Air Dives Research Articles

Selecting optimal air diving gradient factors for Belgian military divers: more conservative settings are not necessarily safer.

In 2018, the Belgian Defence introduced a commercial off-the-shelf dive computer (Shearwater Perdix™) for use by its military divers. There were operational constraints when using its default gradient factors (GF). We aimed to provide guidelines for optimal GF selection. The Defence and Civil Institute of Environmental Medicine (DCIEM) dive tables and the United States Navy (USN) air decompression tables are considered acceptably safe by the Belgian Navy Diving Unit. The decompression model used in the Shearwater Perdix (Bühlmann ZH-L16C algorithm with GF) was programmed in Python. Using a sequential search of the parameter space, the GF settings were optimised to produce decompression schedules as close as possible to those prescribed by the USN and DCIEM tables. All reference profiles are approached when GFLO is kept equal to 100 and only GFHI is reduced to a minimum of 75 to prolong shallower stop times. Using the Perdix default settings (GFLO = 30 and GFHI = 70) yields deeper initial stops, leading to increased supersaturation of the 'slower' tissues, which potentially leads to an increased DCS risk. However, Perdix software does not currently allow for the selection of our calculated optimal settings (by convention GFLO < GFHI). A sub-optimal solution would be a symmetrical GF setting between 75/75 and 95/95. For non-repetitive air dives, the optimal GF setting is GFLO 100, with only the GFHI parameter lowered to increase safety. No evidence was found that using the default GF setting (30/70) would lead to a safer decompression for air dives as deep as 60 metres of seawater; rather the opposite. Belgian Navy divers have been advised against using the default GF settings of the Shearwater Perdix dive computer and instead adopt symmetrical GF settings which is currently the optimal achievable approach considering the software constraints.

Proposed Thalmann Algorithm Air Diving Decompression Table for the Swedish Armed Forces Based on Logistic Probabilistic Modeling of No-Stop Air Diving Data

The Swedish Armed Forces (SwAF) air dive tables are under revision. Currently, the air dive table from US Navy (USN) Diving manual (DM) Rev.6 is used with a msw to fsw conversion. Since 2017, USN has been diving according to USN DM rev. 7, which incorporates updated air dive tables derived from the exponential linear decompression algorithm (EL-DCM) Thalmann with VVAL79 parameters. The SwAF decided to replicate and analyze the USN table development methodology before revising their current tables. The ambition was to potentially find a table that correlates with the desired risk of decompression sickness. New compartmental parameters for the EL-DCM algorithm, called SWEN21B, were developed by applying maximum likelihood methods on 2953 scientifically controlled direct ascent air dives with known outcomes of decompression sickness (DCS). The targeted probability of DCS for direct ascent air dives was ≤1% overall and ≤1‰ for neurological DCS (CNS-DCS). 154 wet validation dives were performed with air between 18-57 msw. Both direct ascent and decompression stop dives were conducted, resulting in two joint pain DCS (18msw/59min), one leg numbness CNS-DCS (51msw/10min with deco-stop), and nine marginal DCS, such as rashes and itching. A total of three DCS, including one CNS-DCS, yield a predicted risk level (95% confidence interval) of 0.4-5.6% for DCS and 0.0-3.6% for CNS-DCS. Two out of three divers with DCS had patent foramen ovale, PFO. The SWEN21 table is recommended for the SwAF for air diving as it, after results from validation dives, suggests being within the desired risk levels for DCS and CNS-DCS.

The protective effects of 1,3-butanediol acetoacetate diester on decompression sickness in rats.

