In Denver, CO with an atmospheric pressure less than sea level, how should the ordering physician compensate when prescribing hyperbaric oxygen therapy for a standard wound care table 2.0 ATA for 90 minutes? Please provide a detailed explanation.
Thank you for your question. The UHMS HBO2 safety committee can provide information to assist you in answering your question, but the ultimate responsibility for these types of questions rests with the medical director and safety director of your facility.
The decision whether or not to compensate for differential pressure due to altitude is the medical director’s responsibility. The US Navy requires altitude corrections for undersea divers starting at 1000 feet of altitude. We are not aware of conclusive data that indicates the outcomes are better or worse for patients because they were treated using gauge pressure versus atmospheres absolute (ATA) at altitude. In addition, our patients are fully acclimated to the altitude prior to starting treatment. We suggest that the chamber pressure should be as accurate as possible. There are certainly physicians/medical directors who choose to compensate for the differential pressure at altitude and those that do not.
We are not aware of any hyperbaric chambers with absolute pressure monitoring installed by the manufacturer. Most chamber pressure monitors use a gauge scale such as: feet of sea water/fsw, pounds per square inch/psig, atmospheres/atm, bar, kilo pascals/Kpa). These gauges inform the user what the pressure is inside the chamber with no regard to the outside ambient pressure. (note: the ATA or ATM marking from the manufacturer on your chamber is most likely a gauge pressure and not completely accurate above sea level). To calculate absolute pressure, one needs to include the surrounding ambient pressure. This pressure differs from facility to facility due to differences in altitude and a constantly varying local barometric pressure. Therefore, if one is to be accurate in the use of atmospheres absolute (ATA), an absolute gauge must be added to the chamber.
Denver is at an altitude of 5,924 feet and has an average barometric pressure of 12.2 pounds per square inch absolute (psia), 0.83 ATA. If a chamber at that altitude is compressed to 14.7 pounds per square inch gauge (psig) to achieve 2.0 ATA there will be an error of 2.5 psig or 0.17 ATA. The absolute pressure in the chamber would be 26.9 psia/1.83 ATA not 29.4 psia/2.0 ATA. The physician in Denver, CO wanting to achieve 2.0 ATA in the chamber will need to compensate for altitude by adding back the differential between altitude and sea-level by compressing the chamber to 17.2 psig.
We have seen facilities that have calculated the pressure to equal ATA for the various treatment schedules and posted these at the control panel. These are based on the average barometric pressure in their area. There are other facilities that have added absolute gauges to the chamber systems and verify the absolute pressure in the chamber daily.
Confirming pressure by calculation:
When limited to a pressure gauge using a gauge scale, the absolute pressure can be calculated (with reasonable accuracy) by adding the average barometric pressure for your facility found from a reliable source such as the blood gas lab, TcpO2 monitors, or weather sources. If using weather sources, confirm that the pressure scale they are using is the same as your gauge. Please note that airports report a standardized barometric pressure relative to sea level and it may not be the actual barometric pressure for your location.
Confirming pressure mechanically:
The most accurate way to confirm your chamber is at the absolute pressure prescribed is to install a calibrated absolute pressure gauge.
Things to consider:
1. The pressure relief valves should be checked to make sure that they will accommodate the increased pressure. Example, Denver to achieve 44.1 psia/3.0 ATA, the chamber will be compressed to approximately 32 psig. Monoplace chambers are designed for 30 psig working pressure.
2. The chamber manufacturer may need to be engaged as your chamber gauges are designed for sea level use: a monoplace chamber gauge may not display values above 30 psig and, in fact, the controls for a monoplace may not allow a pressure greater than 30 psig without adjustment.
3. If this is a multiplace chamber the increased chamber pressure to achieve ATA will affect the inside attendant’s sea level equivalent depth, and this must be accounted for in your procedures. Failure to take this into account may result in decompression illness risk for the inside attendants.
4. Is this undertaking warranted for your facility’s altitude? There is no right answer. Each facility needs to determine what is best for their situation.
5. Perform a risk assessment if chamber modifications are not supported by the chamber manufacturer.
6. Will this modification void your warranty?
7. If you undertake these changes, there is a need for enhanced staff training.
8. Update your maintenance program to address the additional gauge(s), as appropriate.
9. There may be issues not discussed here that are specific to your facility’s chamber(s).
• U.S. Navy Diving Manual. Revision 7 Washington, D. C.: U.S. Government Printing Office; 2016.
• UHMS 2013 ASM, Abstract, Absolute Pressures for monoplace and multiplace chambers, Bell JE, Weaver LK, Churchill S, Haberstock D
• UHM 2014, Vol. 41, No. 6 – Altitude Decompression tables for HBO2 Attendants, James Bell, CHT/EMT/CFPS, Paul A Thombs, M.D., William J. Davison, CHT, Lindell K Weaver, M.D.,FACP, FCCP, FCCM, FUHM
The UHMS Safety Committee
Comment on FAQ: “How would an MD compensate for altitude in a monoplace chamber”
Absolute Pressures for monoplace and multiplace chambers
Neither the Undersea and Hyperbaric Medical Society (UHMS) staff nor its members are able to provide medical diagnosis or recommend equipment over the internet. If you have medical concerns about hyperbaric medicine you need to be evaluated by a doctor licensed to practice medicine in your locale, who can provide you professional recommendations for hyperbaric medicine based upon your condition. The responsibility of approving the use of equipment resides with the physician and safety director of the facility. Information provided on this forum is for general educational purposes only. It is not intended to replace the advice of your own health care practitioner and you should not rely upon it as though it were specific medical advice given to you personally.