Risk assessment of SWEN21 a suggested new dive table for the Swedish armed forces: bubble grades by ultrasonography.

Introduction: To develop the diving capacity in the Swedish armed forces the current air decompression tables are under revision. A new decompression table named SWEN21 has been created to have a projected risk level of 1% for decompression sickness (DCS) at the no stop limits. The aim of this study was to evaluate the safety of SWEN21 through the measurement of venous gas emboli (VGE) in a dive series. Methods: A total 154 dives were conducted by 47 divers in a hyperbaric wet chamber. As a proxy for DCS risk serial VGE measurements by echocardiography were conducted and graded according to the Eftedal-Brubakk scale. Measurements were done every 15 minutes for approximately 2 hours after each dive. Peak VGE grades for the different dive profiles were used in a Bayesian approach correlating VGE grade and risk of DCS. Symptoms of DCS were continually monitored. Results: The median (interquartile range) peak VGE grade after limb flexion for a majority of the time-depth combinations, and of SWEN21 as a whole, was 3 (3-4) with the exception of two decompression profiles which resulted in a grade of 3.5 (3-4) and 4 (4-4) respectively. The estimated risk of DCS in the Bayesian model varied between 4.7-11.1%. Three dives (2%) resulted in DCS. All symptoms resolved with hyperbaric oxygen treatment. Conclusions: This evaluation of the SWEN21 decompression table, using bubble formation measured with echocardiography, suggests that the risk of DCS may be higher than the projected 1%.

Measuring whole body inert gas wash-out.

Introduction: Quantifying inert gas wash-out is crucial to understanding the pathophysiology of decompression sickness. In this study, we developed a portable closed-circuit device for measuring inert gas wash-out and validated its precision and accuracy both with and without human subjects. Methods: We developed an exhalate monitor with sensors for volume, temperature, water vapor and oxygen. Inert gas volume was extrapolated from these inputs using the ideal gas law. The device's ability to detect volume differences while connected to a breathing machine was analysed by injecting a given gas volume eight times. One hundred and seventy-two coupled before-and-after measurements were then compared with a paired t-test. Drift in measured inert gas volume during unlabored breathing was evaluated in three subjects at rest using multilevel linear regression. A quasi-experimental cross-over study with the same subjects was conducted to evaluate the device's ability to detect inert gas changes in relation to diving interventions and simulate power. Results: The difference between the injected volume (1,996 ml) and the device's measured volume (1,986 ml) was -10 ml. The 95% confidence interval (CI) for the measured volume was 1,969 to 2,003 ml. Mean drift during a 43 min period of unlaboured breathing was -19 ml, (95% CI, -37 to -1). Our power simulation, based on a cross-over study design, determined a sample size of two subjects to detect a true mean difference of total inert gas wash-out volume of 100 ml. Conclusions: We present a portable device with acceptable precision and accuracy to measure inert gas wash-out differences that may be physiologically relevant in the pathophysiology of decompression sickness.

Proposed Thalmann algorithm air diving decompression table for the Swedish Armed Forces.

The Swedish Armed Forces (SwAF) air dive tables are under revision. Currently, the air dive table from the U.S. Navy (USN) Diving Manual (DM) Rev. 6 is used with an msw-to-fsw conversion. Since 2017, the USN has been diving according to USN DM rev. 7, which incorporates updated air dive tables derived from the Thalmann Exponential Linear Decompression Algorithm (EL-DCM) 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 2,953 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). One hundred fifty-four wet validation dives were performed with air between 18 to 57 msw. Both direct ascent and decompression stop dives were conducted, resulting in incidences of two joint pain DCS (18 msw/59 minutes), one leg numbness CNS-DCS (51 msw/10 minutes with deco-stop), and nine marginal DCS cases, such as rashes and itching. A total of three DCS incidences, 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. 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.

Agreement between ultrasonic bubble grades using a handheld self-positioning Doppler product and 2D cardiac ultrasound.

