The obtained biochemical data that indicated the simulated extravehicular activity (EVA) condition is accompanied by the certain changes in the blood lipid metabolism, structural and functional state of erythrocyte membranes, which are reversible. The most pronounced changes are found in subjects with DCS symptoms.

Gas Embolism: Part II. Arterial Gas Embolism and Decompression Sickness

Gas emboli syndromes are known to occur in many different settings, and they may result in life-threatening emergencies. Venous gas embolization was discussed previously in Part I of this review. Gas emboli that gain access to the arterial circulation or that result from exposures to decreased ambient pressures in the environment are discussed in Part II. The prevalence of arterial gas emboli and decompression sickness are likely not as high as for venous gas emboli. Most cases are preventable, and prompt treatment is frequently effective. Once present, gas bubbles generally distribute themselves throughout the body based on the relative blood flow at the time, thus making the nervous system, heart, lung, and skin the primary organ systems involved. Both mechanical and biophysical effects lead to intravascular and extracellular alterations that result in tissue injury. The clinical manifestations of these disorders are varied, and a high index of suspicion in the appropriate settings will aid health care providers in prompt recognition of these problems and allow timely intervention with specific therapy. Management of arterial gas emboli and decompression sickness is similar, with a focus on hyberbaric chamber therapy and intermittent hyperoxygenation. Recompression schedules in current use have withstood the test of time. Research continues to refine our understanding of these diseases and to optimize the treatment regimens available.

Blood biochemical and cellular changes during decompression and simulated extravehicular activity

Blood biochemical and cellular parameters were measured in human subjects before and after exposure to a decompression schedule involving 6 h of oxygen prebreathing. The exposure was designed to simulate extravehicular activity for 6 h (subjects performed exercise while exposed to 29.6 kPa). There were no significant differences between blood samples from subjects who were susceptible (n = 11) versus those who were resistant (n = 27) to formation of venous gas emboli. Although several statistically significant (P less than 0.05) changes in blood parameters were observed following the exposure (increases in white blood cell count, prothrombin time, and total bilirubin, and decreases in triglycerides, very-low-density lipoprotein cholesterol, and blood urea nitrogen), the changes were small in magnitude and blood factor levels remained within normal clinical ranges. Thus, the decompression schedule used in this study is not likely to result in blood changes that would pose a threat to astronauts during extravehicular activity.

Effects of exercise on the incidence of decompression sickness: a review of pertinent literature and current concepts

The effects of exercise on the incidence of decompression sickness (DCS) are not completely understood. This paper reviews studies that have addressed this question. Studies have involved exposures of animals and human subjects to high pressures, which would occur in SCUBA diving operations, and to low pressures, which exist during high-altitude aircraft flights and extravehicular activities during space flight. The temporal course of the exercise in relation to the decompression procedure and the nature of the exercise may influence effects on DCS incidence. Effects of exercise on the uptake and elimination of nitrogen, the production of bubbles due to limb movements, the potential role of carbon dioxide, and current plans for further research are discussed.

Hematological changes following repetitive decompressions during simulated extravehicular activity

A study was performed to evaluate decompression procedures suggested for use prior to Space Shuttle extravehicular activity. Hematological parameters were measured in 12 male human subjects before and after exposure in an altitude chamber to a 3-day staged decompression schedule, with simulated extravehicular activity. Following the exposure, significant increases occurred in white blood cell count and activated partial thromboplastin time, and platelet aggregate ratio was significantly decreased. Pre-exposure samples from subjects who were susceptible to formation of venous gas emboli (VGE) exhibited a significantly lower degree of ADP-induced platelet aggregation and a significantly higher amount of lymphocyte blastogenic transformation in response to the mitogen phytohemagglutinin than samples from VGE-resistant subjects. The results indicate that, following this decompression profile, small but significant changes occur in several hematological parameters, and that levels of certain parameters may be related to susceptibility to VGE formation during decompression.