Hyperbaric oxygen (HBO) therapy is a treatment modality whereby a person breathes 100% O2 while exposed to increased atmospheric pressure. Treatments are performed in either a monoplace (single patient) or a multiplace (typically two to 14 patients) chamber. Pressures applied while patients are in the chamber usually are two to three atmospheres absolute (ATA). Treatments vary from 1.5 to 8 hours, depending on the indication, and may be performed one to three times daily. Monoplace chambers usually are compressed with pure oxygen. Multiplace chambers are pressurized with air, and patients breathe pure oxygen through a tight-fitting face mask, a head tent, or endotracheal tube.
Therapeutic mechanisms of action for HBO are based on elevation of both hydrostatic pressure and the partial pressure of oxygen. Elevation of the hydrostatic pressure causes a reduction in the volume of gas according to Boyle's law. This action has direct relevance to pathologic conditions in which gas bubbles are present in the body, such as arterial gas embolism and decompression sickness. During treatment, the arterial oxygen tension typically exceeds 1500 mm Hg, and tissue oxygen tensions of 200 to 400 mm Hg.153 Under normal environmental conditions, hemoglobin is virtually saturated with oxygen on passage through the pulmonary microvasculature, so the primary effect of HBO is to increase the dissolved oxygen content of plasma. Application of each additional atmosphere of pressure while breathing 100% oxygen increases the dissolved oxygen concentration in the plasma by 2.2 mL O2/dL (vol%) (Chap. 21).
Mechanisms of HBO relevant to its use as an antidote are its ability to diminish hypoxic stress by increasing tissue oxygen tension and decreasing the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS).74,130,142,150,151 Reactive species are recognized to serve as signaling molecules in transduction cascades.2,25,166 Reactive species have positive and negative effects depending on their concentration and intracellular localization. Hence, with regard to HBO as with any xenobiotic, one must contend with a dosing issue in order to achieve benefit versus injury. Because exposure to hyperoxia in typical clinical HBO protocols is rather brief, studies show that antioxidant defenses are adequate so that tissue injuries can be avoided.44,45,123,164
Although elevating tissue O2 tension is a primary effect of HBO, lasting benefits are related to abatement of the underlying pathophysiological processes. In the context of an antidote, HBO is most commonly used for treatment of carbon monoxide (CO) poisoning. Experience using HBO for life-threatening poisonings from cyanide (CN), hydrogen sulfide (H2S), or carbon tetrachloride (CCl4) and in patients with high methemoglobin levels is limited. HBO has been suggested for management of diverse poisonings, but discussion of these applications is beyond the scope of this Antidotes in Depth because supporting clinical and experimental evidence is sparse.161
Administration of supplemental oxygen is the cornerstone for treatment of ...