From UPTD to ESOT: Monitoring hyperoxic exposure in surface-oriented diving.

A recent review suggested that the measure K = t² x pO2 [4.57] (t is exposure time in h, pO in atm) should replace unit pulmonary toxic dose (UPTD) as an exposure index for pulmonary oxygen toxicity (POT) in surface-oriented diving. K would better predict reduction in vital capacity (VC) during exposure and allow prediction of recovery. Although K is more accurate estimating VC changes than UPTD, the calculation of K is more extensive, particularly when estimating hyperoxic exposure for dives with multiple pO2 segments. Furthermore, and in contrast with UPTD, K is difficult to interpret on its own given its non- linear dimension of time. We suggest that a new metric: ESOT (equivalent surface oxygen time) should be used to replace UPTD. ESOT = t x pO2 [2.285] (t is exposure time in minutes, pO in atm). ESOT=1 is thus the hyperoxic exposure reached after one minute of breathing 100% O2 at surface pressure. Hyperoxic monitoring by ESOT is more practical than K to apply in an operational environment, with no loss of accuracy in POT prediction. In addition, it intuitively allows interpreting hyperoxic exposures on its own, analogous to UPTD. The daily hyperoxic threshold limits suggested by Risberg and van Ooij for two, five and an unlimited number of successive diving days would translate to ESOTs of 650, 500 and 420 respectively.

Hyperbaric oxygen therapy awareness within a doctor population.

Hyperbaric medicine is a relatively young specialty that remains in the blind spot of most doctors' awareness. This study endeavors to identify the level of awareness of the indications for hyperbaric oxygen (HBO2) therapy among a doctor population in a developed country and factors which may improve referral rates. An anonymized questionnaire was distributed to doctors licensed to practice in Malta. Questions included physician specialty, demographics and previous exposure to diving and/or hyperbaric medicine. Moreover, two scoring systems were used to score subjects on HBO2-related topics. Binomial logistic regression models and generalized linear models were used in the statistical analysis. A total of 152 full replies were obtained and analyzed. Respondents who had visited a hyperbaric unit (HBU) (p=0.002) or attended a lecture on HBO2 (p=0.006) scored better than their counterparts, indicating better awareness of HBO2 indications and local chamber location. A previous HBU visit (p=0.001), being a hospital-based doctor (p=0.027) and a history of scuba diving (p=0.03) were associated with willingness to refer patients for HBO2 in the future. Encouraging visits to an HBU has been shown to be associated with multiple factors, which are expected to result in improved referral rates. Targeted educational sessions to doctors and medical students are likely to be beneficial in improving correct referral of patients for HBO2. The findings from this study may prove useful in improving appropriate referral rates of patients who may benefit from this useful treatment modality.

Review of saturation decompression procedures used in commercial diving.

Introduction: This is a review of commercial heliox saturation decompression procedures. The scope does not include compression, storage depth or bell excursion dive procedures. The objectives are to: identify the sources of the procedures; trace their evolution; describe the current practice; and detect relevant trends. Methods: Eleven international commercial diving companies provided their diving manuals for review under a confidentiality agreement. Results: Modern commercial diving saturation procedures are derived from a small number of original procedures (United States Navy, Comex, and NORSOK). In the absence of relevant scientific studies since the late 80's, the companies have empirically adapted these procedures according to their needs and experience. Such adaptation has caused differences in decompression rates shallower than 60 msw, decompression rest stops and the decision to decompress linearly or stepwise. Nevertheless, the decompression procedures present a remarkable homogeneity in chamber PO2 and daily decompression rates when deeper than 60 msw. The companies have also developed common rules of good practice; no final decompression should start with an initial ascending excursion; a minimum hold is required before starting a final decompression after an excursion dive. Recommendation is made for the divers to exercise during decompression. Conclusions: We observed a trend towards harmonisation within the companies that enforce international procedures, and, between companies through cooperation inside the committees of the industry associations.

Atypical distally distributed cutis marmorata decompression sickness associated with unconventional use of thermal protection in a diver with persistent foramen ovale.