Inert gas bubbles are widely accepted as the causative factor of decompression sickness (DCS), resulting in gas embolism and systemic inflammatory responses. The anticonvulsive ketone ester 1,3-butanediol acetoacetate diester (BD-AcAc2) was reported to have the characteristics of increasing blood oxygen partial pressure (ppO2) and anti-inflammation and was thought to have the potential to reduce bubble formation and alleviate the pathological process of DCS. This study aims to investigate the potential protection of BD-AcAc2 against DCS in a rat model. A single dose of BD-AcAc2 was administered orally to adult male rats (5 g/kg body wt), followed by pharmacokinetic analysis or simulated air dives. After decompression, signs of DCS were monitored, and blood was sampled for biochemical measurements. Blood ketosis peaked at 2 h and lasted for more than 4 h. The incidence of DCS was decreased and postponed significantly in rats treated with BD-AcAc2 compared with those treated with saline (P < 0.05). Although BD-AcAc2 failed to reduce bubble load (P > 0.05), it showed an obvious decreasing trend. BD-AcAc2 significantly increased blood ppO2 and ameliorated oxidative and inflammatory responses, represented by increased plasma malondialdehyde (MDA), IL-1, IL-6, and TNF-α and decreased glutathione thiol (P < 0.05) levels, whereas blood pH remained unchanged (P > 0.05). These results suggest that BD-AcAc2 exerted beneficial effects on DCS rats mainly related to increasing ppO2 and anti-inflammatory and antioxidant properties. Together with its capacity for delaying central nervous system (CNS) oxygen toxicity seizures, BD-AcAc2 might be an ideal drug candidate for DCS prevention and treatment.NEW & NOTEWORTHY This is the first study exploring the effects of BD-AcAc2 on DCS prevention, and it was proven to be an efficient and simple method. The role of BD-AcAc2 in increasing ppO2, anti-inflammatory and antioxidant properties was thought to be the critical mechanism in DCS prevention.

Pre-hydration strongly reduces decompression sickness occurrence after a simulated dive in the rat.

Hydration status is considered a parameter likely to influence the risk of decompression sickness (DCS), but scientific evidence is scarce and conflicting. This experiment aimed to analyse the influence of pre-hydration on DCS occurrence in a rat model. Intra-peritoneal injections of saline solution were administered to rats (NaCl 0.9% 0 ml (Control), 0.1 ml (Group 1), or 1 ml·100g-1 body mass (Group 2) at each of 24 h, 12 h, and 30 min prior to simulated air dives (45 min at 1,010 kPa; compression and decompression rates 101 kPa·min-1; stops 5 min at 202 kPa, 5 min at 160 kPa, 10 min at 130 kPa). Evaluation of DCS occurrence and severity was made after decompression. Pre-dive hydration reduced severe DCS from 47% (Control) to 29% (Group 1) and 0% (Group 2), and increased the proportion of animals without any signs of DCS from 40 (Control) to 57% (Group 1) and 93% (Group 2); Chi2 P = 0.041. This experiment demonstrated that pre-hydration can drastically reduce the DCS occurrence in an animal model. In the context of scuba diving, this result highlights the importance of elucidating the mechanisms linking hydration status and DCS risk.

The effects of heliox non-saturation diving on the cardiovascular system and cognitive functions

The purpose of this study was to investigate the effects of a single bout of heliox non-saturation diving on the cardiovascular system and cognitive function. Ten recreational scuba divers (10 males, ∼35 years old) participated in this study. These subjects made two pool dives within a one-week interval, alternating gases with compressed air (21% O2, 79% N2) and with heliox (21% O2 and 79% He). The depth was to 26 meters over a 20-minute duration. The results showed that heliox diving significantly increased blood O2 saturation by 1.15% and significantly decreased blood lactate levels by ∼57% when compared with air diving (P ≺ 0.05). However, there were no significant differences in resting heart rate, systolic or diastolic pressure, core body blood pressure, and pulse wave velocity between the heliox and air dives. The Stroop test showed that the heliox dive significantly increased cognitive function compared with the air dive in both the simple test (Offtime) and interference test (Ontime) (P ≺ 0.05). It was concluded that the heliox dive increases blood O2 saturation and decreases blood lactate concentration when compared with air dives. These conditions are likely to help divers reduce hypoxia in the water, reduce the risk of loss of consciousness, reduce fatigue and allow them to dive for longer. Heliox diving may also help judgment and risk coping skills in the water due to the improvement of cognitive ability as compared to air breathing dives.

