The first recommendation is to go directly to the manufacturer of any equipment to be used in and around hyperbaric chambers. The second guideline is to contact regulatory agencies such as the FDA in the U.S. and the Notified Bodies certifying devices to the European Medical Directive. Other agencies include the U.S. Navy and the Norwegian Underwater Technology Center. Other sources of information include hyperbaric medical journals including those of the Undersea and Hyperbaric Medical Society (UHMS), the South Pacific Underwater Medicine Society (SPUMS), and the European Underwater and Baromedical Society (EUBS).
Inside of any hyperbaric chamber there is sufficient oxygen to allow combustion. With the higher partial pressure of oxygen (pO2) a fire will spread quickly and produce more heat than under normal conditions. The primary risk factor is ignition with the single greatest influence on ignition temperature being the oxygen percentage (not the pO2). As the oxygen percentage rises, ignition temperature decreases.
Preventing O2 from reacting with fuel sources or energized equipment components is an important concern. Additional oxygen sources may come from increased levels within the chamber, leakage of a hood or mask near the equipment, equipment that contains piped oxygen, for example, a ventilator or gas analyzer. Active ventilation, also called purging with either air or nitrogen, is necessary to prevent the oxygen percentage from rising. In a monoplace chamber the concentration of oxygen is 100%. The entire chamber is pressurized with 100% oxygen. In a multiplace chamber the pressurization is done with regular air and 100% oxygen is supplied by hood or mask.
Fuel sources inside equipment include any material or ignitable gas. Almost any fuel source will burn as a vapor combustion reaction. When a fuel source is heated to a temperature where a sufficient amount of vapor is released (the flashpoint) ignition occurs. Materials with low flashpoints are a concern. Volatile and flammable materials like alcohol and acetone are of great concern. Also, fuel vapors may be introduced into the chamber or trapped inside equipment which should be avoided. Even dust particles can behave like a vapor and may easily ignite. An emphasis on rigorous housekeeping is extremely important.
Another area of safety concern is preventing ignition by controlling energy sources. These can include static charge, exothermic reactions, current leakage, and powered components. Even if equipment does not have a power source, it still may have a static charge. This is addressed by using static-dissipating materials and grounding.
Additionally, the higher pO2 in the chamber may cause accelerated oxidation of materials. This makes oxygen compatibility of materials and materials intended to oxidize (for example, fuel cells or analyzer cells) a very real concern.
When equipment has a power source, all potential sources of heat or sparks must be considered. This includes motors, relays, thermostats, switches, batteries, power supplies, connections and wiring. All of the sources need to be inspected for their sparking and arcing potential.
Motors, lamps, circuit boards and ionizing filaments are other sources of heat. Battery life may be shorter with some components becoming hotter under hyperbaric conditions.
Purging with nitrogen, removing sparking components active ventilation, addition of a heat shield (thermal barrier), or addition of a heat sink are potential means to address these concerns.
The flow of gas is another potential heat source. This becomes a concern when gas flow speed exceeds safe limits for oxygen and air, where there are cycling components, where there are solenoid valves, where shock waves or resonant cavities exist, and where there are particles or dirt in the gas stream.
In summary, it is critical that sparks do not interact with oxygen and fuel sources and that heat generated by the equipment is strictly limited to prevent ignition of materials inside or near the equipment.
Other concerns involve the increase or decrease in ambient pressure that can affect both physical and operational failures leading to possible implosion, explosion, failure, and/or false information. The ability of equipment to resist and/or compensate for pressure changes needs to be evaluated. This includes equipment housings and sealed volumes (for example, keypads, internal sealed components relay housings, etc.)
Gas flow components are affected by pressure changes, causing variation in flow meter reading, and volumetric changes in ventilators. Increased gas density in the chamber increases the work load on motors, bearings and moving parts. This can result in faulty readings on measuring devices especially with blood pressure monitors, gas analyzers, or pressure gauges.
In the confined space of a hyperbaric chamber even small amounts of toxic gas or vapor may be of concern. The protocol is to off-gas any toxic or volatile chemicals from analyzer cells, batteries, lubricants, sealing compounds, lamps, sealed devices and any materials reactive with oxygen.
Codes, standards, and guidelines are a helpful source of information and serve a regulatory function. The following publications contain information relevant to hyperbaric medical equipment. Some provide guidelines and others are mandatory in some jurisdictions:
NFPA99: Standard for Health Care Facilities includes requirements for medical gas piping, medical equipment, and hyperbaric facilities. Current standards for clinical hyperbaric facilities are in Chapter 19. Eight more publications are listed on p. 142. Following it is a list of general guidelines for safe practice.
32UHM 2009, Vol. 36 No. 2 on choosing equipment for use in hyperbaric environments. “Decision process to assess medical equipment for hyperbaric use.” F. Burman, R. Sheffield, K. Posey