Cutis marmorata is a mottled, marbling, livedoid rash caused by vascular inflammation and congestion in cutaneous decompression sickness. It may occur during or after ascent due to the formation of bubbles from dissolved nitrogen accumulated throughout the dive. It is strongly associated with the presence of right to left shunts, particularly persistent (patent) foramen ovale (PFO). We report a case of cutis marmorata decompression sickness of an unusual pattern associated with unconventional use of thermal protection (a 'shorty' wetsuit worn over full suit) by a diver with a PFO. The patient also had neurological manifestations of decompression sickness. The distal lower limb pattern of involvement favours the hypothesis that cutis marmorata in humans is likely to be due to bubbles in the skin itself and/or adjacent tissues rather than cerebrally mediated.

Effectiveness of hyperbaric chamber ventilation.

INTRODUCTION: Hyperbaric chamber ventilation (HCV) refers to the intentional introduction of fresh gas, whether air, oxygen, or heliox, into a pressurised hyperbaric chamber in order to remove stale or otherwise compromised gas. The minimum required continuous HCV rate is usually determined by mathematical models derived from the contaminant mass balance within a well-stirred compartment. Non-uniform contaminant distribution patterns inside a hyperbaric chamber could emerge and invalidate the predictions of well-stirred models. METHODS: Contaminant distribution was investigated inside a clinical hyperbaric chamber with the aim of comparing well-stirred model predictions with the actual contaminant concentration measurements. RESULTS: Local ventilation effectiveness inside a clinical hyperbaric chamber may be compromised, leading to higher contaminant concentration values compared to the predictions of a mathematical model with a well-stirred assumption. CONCLUSIONS: A well-stirred assumption in mathematical models is a useful simplification that allows reasonably accurate estimates of HCV requirements. However, local ventilation effectiveness values in a particular hyperbaric chamber might vary, with the potential for hazardous contaminant accumulation in under-ventilated zones.

A review of accelerated decompression from heliox saturation in commercial diving emergencies.

INTRODUCTION: Saturation diving is a specialised method of intervention in offshore commercial diving. Emergencies may require the crew to be evacuated from the diving support vessel. Because saturation divers generally need several days to reach surface, the emergency evacuation of divers is based on dedicated hyperbaric rescue systems. There are still potential situations for which these systems cannot be used or deployed, and where an emergency decompression provides an alternative solution. METHODS: Our objective was to describe historical cases and assess the benefit of emergency decompressions, with the collection of data from the authors' direct experience and networks, providing witness or first-hand information. RESULTS: We documented three cases of emergency decompression following bell evacuations, and six cases of accelerated decompression performed in the chamber or hyperbaric rescue chamber. Review of these cases showed: 1) the complicated nature of such emergencies that make decisions difficult; 2) the variety of solutions implemented; and 3) the surprisingly safe and successful outcomes of several operations. Analysis of the accelerated decompression occurrences allowed derivation of the options used; upward initial excursion, increased chamber partial pressure of oxygen associated to increased ascent rates, and inert gas switching. We identified four published procedures for accelerated decompression. CONCLUSIONS: Despite modern hyperbaric rescue systems, accelerated decompression remains an essential tool in case of emergency. The diving industry needs clear guidance on what can be achieved, depending on the saturation depth and the level of emergency.

Acute Effects on the Human Peripheral Blood Transcriptome of Decompression Sickness Secondary to Scuba Diving.