Comparison of argon and air as thermal insulating gases in drysuit dives during military Arctic diving equipment development tests

Introduction: It is vital to protect divers from the cold, particularly in Arctic conditions. The insulating gas layer within the drysuit is crucial for reducing heat loss. The technical diving community has long claimed the superiority of argon over air as an insulating gas. Although argon is widely used, previous studies have shown no significant differences between the two gases. Owing to its lower heat conductivity, argon should be a better thermal insulating gas than air. Methods: The study aimed to determine whether argon is beneficial for reducing heat loss in divers during development of military drysuit diving equipment in Arctic water temperatures. Four divers completed 14 dives, each lasting 45 minutes: seven dives used air insulation and seven used argon insulation. Rectal and eight skin temperatures were measured from which changes in calculated mean body temperature (MBT) were assessed. Results: There was a significant reduction in area weighted skin temperature over time (0-45 minute) on air dives (ΔTskin = -4.16ÅãC, SE = 0.445, P &lt; 0.001). On argon dives the reduction was significantly smaller compared to air dives (difference between groups = 2.26ÅãC, SE = 0.358, P&lt;0.001). There were no significant changes in rectal temperatures, nor was a significant difference seen between groups. Conclusion: Compared to air, argon may be superior as a drysuit insulating gas in Arctic water temperatures for some divers. Argon used as insulating gas can make diving safer and may diminish the risks of fatal diving accidents and occupational hazard risks in professional diving.

Effect of helium preconditioning on neurological decompression sickness in rats.

Decompression sickness (DCS) occurs because of an excessively rapid and extensive reduction of the ambient pressure. Bubble-induced spinal cord ischemia is generally considered as a part of neurological DCS pathogenesis. Because helium preconditioning (HPC) recently demonstrated beneficial properties against ischemic damage, we hypothesized that HPC may decrease the neurological deficits of DCS in rats. Seventy-five male Sprague-Dawley rats were divided into a non-HPC group ( n = 25) and a HPC group ( n = 25) and 25 naive animals that were euthanized for histological examination ( n = 5) or anesthetized for baseline somatosensory evoked potential (SSEP) recordings ( n = 20). To induce DCS, rats were compressed with air to a pressure of 709 kPa for 60 min and decompressed at a rate of 203 kPa/min. HPC was administered as three episodes of 79% helium-21% oxygen mixture inhalation for 5 min interspersed with 5 min of air breathing. We found that HPC resulted in significantly decreased DCS incidence and delay of DCS onset. HPC also improved animal performance on the grip test after decompression and significantly ameliorated decompression-induced decrease of platelet number. Furthermore, the incidence of abnormal SSEP waves and histological spinal lesions was significantly reduced by HPC. We conclude that HPC can decrease the occurrence of DCS and ameliorate decompression-induced neurological deficits. NEW & NOTEWORTHY Helium preconditioning ameliorates decompression-induced neurological deficits in rats. Helium breathing before air dives may prevent neurological deficit and attenuate symptoms after decompression.

The probability and severity of decompression sickness.

Decompression sickness (DCS), which is caused by inert gas bubbles in tissues, is an injury of concern for scuba divers, compressed air workers, astronauts, and aviators. Case reports for 3322 air and N2-O2 dives, resulting in 190 DCS events, were retrospectively analyzed and the outcomes were scored as (1) serious neurological, (2) cardiopulmonary, (3) mild neurological, (4) pain, (5) lymphatic or skin, and (6) constitutional or nonspecific manifestations. Following standard U.S. Navy medical definitions, the data were grouped into mild—Type I (manifestations 4–6)–and serious–Type II (manifestations 1–3). Additionally, we considered an alternative grouping of mild–Type A (manifestations 3–6)–and serious–Type B (manifestations 1 and 2). The current U.S. Navy guidance allows for a 2% probability of mild DCS and a 0.1% probability of serious DCS. We developed a hierarchical trinomial (3-state) probabilistic DCS model that simultaneously predicts the probability of mild and serious DCS given a dive exposure. Both the Type I/II and Type A/B discriminations of mild and serious DCS resulted in a highly significant (p << 0.01) improvement in trinomial model fit over the binomial (2-state) model. With the Type I/II definition, we found that the predicted probability of ‘mild’ DCS resulted in a longer allowable bottom time for the same 2% limit. However, for the 0.1% serious DCS limit, we found a vastly decreased allowable bottom dive time for all dive depths. If the Type A/B scoring was assigned to outcome severity, the no decompression limits (NDL) for air dives were still controlled by the acceptable serious DCS risk limit rather than the acceptable mild DCS risk limit. However, in this case, longer NDL limits were allowed than with the Type I/II scoring. The trinomial model mild and serious probabilities agree reasonably well with the current air NDL only with the Type A/B scoring and when 0.2% risk of serious DCS is allowed.

