RR598 - Evaluation of Doppler monitoring for the control of hyperbaric exposure in tunnelling

Decompression illness occurs either as symptoms arising soon after the hyperbaric exposure (decompression sickness (DCS)) or as chronic effects (such as dysbaric osteonecrosis) that do not become apparent until many years later. After hyperbaric exposures, the return to atmospheric pressure is routinely achieved by gradual decompression following set tables, and in modern times with breathing of oxygen (eg oxygenated Blackpool Table). The tables are designed to allow for the hyperbaric exposure pressure and duration, but the health risks are not fully controlled for all exposure conditions. Therefore there is a need to be able to monitor and improve the effectiveness of decompression procedures under routine operational conditions in compressed air tunnelling. Doppler monitoring of gas bubbles in the venous blood might fulfil that need.

This potential application of Doppler monitoring was evaluated by assessing the theoretical and practical issues in using Doppler, analysing the published studies that compare Doppler scores to related incidence of DCS, quantifying the relationship of Doppler scores to predicted level of gas in venous blood, and assessing the practical issues and experience of Doppler monitoring in tunnelling work. Doppler scores correlate with risk of DCS, and DCS has been associated with long term health effects. Data from diving trials and hypobaric exposure trials indicate that the correlation of Doppler grades with risk of DCS is not the same for all situations, which may be because the monitored bubbles are on their way out of the body and therefore are unlikely to be the ones causing the DCS.

Only a small part of the available data (relating to Doppler scores) were from compressed air work or compressed air work simulations. The variability of Doppler results between individuals and between small groups means that Doppler scores have limitations in routine operational use. However, they are indicative of relative risk. The role of Doppler is likely to be limited for routine operational use at the current level of knowledge, but we make recommendations on what would be needed to make Doppler monitoring suitable for routine use in compressed air work in the UK.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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Updated 2021-04-21