Decompression sickness (DCS) develops due to inert gas bubble formation in bodily tissues and in the circulation, leading to a wide range of potentially serious clinical manifestations. Its pathophysiology remains incompletely understood. In this study, we aim to explore changes in the human leukocyte transcriptome in divers with DCS compared to closely matched unaffected controls after uneventful diving. Cases (n = 7) were divers developing the typical cutis marmorata rash after diving with a confirmed clinical diagnosis of DCS. Controls (n = 6) were healthy divers who surfaced from a ≥25 msw dive without decompression violation or evidence of DCS. Blood was sampled at two separate time points-within 8 h of dive completion and 40-44 h later. Transcriptome analysis by RNA-Sequencing followed by bioinformatic analysis was carried out to identify differentially expressed genes and relate their function to biological pathways. In DCS cases, we identified enrichment of transcripts involved in acute inflammation, activation of innate immunity and free radical scavenging pathways, with specific upregulation of transcripts related to neutrophil function and degranulation. DCS-induced transcriptomic events were reversed at the second time point following exposure to hyperbaric oxygen. The observed changes are consistent with findings from animal models of DCS and highlight a continuum between the responses elicited by uneventful diving and diving complicated by DCS. This study sheds light on the inflammatory pathophysiology of DCS and the associated immune response. Such data may potentially be valuable in the search for novel treatments targeting this disease.

Increasing prevalence of vestibulo-cochlear decompression illness in Malta - an analysis of hyperbaric treatment data from 1987-2017.

INTRODUCTION: Scuba diving is a big part of the tourism sector in Malta, and all the cases of decompression illness (DCI)are treated within the single hyperbaric referral centre in the country. METHODS: This retrospective analysis reviews all the medical records of divers with DCI in Malta within the 30-year period between 1987 to 2017 who required recompression therapy with hyperbaric oxygen. RESULTS: There were 437 discrete cases of DCI managed with recompression therapy. Amongst DCI subtypes, the prevalence of musculo-skeletal DCI is decreasing, whereas that of vestibulo-cochlear DCI is increasing. CONCLUSION: The increasing prevalence of vestibulo-cochlear DCI may be due to a change in diving practices in Malta.

Early detection of diving-related cognitive impairment of different nitrogen-oxygen gas mixtures using critical flicker fusion frequency.

INTRODUCTION: Cognitive impairment related to inert gas narcosis (IGN) is a threat to diving safety and operations at depth that might be reduced by using enriched air nitrox (EANx) mixtures. Using critical flicker fusion frequency (CFFF), a possible early detection of cognitive abilities/cerebral arousal impairment when breathing different oxygen (O2) fractions was investigated. METHODS: Eight male volunteers performed, in random order, two dry chamber dives breathing either air or EANx40 (40% O2-60% nitrogen) for 20 minutes (min) at 0.4 MPa. Cognition and arousal were assessed before the dive; upon arrival at 0.4 MPa; after 15 min exposure at 0.4 MPa; on surfacing and 30 min post-dive using behavioural computer-based testing psychology experiment building language (PEBL) and by CFFF while continuously recording brain oxygenation with near-infrared spectroscopy. RESULTS: In both breathing conditions, CFFF and PEBL demonstrated a significant inverse correlation (Pearson r of -0.90, P < 0.0001), improved cognitive abilities/cerebral arousal occurred upon arrival at 0.4 MPa followed by a progressive deterioration. Initial brain activation was associated with a significant increase in oxyhaemoglobin (HbO2) and a simultaneous decrease of deoxyhaemoglobin (HHb). The magnitude of the changes was significantly greater under EANx (P = 0.038). CONCLUSIONS: Since changes were not related to haemodynamic variables, HbO2 and HHb values indicate a significant, O2-dependent activation in the prefrontal cortex. Owing to the correlation with some tests from the PEBL, CFFF could be a convenient measure of cognitive performance/ability in extreme environments, likely under the direct influence of oxygen partial pressure, a potent modulator of IGN symptoms.

Anton's syndrome as a presentation of decompression illness.

We present a case of a patient with Anton's syndrome due to decompression illness (DCI) after recreational scuba diving. Visual anosognosia, or denial of loss of vision, which is associated with lack of awareness regarding visual loss in the setting of cortical blindness, is known as Anton's syndrome (also termed Anton-Babinski syndrome). Our patient presented with progressive neurological DCI treated with repeated recompression. The anosogosia resolved after 48 h. Subsequent echocardiography revealed a persistent (patent) foramen ovale.