Small Airway Function after Repeated Dives with Elevated PO2

Repetitive diving with mildly hyperbaric oxygen pressures (oxygen partial pressure (PO2) approximately 130 kPa) may cause small airway dysfunction, but the relative importance of immersion, PO2, and diver activity has not been elucidated. PURPOSE: To differentiate effects of immersion and elevated PO2 at rest and during exercise, parameters of small airway function were measured in conjunction with five series of dives: rest or cycle ergometer exercise in a 15-foot deep pool with O2 (PO2 = 130 kPa) or air (PO2 = 30 kPa), and rest in a dry hyperbaric chamber with PO2 = 130 kPa. METHODS: Each dive series consisted of five consecutive days with six hours of exposure and 18 hours between dives (same daily starting time). Forced expired volume in 1 second (FEV1), and maximum forced expired flow between 25% and 75% of volume expired (FEF25–75) were measured (CPL, nSpire Medical) the prior week and before and after each dive, plus on the morning after and three days after the fifth dive. The averages of three ATS-valid measures at each time were compared to the baseline average (pre-diving and pre-first dive). The numbers of divers for each condition were wet resting O2:10; wet resting air: 10; dry resting O2:12; wet exercise O2:10; wet exercise air: 14. RESULTS: After surfacing no parameters decreased from baseline. FEV1 and FEF25–75 increased after exercise air dives (Maximum changes: FEV1: 2.3%, FEF25–75: 10.2%, p< 0.001). However, FEV1 and FEF25–75 were low on mornings following resting and exercise O2 dives (Maximum changes: resting: FEV1 -5.2%, FEF25–75: −10.6%, both p< 0.01; exercise: FEV1: −5.2%, p < 0.02; FEF25–75: −15.4%, p< 0.001). Changes resolved after the next dive, but persisted on the third day after the fifth wet O2 dives: resting O2: FEV1 -4.2%, p<0.03; exercise O2: FEV1: −3.9%, p < 0.02; FEF25–75: −10%, p < 0.005. CONCLUSION: Decreases in indices of small airway function appear to follow from a combination of volume shifts (immersion) and elevated PO2. Effects are delayed after the end of exposure, suggesting inflammation. Without a later immersion, small airway function remains depressed for three days following wet O2 at rest or exercise, while volume shifts with elevated pulmonary blood flow (exercise) and minimally- elevated PO2 (air) appear to improve function. Supported by NAVSEA DSBDP and ONR.

Reactive Oxygen Species, Mitochondria, and Endothelial Cell Death during In Vitro Simulated Dives.

Excessive reactive oxygen species (ROS) is considered a consequence of hyperoxia and a major contributor to diving-derived vascular endothelial damage and decompression sickness. The aims of this work were: 1) to directly observe endothelial ROS production during simulated air dives as well as its relation with both mitochondrial activity and cell survival; and 2) to determine which ambient factor during air diving (hydrostatic pressure or oxygen and/or nitrogen partial pressure) is responsible for the observed modifications. In vitro diving simulation was performed with bovine arterial endothelial cells under real-time observation. The effects of air diving, hydrostatic, oxygen and nitrogen pressures, and N-acetylcysteine (NAC) treatment on mitochondrial ROS generation, mitochondrial membrane potential and cellular survival during simulation were investigated. Vascular endothelial cells performing air diving simulation suffered excessive mitochondrial ROS, mitochondrial depolarization, and cell death. These effects were prevented by NAC: after NAC treatment, the cells presented no difference in damage from nondiving cells. Oxygen diving showed a higher effect on ROS generation but lower impacts on mitochondrial depolarization and cell death than hydrostatic or nitrogen diving. Nitrogen diving had no effect on the inductions of ROS, mito-depolarization, or cell death. This study is the first direct observation of mitochondrial ROS production, mitochondrial membrane potential and cell survival during diving. Simulated air SCUBA diving induces excessive ROS production, which leads to mitochondrial depolarization and endothelial cell death. Oxygen partial pressure plays a crucial role in the production of ROS. Deleterious effects of hyperoxia-induced ROS are potentiated by hydrostatic pressure. These findings hold new implications for the pathogenesis of diving-derived endothelial dysfunction.

