To the Editor:

The indications for, contraindications to, and adverse effects of hyperbaric oxygen have been established by the Committee on Hyperbaric Oxygenation of the Undersea and Hyperbaric Medical Society (Kensington, MD) and are revised every 3 yr, with the last edition published in 1999. 1All hyperbaric chambers and related support systems must be designed, fabricated, installed, and operated in accordance with the requirements of the American National Standards Institute and American Society of Mechanical Engineers Pressure Vessels for Human Occupancy 1. For fire safety reasons, certain electrical devices without formal Food and Drug Administration approval for use in hyperbaric conditions are prohibited in hyperbaric oxygenation chambers. 2This includes some patient-controlled analgesia devices. For that reason, patients receiving continuous, intravenous analgesic treatment may need to receive alternative medication during long hyperbaric oxygenation sessions. Oral analgesic compounds, such as acetaminophen, are often sufficient. However, use of analgesics prepared as effervescent tablets is potentially dangerous if administered during the period of compression. The problem relates to the release of dissolved carbon dioxide during decompression. After a tablet is dissolved in water, the liquid is saturated with carbon dioxide; Henry’s law indicates that as ambient pressure is reduced, this dissolved gas leaves the solution. We assessed gas production over a 60-min period beginning at the moment of dissolution of a single 500-mg effervescent analgesic tablet in 200 ml water (Dafalgan®; UPSAMEDICA, Brussels, Belgium).

In the 1 atmosphere absolute (ATA) group (outside the chamber), 75 ml gas was released during the initial “bubbling phase” (lasting approximately 2 min), and an extra 15 ml was released over the remainder of the hour. When identical tablets were dissolved at 2.5 ATA, only 15 ml gas was released during the initial bubbling phase. Then, the chamber was decompressed, and an additional 75 ml gas left the solution. It is not difficult to imagine the possible consequences of similar gas release occurring during decompression in patients to whom dissolved effervescent tablets are administered during hyperbaric therapy, particularly if two or more tables are used.

UHMS Operations Committee 1999 Report. Kensington, Maryland, Undersea and Hyperbaric Medical Society, 1999
Cook GA, Meierer RE, Schields BM: Screening of flame-resistant materials and comparison of helium with nitrogen for use in diving atmospheres. First Summary Report under ONR Contract N00014-66-C-014. Tonawanda, New York, Union Carbide, 1967