Comparing the EMMA capnograph with sidestream capnography and arterial carbon dioxide pressure at 284 kPa.

Introduction: Capnography aids assessment of the adequacy of mechanical patient ventilation. Physical and physiological changes in hyperbaric environments create ventilation challenges which make end-tidal carbon dioxide (ETCO2) measurement particularly important. However, obtaining accurate capnography in hyperbaric environments is widely considered difficult. This study investigated the EMMA capnograph for hyperbaric use. Methods: We compared the EMMA capnograph to sidestream capnography and the gold standard arterial carbon dioxide blood gas analysis in a hyperbaric chamber. In 12 resting subjects breathing air at 284 kPa, we recorded ETCO2 readings simultaneously derived from the EMMA and sidestream capnographs during two series of five breaths (total 24 measurements). An arterial blood gas sample was also taken simultaneously in five participants. Results: Across all measurements there was a difference of about 0.1 kPa between the EMMA and sidestream capnographs indicating a very slight over-estimation of ETCO2 by the EMMA capnograph, but fundamentally good agreement between the two end-tidal measurement methods. Compared to arterial blood gas pressure the non-significant difference was about 0.3 and 0.4 kPa for the EMMA and sidestream capnographs respectively. Conclusions: In this study, the EMMA capnograph performed equally to the sidestream capnograph when compared directly, and both capnography measures gave clinically acceptable estimates of arterial PCO2.

Does hyperbaric oxygen cause narcosis or hyperexcitability? A quantitative EEG analysis.

Divers breathe higher partial pressures of oxygen at depth than at the surface. The literature and diving community are divided on whether or not oxygen is narcotic. Conversely, hyperbaric oxygen may induce dose-dependent cerebral hyperexcitability. This study evaluated whether hyperbaric oxygen causes similar narcotic effects to nitrogen, and investigated oxygen's hyperexcitability effect. Twelve human participants breathed "normobaric" air and 100% oxygen, and "hyperbaric" 100% oxygen at 142 and 284 kPa, while psychometric performance, electroencephalography (EEG), and task load perception were measured. EEG was analyzed with functional connectivity and temporal complexity algorithms. The spatial functional connectivity, estimated using mutual information, was summarized with the global efficiency network measure. Temporal complexity was calculated with a "default-mode-network (DMN) complexity" algorithm. Hyperbaric oxygen-breathing caused no change in EEG global efficiency or in the psychometric test. However, oxygen caused a significant reduction of DMN complexity and a reduction in task load perception. Hyperbaric oxygen did not cause the same changes in EEG global efficiency seen with hyperbaric air, which likely related to a narcotic effect of nitrogen. Hyperbaric oxygen seemed to disturb the time evolution of EEG patterns that could be taken as evidence of early oxygen-induced cortical hyperexcitability. These findings suggest that hyperbaric oxygen is not narcotic and will help inform divers' decisions on suitable gas mixtures.

EEG functional connectivity is sensitive for nitrogen narcosis at 608 kPa.

Divers commonly breathe air, containing nitrogen. Nitrogen under hyperbaric conditions is a narcotic gas. In dives beyond a notional threshold of 30 m depth (405 kPa) this can cause cognitive impairment, culminating in accidents due to poor decision making. Helium is known to have no narcotic effect. This study explored potential approaches to developing an electroencephalogram (EEG) functional connectivity metric to measure narcosis produced by nitrogen at hyperbaric pressures. Twelve human participants (five female) breathed air and heliox (in random order) at 284 and 608 kPa while recording 32-channel EEG and psychometric function. The degree of spatial functional connectivity, estimated using mutual information, was summarized with global efficiency. Air-breathing at 608 kPa (experienced as mild narcosis) caused a 35% increase in global efficiency compared to surface air-breathing (mean increase = 0.17, 95% CI [0.09-0.25], p = 0.001). Air-breathing at 284 kPa trended in a similar direction. Functional connectivity was modestly associated with psychometric impairment (mixed-effects model r2 = 0.60, receiver-operating-characteristic area, 0.67 [0.51-0.84], p = 0.02). Heliox breathing did not cause a significant change in functional connectivity. In conclusion, functional connectivity increased during hyperbaric air-breathing in a dose-dependent manner, but not while heliox-breathing. This suggests sensitivity to nitrogen narcosis specifically.