Hyperbaric oxygen diving affects exhaled molecular profiles in men

Exhaled breath contains volatile organic compounds (VOCs) that are associated with respiratory pathophysiology. We hypothesized that hyperbaric oxygen exposure (hyperoxia) generates a distinguishable VOC pattern. This study aimed to test this hypothesis in oxygen-breathing divers. VOCs in exhaled breath were measured in 10 male divers before and 4h after diving to 9msw (190kPa) for 1h. During the dive they breathed 100% oxygen or air in randomized order. VOCs were determined using two-dimensional gas chromatography with time-of-flight mass spectrometry. Compared to air dives, after oxygen dives there was a significant increase in five VOCs (predominately methyl alkanes). Furthermore, a strong, positive correlation was found between increments in 2,4-dimethyl-hexane and those of 4-ethyl-5-methyl-nonane. Although non-submerged hyperoxia studies on VOCs have been performed, the present study is the first to demonstrate changes in exhaled molecular profiles after submerged oxygen diving. The pathophysiological background might be attributed to either a lipid peroxidation-induced pathway, an inflammatory pathway, or to both.

Nitric oxide and carbon monoxide diffusing capacity after a 1‐h oxygen dive to 9 m of sea water

To prevent extensive pulmonary lesions in submerged oxygen divers lung function like the forced vital capacity (FVC) or the diffusing capacity for carbon monoxide (DL,co) are used to monitor pulmonary oxygen toxicity (POT). As the diffusing capacity for nitric oxide (DL,no) measures more accurately the membrane diffusing capacity compared to DL,co we hypothesized that DL,no is superior in monitoring the onset of physiological changes indicative of POT as compared to DL,co or FVC. 26 healthy divers (mean age 30.7 ± 6.2 years) made two submerged dives to 190 kPa for 1 h on two randomized separate days, whilst breathing 100% oxygen or compressed air. FVC, DL,no, DL,co and alveolar volume (VA ) were measured 6 times during a 26-h period. Up to 8 h no significant differences in outcomes were found between the oxygen and air dives. However, at 8 h after the oxygen dives there was a significant reduction in DL,no, DL,co and VA as compared with air dives. In contrast, the reduction in FVC was significantly greater after the air dive. At 22 h there were no longer differences in outcomes between the dives. These data show that DL,no and DL,co are significantly reduced 8 h after submerged oxygen dives as compared to similar air dives. Together with the reduction in VA this may be indicative of interstitial edema as an early sign of POT. Our data warrant validation of the superiority of DL,no and DL,co over FVC in the practical monitoring of divers.

Measuring postural-related changes of spontaneous baroreflex sensitivity after repeated long-duration diving: Frequency domain approaches

Sustained water immersion is thought to modulate orthostatic tolerance to an extent dependent on the duration and repetition over consecutive days of the diving sessions. We tested this hypothesis investigating in ten healthy subjects the potential changes in the cardiovascular response to head-up tilt induced by single and multiple resting air dives. Parametric cross-spectral analysis of spontaneous RR interval and systolic arterial pressure variability was performed in three experimental sessions: before diving (BD), after single 6-hour dive (ASD), and after multiple 6-hour dives (AMD, 5 consecutive days with 18-hour surface interval). From this analysis, baroreflex sensitivity (BRS) was computed as spectral power ratio (αBRS), non-causal transfer function gain (tfBRS) and causal transfer function gain (γBRS) evaluated at low frequency (0.04-0.14Hz) in the supine position (su) as well as in the standing upright position in the early tilt (et) and late tilt (lt) epochs. We found that, while αBRS decreased significantly in et and lt compared to su during all sessions, tfBRS and γBRS decreased during ASD and AMD but not during BD; moreover γBRS evidenced a progressive decrease from BD to ASD and to AMD in both et and lt epochs. These results indicate the necessity of following a causal approach for the estimation of BRS in the frequency domain, and suggest a progressive impairment of the baroreflex response to postural stress after single and multiple dives, which may reflect symptoms of increasing orthostatic intolerance.

