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INTRODUCTION

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Hyperbaric oxygen (HBO) is used therapeutically in poisoning by carbon monoxide (alone or if complicated by cyanide poisoning), methylene chloride, hydrogen sulfide, and carbon tetrachloride. It is also a recognized therapy for air or gas embolism, such as may arise from ingestion of hydrogen peroxide, and for anemia, a functional form of which arises from oxidants that induce methemoglobinemia.

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HISTORY

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Hyperbaric medicine became established as a clinical discipline in the later half of the 20th century with a focus on treatment of decompression sickness. Utilization of hyperbaric chambers has expanded in the past 60 years with improved understanding of basic mechanisms of action. The first case reports of HBO for documented CO poisoning appeared in 1960,192 and the consistent application of HBO in CO poisoning began in many centers at that time.190 The first report presenting statistical evidence of the superiority of HBO compared to normobaric oxygen in CO poisoning, as well as a description of the “late syndrome,” was published in 1969.74

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PHARMACOLOGY

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Chemistry and Preparation

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In hyperbaric oxygen (HBO) therapy the patient breathes 100% O2 while exposed to increased atmospheric pressure. Treatments are performed in either a monoplace (single patient) or a multiplace (typically 2–14 patients) chamber. Pressures applied while patients are in the chamber usually are 2 to 3 atmospheres absolute (ATA), where sea level air pressure equals 1 ATA. Treatments generally last for 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 as clinically indicated.

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Mechanisms of Action

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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 reduces the volume of a gas according to Boyle’s law (within a closed system, absolute pressure and volume of a given mass of a confined gas are inversely proportional, if the temperature remains unchanged). This action has direct relevance to pathologic conditions in which gas bubbles are present in the body, such as arterial gas embolism, decompression sickness, and ingestion of concentrated solutions of hydrogen peroxide (H2O2). During treatment, the arterial oxygen tension typically exceeds 1500 mm Hg and achieves tissue oxygen tensions of 200 to 400 mm Hg—over fivefold higher than when breathing air.203 While one is breathing air under normal environmental conditions, hemoglobin is saturated with oxygen on passage through the pulmonary microvasculature, and the primary effect of HBO is to increase the dissolved oxygen content of plasma. In addition, HBO affects neutrophil adhesion to blood vessels and restores mitochondrial, neutrophil, and immunologic disturbances caused by CO poisoning.

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