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.

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%.

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.

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.