Body fat does not affect venous bubble formation after air dives of moderate severity: theory and experiment

For over a century, studies on body fat (BF) in decompression sickness and venous gas embolism of divers have been inconsistent. A major problem is that age, BF, and maximal oxygen consumption (Vo2max) show high multicollinearity. Using the Bühlmann model with eight parallel compartments, preceded by a blood compartment in series, nitrogen tensions and loads were calculated with a 40 min/3.1 bar (absolute) profile. Compared with Haldanian models, the new model showed a substantial delay in N2 uptake and (especially) release. One hour after surfacing, an increase of 14-28% in BF resulted in a whole body increase of the N2 load of 51%, but in only 15% in the blood compartment. This would result in an increase in the bubble grade of only 0.01 Kisman-Masurel (KM) units at the scale near KM = I-. This outcome was tested indirectly by a dry dive simulation (air breathing) with 53 male divers with a small range in age and Vo2max to suppress multicollinearity. BF was determined with the four-skinfold method. Precordial Doppler bubble grades determined at 40, 80, 120, and 160 min after surfacing were used to calculate the Kisman Integrated Severity Score and were also transformed to the logarithm of the number of bubbles/cm(2) (logB). The highest of the four scores yielded logB = -1.78, equivalent to KM = I-. All statistical outcomes of partial correlations with BF were nonsignificant. These results support the model outcomes. Although this and our previous study suggest that BF does not influence venous gas embolism (Schellart NAM, van Rees Vellinga TP, van Dijk FH, Sterk W. Aviat Space Environ Med 83: 951-957, 2012), more studies with different profiles under various conditions are needed to establish whether BF remains (together with age and Vo2max) a basic physical characteristic or will become less important for the medical examination and for risk assessment.

Effects of successive air and nitrox dives on human vascular function

SCUBA diving is regularly associated with asymptomatic changes in cardiac, pulmonary and vascular function. The aim of this study was to evaluate the changes in vascular/endothelial function following SCUBA diving and to assess the potential difference between two breathing gases: air and nitrox 36 (36% oxygen and 64% nitrogen). Ten divers performed two 3-day diving series (no-decompression dive to 18 m with 47 min bottom time with air and nitrox, respectively), with 2 weeks pause in between. Arterial/endothelial function was assessed using SphygmoCor and flow-mediated dilation measurements, and concentration of nitrite before and after diving was determined in venous blood. Production of nitrogen bubbles post-dive was assessed by ultrasonic determination of venous gas bubble grade. Significantly higher bubbling was found after all air dives as compared to nitrox dives. Pulse wave velocity increased slightly (~6%), significantly after both air and nitrox diving, indicating an increase in arterial stiffness. However, augmentation index became significantly more negative after diving indicating smaller wave reflection. There was a trend for post-dive reduction of FMD after air dives; however, only nitrox diving significantly reduced FMD. No significant differences in blood nitrite before and after the dives were found. We found that nitrox diving affects systemic/vascular function more profoundly than air diving by reducing FMD response, most likely due to higher oxygen load. Both air and nitrox dives increased arterial stiffness, but decreased wave reflection suggesting a decrease in peripheral resistance due to exercise during diving. These effects of nitrox and air diving were not followed by changes in plasma nitrite.

The effects of successive air and trimix dives on human cardiovascular function

The use of trimix has significantly increased among diver population. However, the data indicating how the field trimix dives at most common depths affect the cardiovascular function are sparse. The purpose of this study was to investigate the cardiovascular effects of trimix dives and compare them with the air dives, as well as determine whether the repetition of dives in successive days affects their extent. Eight professional divers performed four dives in consecutive days where the dive depth was progressively increased to the maximum of 55 m, and the divers used air in the 1st dive, nitrox 25 for the 2nd, and trimix 20/30 for the 3rd and 4th dive. After each dive, a significantly decreased left ventricular ejection fraction and fractional shortening, as well as increased end‐systolic volume without a change in end‐diastolic volume were found, indicating a depressed systolic function of the left heart. Assessment of the ratio between pulmonary artery acceleration time and right ventricular ejection time revealed an increase in PAP after all the dives. The current study shows that the cardiovascular effects of trimix dives do not differ from those of the dives with compressed air. However, it suggests that even a very safe and conservative trimix diving profile exerts significant cardiovascular effects.This study was supported by the Croatian Ministry of Science, Education and Sports.