Professional diver routine health surveillance and certification: an internet-based satisfaction survey of New Zealand divers.

INTRODUCTION: Professional divers, like many other specialised occupational groups, are subject to regulatory constraints that include mandatory initial medical certification and routine recertification. The New Zealand system of diver certification and health surveillance has undergone modifications in recent years, but its acceptance among end-users has never been formally assessed. Because of the wide variety of tasks, circumstances and personalities encountered in the diving industry, unanimous satisfaction is an unrealistic expectation, but establishing the current mood of divers in this regard and canvassing opinions on possible improvements is an important step towards optimising the certification process. METHOD: A multi-choice satisfaction questionnaire was added, as a quality assurance measure, to the on-line health questionnaire completed annually by all New Zealand professional divers. A complete 12-month dataset was analysed to determine levels of satisfaction, areas of dissatisfaction and suggestions for improvement. Comparison of the opinions of various diver groups was achieved by stratification into employment-type sub-groups and those working locally, overseas or both. RESULTS: The responses of 914 divers who completed the survey established an 85% satisfaction rate with the existing diver certification system. Dissatisfaction was independent of diving locality. Compliance cost was the most common area of dissatisfaction, particularly among recreational diving instructors. CONCLUSIONS: Most New Zealand professional divers consider the current certification system satisfactory. Effective communication between the regulating authority and divers was identified as an important area for further development.

The impact of health on professional diver attrition.

INTRODUCTION: Approximately 77% of professional divers leave the industry within five years of entry, for reasons that are uncertain. One possibility is that attrition is due to ill-health. The health of New Zealand occupational divers is surveyed by a comprehensive medical examination every five years and by a health questionnaire in the intervening years. Divers are thereby confirmed 'fit' annually. The aim of this study was to determine if divers quit the industry due to a health problem not identified by this health surveillance system. METHOD: 601 divers who had left the industry within five years of entry medical examination ('quitters') were identified from a computerised database. One hundred and thirty-six who could be contacted were questioned about their principal reason for quitting. Comparison was made between the health data of all those defined as 'quitters' and a group of 436 'stayers' who have remained active in the industry for over 10 years. RESULTS: Health was the principal reason for abandoning a diving career for only 2.9% of quitters. The overwhelming majority (97.1%) quit because of dissatisfaction with aspects of the work, such as remuneration and reliability of employment. Besides gender, the only significant difference between the health data of quitters and stayers was that smoking was four times more prevalent among quitters. CONCLUSIONS: The key determinant of early attrition from the New Zealand professional diver workforce is industry-related rather than health-related. The current New Zealand diver health surveillance system detects the medical problems that cause divers to quit the industry.

The impact of diving on hearing: a 10-25 year audit of New Zealand professional divers.

INTRODUCTION: Surveillance of professional divers' hearing is routinely undertaken on an annual basis despite lack of evidence of benefit to the diver. The aim of this study was to determine the magnitude and significance of changes in auditory function over a 10-25 year period of occupational diving with the intention of informing future health surveillance policy for professional divers. METHODS: All divers with adequate audiological records spanning at least 10 years were identified from the New Zealand occupational diver database. Changes in auditory function over time were compared with internationally accepted normative values. Any significant changes were tested for correlation with diving exposure, smoking history and body mass index. RESULTS: The audiological records of 227 professional divers were analysed for periods ranging from 10 to 25 years. Initial hearing was poorer than population norms, and deterioration over the observation period was less than that predicted by normative data. Changes in hearing were not related to diving exposure, or smoking history. CONCLUSION: Audiological changes over 10 to 25 years of occupational diving were not found to be significantly different from age-related changes. Routine annual audiological testing of professional divers does not appear to be justifiable.