INTRODUCTION: Intravascular bubble load after decompression can be detected and scored using ultrasound techniques that measure venous gas emboli (VGE). The aim of this study was to analyse the agreement between ultrasonic bubble grades from a handheld self-positioning product, the O'Dive™, and cardiac 2D ultrasound after decompression. METHODS: VGE were graded with both bilateral subclavian vein Doppler ultrasound (modified Spencer scale) and 2D cardiac images (Eftedal Brubakk scale). Agreement was analysed using weighted kappa (Kw). Analysis with Kw was made for all paired grades, including measurements with and without zero grades, and for each method's highest grades after each dive. RESULTS: A total of 152 dives yielded 1,113 paired measurements. The Kw agreement between ultrasound VGE grades produced by cardiac 2D images and those from the O'Dive was 'fair'; when zero grades were excluded the agreement was 'poor'. The O'Dive was found to have a lower sensitivity to detect VGE compared to 2D cardiac image scoring. CONCLUSIONS: Compared to 2D cardiac image ultrasound, the O'Dive yielded generally lower VGE grades, which resulted in a low level of agreement (fair to poor) with Kw.

Randomised, controlled, open label, multicentre clinical trial to explore safety and efficacy of hyperbaric oxygen for preventing ICU admission, morbidity and mortality in adult patients with COVID-19.

INTRODUCTION: COVID-19 may cause severe pneumonitis and trigger a massive inflammatory response that requires ventilatory support. The intensive care unit (ICU)-mortality has been reported to be as high as 62%. Dexamethasone is the only of all anti-inflammatory drugs that have been tested to date that has shown a positive effect on mortality. We aim to explore if treatment with hyperbaric oxygen (HBO) is safe and effective for patients with severe COVID-19. Our hypothesis is that HBO can prevent ICU admission, morbidity and mortality by attenuating the inflammatory response. The primary objective is to evaluate if HBO reduces the number of ICU admissions compared with best practice treatment for COVID-19, main secondary objectives are to evaluate if HBO reduces the load on ICU resources, morbidity and mortality and to evaluate if HBO mitigates the inflammatory reaction in COVID-19. METHODS AND ANALYSIS: A randomised, controlled, phase II, open label, multicentre trial. 200 subjects with severe COVID-19 and at least two risk factors for mortality will be included. Baseline clinical data and blood samples will be collected before randomisation and repeated daily for 7 days, at days 14 and 30. Subjects will be randomised with a computer-based system to HBO, maximum five times during the first 7 days plus best practice treatment or only best practice treatment. The primary endpoint, ICU admission, is defined by criteria for selection for ICU. We will evaluate if HBO mitigates the inflammatory reaction in COVID-19 using molecular analyses. All parameters are recorded in an electronic case report form. An independent Data Safety Monitoring Board will review the safety parameters. ETHICS AND DISSEMINATION: The trial is approved by The National Institutional Review Board in Sweden (2020-01705) and the Swedish Medical Product Agency (5.1-2020-36673). Positive, negative and any inconclusive results will be published in peer-reviewed scientific journals with open access. TRIAL REGISTRATION: NCT04327505. EudraCT number: 2020-001349-37.

The performance of 'temperature stick' carbon dioxide absorbent monitors in diving rebreathers.