Respiratory pattern after wet and dry chamber dives to 0.6 MPa ambient pressure in healthy males

The purpose of this study was to evaluate respiratory effects of wet and dry hyperbaric chamber dives to 0.6 MPa ambient pressure in healthy males. There were 19 and 22 subjects who participated in two series of dives with a bottom time of 15 min and decompression times of 28 and 17 min, respectively. Airways conductance, residual volume, forced vital capacity, forced expiratory volume in 1 sec, mid expiratory flow at 25, 50 and 75% of FVC, and diffusion capacity for CO were measured before the dives, after 3 h, and after 24 h. Multivariate analysis of covariance revealed no statistically significant effects of time or the interaction between time and dry or wet environment on the measured lung function parameters. These findings suggest first that even deep air dives may not necessarily affect pulmonary function, and second, that factors related to the particular wet environment do not seem to contribute to lung function changes after dives.

A model of extravascular bubble evolution: effect of changes in breathing gas composition.

Observations of bubble evolution in rats after decompression from air dives (O. Hyldegaard and J. Madsen. Undersea Biomed. Res. 16: 185-193, 1989; O. Hyldegaard and J. Madsen. Undersea Hyperbaric Med. 21: 413-424, 1994; O. Hyldegaard, M. Moller, and J. Madsen. Undersea Biomed. Res. 18: 361-371, 1991) suggest that bubbles may resolve more safely when the breathing gas is a heliox mixture than when it is pure O(2). This is due to a transient period of bubble growth seen during switches to O(2) breathing. In an attempt to understand these experimental results, we have developed a multigas-multipressure mathematical model of bubble evolution, which consists of a bubble in a well-stirred liquid. The liquid exchanges gas with the bubble via diffusion, and the exchange between liquid and blood is described by a single-exponential time constant for each inert gas. The model indicates that bubbles resolve most rapidly in spinal tissue, in adipose tissue, and in aqueous tissues when the breathing gas is switched to O(2) after surfacing. In addition, the model suggests that switching to heliox breathing may prolong the existence of the bubble relative to breathing air for bubbles in spinal and adipose tissues. Some possible explanations for the discrepancy between model and experiment are discussed.

Complement activation in divers after repeated air/heliox dives and its possible relevance to DCS

Plasma levels of the anaphylatoxin C5a were measured in 19 divers performing repeated air dives. Blood samples were collected immediately before the first dive and 2 h after the first and the second or third dive. Serum obtained at the same times was subjected to complement activation in vitro by air bubbles. Six divers developed symptoms of decompression sickness (DCS). Most intravascular bubbles were observed in divers with the lowest plasma levels of C5a. Postdive plasma levels of C5a did not increase compared with predive levels, nor were postdive levels significantly different after two or three dives compared with the first dive. Repeated dives did not influence the amounts of C5a generated in vitro. Neither plasma levels of C5a nor C5a generated in vitro were significantly different in divers who experienced symptoms of DCS vs. divers without symptoms of DCS. We conclude that plasma level of C5a and measurement of C5a generation in vitro cannot be used to predict DCS.

The efficiency of divers breathing oxy-helium.

Eight divers performed an addition test and a screwplate test of manual dexterity in the open sea under four conditions—-breathing either air or an oxy-helium mixture and working at a depth of either 10 or 200 ft. Speed of addition was impaired at depth for both air (19.9 per cent) and helium (14.8 per cent), while errors increased only on air (from 5.9 to 21.1 per cent). The manual dexterity test also showed a decrement in speed for both air (46.7 per cent) and helium (31.8 per cent), and air divers lost more screws at depth (11.1 per cent) than at 10 ft (4.7 per cent). While a decrement at depth was expected in the air dives, the considerable impairment shown on oxy-helium dives was not. A further experiment was therefore run in a dry pressure chamber to study the effects of breathing oxy-helium at pressure when the additional stresses associated with deep diving in the open sea were absent. At a pressure equivalent to 200 ft of water, there was a 10 per cent impairment in speed on both the screwplate (...