Atmosphere Control

Careful control of the chamber's atmosphere is essential to provide a safe environment for the patient and tender. The major components of atmosphere quality control are O₂ , CO₂ , and trace gases.

In a multiplace chamber it is essential that the patient breathe as high a concentration of O₂ as possible (usually 98% or greater) and that the O₂ concentration within the chamber be maintained close to 21% to minimize the fire hazard. In some hyperbaric units, the head tent O₂ concentration is routinely monitored. In others, the concentration is assumed to be high because of a high rate of O₂ flow through the head tent. Leakage of O₂ from head tents, masks, and ventilators will tend to raise the atmospheric O₂ concentration. Typically, an upper limit of around 23% is used as a criterion for ventilating the chamber with air or 100% nitrogen until the O₂ concentration decreases. Monoplace chambers are continuously ventilated at 85 to 240 L/min, a rate that will usually keep the O₂ concentration close to 100%.

CO₂ control, particularly within a head tent, is important because significant elevations in the inspired CO₂ concentration will potentiate O₂ toxicity, as noted earlier. Some hyperbaric physicians believe that a small amount of CO₂ in the inspired gas may be beneficial to vasodilate ischemic tissue and increase O₂ delivery, although no data are available to recommend one approach over the other. A typical standard for the upper limit of head tent CO₂ is 1% "surface-equivalent" CO₂ , equal to a partial pressure of 7.6 mm Hg. Chamber CO₂ is usually limited to around 0.5% surface equivalent (ambient PCO₂ ≅4 mm Hg), although in nonsaturation dives it is very unlikely that CO₂ would approach this value except in a very small chamber. With the use of a nonscrubbing (open circuit) system, head tent O₂ flow rates of 40 to 60 L/min (measured at chamber pressure) are usually adequate to keep CO₂ levels at an appropriately low level.

Trace gases that may enter the environment include CO and hydrocarbons from improperly functioning compressors or from automobile exhaust that may be near the compressor air intake. Volatile gases such as alcohol vapor from skin disinfectant solutions and mercury vapor from spillage of sphygmomanometer columns may also pollute the atmosphere. Trace gases become extremely important in hyperbaric chambers because of the increase in partial pressure that occurs with increasing ambient pressure. For example, a CO concentration of 50 ppm, though acceptable at 1 ATA, will be equivalent to 300 ppm at 6 ATA, a level that is clearly unacceptable. Compressed air from the chamber banks should be periodically submitted for analysis of trace gases. Nonvolatile skin preparation solutions such as benzalkonium or iodine compounds will not pollute the chamber's atmosphere. It is recommended that mercury in any form be excluded from hyperbaric chambers. At least one chamber in the United States had to be decommissioned because of pervasive mercury vapor contamination from spillage. Acute mercury poisoning has also occurred in occupants of a hyperbaric chamber when mercury from a broken thermometer fell on the floor and remained undetected.

Considerations of battery use may have implications for chamber atmosphere control, as well as fire hazards. All batteries release small quantities of hydrogen, though not usually in amounts that would be hazardous. As mentioned previously, lithium-sulfur dioxide batteries carry a theoretical risk of sulfur dioxide discharge. Similarly, there is an objection to the use of mercury cells. Release of inert gas taken up by the battery during the dive might theoretically result in leakage of mercury into the atmosphere. This risk must be weighed against the potential benefits (e.g., of using mercury cell-powered temporary pacemakers). Alkaline cells are considered safe, although temporary failure has been observed at extremely high ambient pressure (40 to 60 ATA). The only battery that has been specifically tested at increased ambient pressure is the Gates lead acid, gelled electrolyte battery, which is rated by the manufacturer up to 8 ATA.^[189] After pressure compensation with mineral oil, such batteries may in fact operate satisfactorily to an ambient pressure of 680 ATA.