INTRODUCTION: Diving rebreathers use canisters containing soda lime to remove carbon dioxide (CO2) from expired gas. Soda lime has a finite ability to absorb CO2. Temperature sticks monitor the exothermic reaction between CO2 and soda lime to predict remaining absorptive capacity. The accuracy of these predictions was investigated in two rebreathers that utilise temperature sticks. METHODS: Inspiration and rEvo rebreathers filled with new soda lime were immersed in water at 19°C and operated on mechanical circuits whose ventilation and CO2-addition parameters simulated dives involving either moderate exercise (6 MET) throughout (mod-ex), or 90 minutes of 6 MET exercise followed by 2 MET exercise (low-ex) until breakthrough (inspired PCO2 [PiCO2] = 1 kPa). Simulated dives were conducted at surface pressure (sea-level) (low-ex: Inspiration, n = 5; rEvo, n = 5; mod-ex: Inspiration, n = 7, rEvo, n = 5) and at 3-6 metres' sea water (msw) depth (mod-ex protocol only: Inspiration, n = 8; rEvo, n = 5). RESULTS: Operated at surface pressure, both rebreathers warned appropriately in four of five low-ex tests but failed to do so in the 12 mod-ex tests. At 3-6 msw depth, warnings preceded breakthrough in 11 of 13 mod-ex tests. The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another. CONCLUSION: When operated at even shallow depth, temperature sticks provided timely warning of significant CO2 breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure.

Measuring Uptake and Elimination of Nitrogen in Humans at Different Ambient Pressures.

BACKGROUND: To measure nitrogen (N2) wash-out and uptake requires elaborate set-ups, especially when doing the measurements at increased or decreased ambient pressure. Here we present a transportable device for quantifying N2 turnover in humans which can be used at different ambient pressures. METHODS: A modified close-circuit electronic rebreather was used to assess N2 turnover. Changes in N2 volume within the rebreathing circuit, reflecting N2 uptake or washout, were derived from the continuously monitored total system volume and the calculated volumes of oxygen and water vapor. The calculation of continuous N2 volume curves was performed off-line using dedicated computer software. RESULTS: Four subjects participated in the proof-of-concept tests. At steady state, the drift in calculated N2 volume in the rebreathing circuit over a 1-h duration was minimal. Three of the subjects participated in additional N2 steady-state measurements where 1019 mL (BTPD) of N2 was injected into the rebreathing circuit over 20 min and the measured volume increase was 1006 ± 32 mL. Lastly, N2 elimination was assessed during decompression to 0.5 atm and while breathing hyperoxic gas. N2 uptake was measured during compression to 1.8 atm. The elimination and uptake curves were deemed to be realistic. DISCUSSION: A method for assessing N2 turnover in humans has been developed and a first evaluation has been performed. It is easy to work with operationally and can be used at different ambient pressures. More research is needed in order to further validate it as a method for assessing N2 turnover in humans.Sundblad P, Frånberg O, Siebenmann C, Gennser M. Measuring uptake and elimination of nitrogen in humans at different ambient pressures. Aerosp Med Hum Perform. 2016; 87(12):1045-1050.

Measurement and modeling of oxygen content in a demand constant mass ratio injection rebreather.

Mechanical semi-closed rebreathers do not need oxygen sensors for their functions, thereby reducing the complexity of the system. However, testing and modeling are necessary in order to determine operational limits as well as the decompression obligation and to avoid hyperoxia and hypoxia. Two models for predicting the oxygen fraction in a demand constant mass ratio injection (DCMRI) rebreather for underwater use were compiled and compared. The model validity was tested with an IS-MIX, Interspiro AB rebreather using a metabolic simulator connected to a breathing machine inside a water-filled pressure chamber. The testing schedule ranged from 0.5-liter (L) to 3-liter tidal volumes, breathing frequencies from five to 25 breaths/minute and oxygen consumptions from 0.5 L/minute to 4 L/minute. Tests were carried out at surface and pressure profiles ranging to 920 kPa(a) (81 meters of sea water, 266 feet of sea water). The root mean squared error (RMSE) of the single-compartment model was 2.4 percent-units of oxygen for the surface test with the 30% dosage setting but was otherwise below 1% unit. For the multicompartment model the RMSE was below 1% unit of oxygen for all tests. It is believed that these models will aid divers in operational settings and may constitute a helpful tool when developing semi-closed rebreathing apparatuses.

A metabolic simulator for unmanned testing of breathing apparatuses in hyperbaric conditions.