Postal survey of fitness-to-dive opinions of diving doctors and general practitioners.

AIM: To determine the consensus and concordance with published standards and expert opinion among New Zealand's designated diving doctors (DDDs) and general practitioners (GPs) regarding medical fitness-to-dive. METHODS: A postal survey canvassed doctors' opinions regarding fitness-to-dive of 20 'real-life' applicants with potentially relevant medical conditions. In 17 cases, a 'desired response' was identified as expert opinion and the relevant published Standard concurred; the remaining three cases were excluded from analysis. Consensus was measured between the groups of doctors, and concordance measured against the 'desired response'. The performance of the DDDs was also correlated with both the number of diver medical assessments conducted annually and time since completing a diving medicine course. RESULTS: Seventy-seven of 98 DDDs (79%) and 75 of 200 GPs (38%) responded to the questionnaire. The mean concordance was 60% and 50% for DDDs and GPs respectively. Consensus between DDDs and GPs was generally high, but was low between these groups and the 'desired response'. DDD's concordance was negatively correlated (r = -0.3) with time since undertaking a diving medicine course, but was positively correlated (r = 0.2) with their annual number of dive medical assessments. Both groups were more likely to differ from the 'desired response' by considering an 'unfit' diver as 'fit' than the converse. CONCLUSIONS: There is poor concordance between doctors assessing fitness to dive and both published recommendations and expert opinion when there is a relevant medical condition. This supports the current New Zealand practice of centralised audit of occupational diver medical fitness prior to certification.

The long-term effects of compressed gas diving on lung function in New Zealand occupational divers: a retrospective analysis.

OBJECTIVES: Long-term effects of occupational diving on lung function are uncertain. No previous study has been conducted on New Zealand occupational divers. The aim of this study was to investigate changes in divers' lung function over a five year period. METHODS: The lung function data of all occupational divers with two spirometric determinations separated by a five year interval (N = 336 out of 1,475 currently registered divers) were entered into a database and analysed for changes (5.6 years mean). The trends were correlated against gender, smoking status and years of diving experience (as more accurate diving exposure data were not available). Spirometric indices were compared with predicted reference values derived from New Zealand (WRS), Australian (Gore), and American (NHANES III and Knudson) populations. RESULTS: Small, but signifi cant, decreases were found in FEV1 (0.27% against predicted per annum, P = 0.02) and PEF (0.47% per annum, P = 0.04) using the NHANES III equations. No other changes in lung function parameters reached statistical significance (P < 0.05) using any of the four sets of prediction equations. No changes correlated significantly with reported years diving. CONCLUSION: Observed changes to occupational divers' lung function tests over 5.6 years were small and unrelated to years of diving, which might be due to a 'healthy worker effect'. Clinical relevance is unlikely, but this requires further evaluation. There was significant disparity in normative values derived from the four sets of prediction equations and there is some consequential concern about the ongoing utility of such surveillance of New Zealand professional divers.

The clinical toxicology of carbon monoxide.

Carbon monoxide (CO) is a dangerous exogenous poison and an essential endogenous neurotransmitter. This gas when inhaled has an anaesthetic effect, which is poorly understood, but which may be fatal if compensatory mechanisms are exhausted, if cardiac oxygen (O(2)) needs exceed myocardial oxygenation and/or if apnoea or asphyxia onsets. Although there is considerable evidence that hypoxia occurs late in CO poisoning, both the treatment of acutely poisoned people and environmental exposure limits are largely based on a hypoxic theory of toxicity. The significance of recent demonstrations of increased endogenous CO and NO production in neurons of animals exposed to exogenous CO, and of a related sequestration of leucocytes along the endothelium and subsequent diapedesis is also not fully understood, but may in part explain both acute and delayed deleterious effects of a CO exposure. Delayed brain injuries due to a CO exposure may be preventable by hyperbaric O(2). However, the ideal dose of O(2) in this context, if any, is unknown and other potential treatments need to be tested.