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Monthly Briefing April 2024

A monoplace-based hyperbaric safety director recently contacted the Board. She advised that her policy is to limit the amount of combustible material entering the chamber, including only allowing  a single blanket. She added that her facility is currently treating a patient who insists on three blankets. Upon explaining why this is not permissible from a safety perspective the patient requested to speak to her medical director. She sought the Board’s guidance which was supportive of her policy and summarized below.  

Limiting in-chamber combustible material should be an essential element of any fire safety plan. In the event of a monoplace chamber fire, it will continue to burn (while generating heat/increasing pressure) until either all combustible material is consumed, or the chamber’s internal pressure limit is exceeded, with resultant catastrophic failure. Excessive combustible material was considered contributory to the February 1996 uncontained monoplace fire and explosion in Yamanishi, Japan. There existed a relatively large “fuel” burden, in terms of the patient’s own clothing, including a heavily insulated acrylic jacket, along with blankets and linens. This contrasted with a fully contained monoplace chamber fire, also in Japan several years earlier, that did not involve the same amount of combustible material. The ignition source of these two fires was a reusable and a disposable pocket warmer, respectively. 

Thermal comfort within a monoplace chamber is best addressed by manipulation of its oxygen flow/purge rate. If a patient complains of being cold, flow should be reduced thereby limiting the “wind tunnel-like” cooling effect associated with higher gas flows, rather than providing an additional blanket(s). This process has long proven effective and will be more so today with the lower available flow rate options. An additional advantage of a lowered flow rate is its impact on relative humidity (RH). The common source of compression gas is a bulk liquid system. Stored liquified oxygen passes through vaporizer coils and “boils off” to a moisture-free gaseous state prior to entering the chamber.  This results in a low chamber RH (we measured it as low as 28% at 400 lpm). The dryer the atmosphere the greater the potential for static accumulation, a fire risk with any static discharge in proximity to volatile hazardous vapors. Reducing the flow rate to 240 lpm (the lowest available level in earlier chamber models) increased RH to the mid-60’s as patient evaporative moisture loss readily accumulates. Higher RH’s also act as a natural conductor to earth for any developing static charge while doing much to prevent its buildup in the first place.       

Room temperature can also be adjusted upwards within the commonly recommended 68-72 F range if there are consistent complaints of patients being too cold.

For patients complaining of being too warm, provide a sheet not a blanket and increase flow rates to achieve the desired cooling effect, while accepting a proportionally lowered RH. Alternatively, adjusting room temperature to the lower and of the above range may prove sufficient.  

The above summary has been condensed into the Board’s most recent Position Statement, at 

Dick Clarke, President

National Board of Diving & Hyperbaric Medicine

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Updated April 1, 2024