BACKGROUND: A major part of testing of rebreather apparatuses for underwater diving focuses on the oxygen dosage system. METHODS: A metabolic simulator for testing breathing apparatuses was built and evaluated. Oxygen consumption was achieved through catalytic combustion of propene. With an admixture of carbon dioxide in the propene fuel, the system allowed the respiratory exchange ratio to be set freely within human variability and also made it possible to increase test pressures above the condensation pressure of propene. The system was tested by breathing ambient air in a pressure chamber with oxygen uptake (Vo2) ranging from 1-4 L · min(-1), tidal volume (VT) from 1-3 L, breathing frequency (f) of 20 and 25 breaths/min, and chamber pressures from 100 to 670 kPa. RESULTS: The measured end-tidal oxygen concentration (Fo2) was compared to calculated end-tidal Fo2. The largest average difference in end-tidal Fo2during atmospheric pressure conditions was 0.63%-points with a 0.28%-point average difference during the whole test. During hyperbaric conditions with pressures ranging from 100 to 670 kPa, the largest average difference in Fo2was 1.68%-points seen during compression from 100 kPa to 400 kPa and the average difference in Fo2during the whole test was 0.29%-points. CONCLUSION: In combination with a breathing simulator simulating tidal breathing, the system can be used for dynamic continuous testing of breathing equipment with changes in VT, f, Vo2, and pressure.

Nitrox permits direct exit for attendants during extended hyperbaric oxygen treatment.

BACKGROUND: Intermittent breathing of oxygen-enriched air, nitrox (1:1 air:oxygen, 60.5% O2), for attendants in multiplace hyperbaric chambers should enable treatment protocols (HOPAN - hyperbaric oxygen protocol attendants' nitrox) of up to 200 minutes at 2.8 atmospheres absolute (ATA), while retaining the option of a direct decompression and exit. METHODS: HOPAN with cycles of 15 minutes of nitrox breathing followed by 10 minutes of chamber air for attendants were occasionally used from 2007-2009. HOPAN vs. LTP (local treatment protocols) were evaluated via an anonymous enquiry among attendants; patients' medical records were followed six months post-HBO2 treatment (HBO2T). RESULTS: 88 HOPANs, with 59 chamber attendants assisting 30 patients, were documented. HOPAN duration ranged from 55-167 minutes (median 140 minutes). 31/59 attendants answered the enquiry. Perceived comfort of each protocol (HOPAN vs. LTP) by attendants was reported as equal. Symptoms, both minor (parestesias) and severe (joint pain), were reported in connection with LTP, while only one occurrence (mild joint pain) was reported in connection with HOPAN. No complications were documented among the attendants or the patients. It is suggested that nitrox breathing for chamber attendants provide flexible HBO2T for patients at 2.8 ATA for up to 200 minutes within no-decompression limits, facilitating future studies of HBO2T dosage.

Investigation of a demand-controlled rebreather in connection with a diving accident.

This paper describes the examination of a Halcyon RB80 semi-closed underwater breathing apparatus used in a diving accident in 2007. The apparatus was supplied with trimix (oxygen, nitrogen and helium) containing 31% oxygen. The duration of the dive was 105 minutes at 28 meters' average depth in fresh water, with a 19-minute oxygen decompression stop at 6 meters. Upon surfacing the diver experienced seizures and signs of severe neurological deficits. The apparatus was tested with regard to the oxygen fraction drop from the supply gas to the breathing loop--i.e., the oxygen fraction inhaled by the diver (FiO2) was investigated. The FiO2 was measured and found to be lower than the value stated on the manufacturer's web page at the time of the accident. This investigation suggests that during the dive, the actual FiO2% was 17.9-25.3%, which is considerably lower than the FiO2% used for decompression calculations (30%). The underestimation of FiO2 resulted in too short and/or too few decompression stops during ascent. The low FiO2 would also put a diver at risk of hypoxia at shallow depths. It is concluded that inadequate information on the performance of the rebreather was a major contributing factor to this accident.