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INTRODUCTION

Hyperbaric oxygen (HBO) is used therapeutically in poisoning by carbon monoxide (CO) (alone or if complicated by cyanide {CN} poisoning), methylene chloride, hydrogen sulfide (H2S), and carbon tetrachloride (CCl4). It is also a recognized therapy for air or gas embolism, such as may arise from ingestion of hydrogen peroxide (H2O2), and for anemia, a functional form of which arises from oxidants that induce methemoglobinemia.

HISTORY

Hyperbaric medicine became established as a clinical discipline in the latter 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 1960,203 and the consistent application of HBO in CO poisoning began in many centers at that time.200 The first report presenting statistical evidence of the superiority of HBO compared with normobaric oxygen in CO poisoning, as well as a description of the “late syndrome,” was published in 1969.76

PHARMACOLOGY

Chemistry and Preparation

Pressures applied while patients are in the hyperbaric chamber usually are 2 to 3 atmospheres absolute (ATA); sea level air pressure equals 1 ATA. Treatments generally last for 1.5 to 8 hours, depending on the indication, and are performed one to three times daily. Monoplace (single-person) chambers usually are compressed with pure oxygen. Multiplace (2–14 patients treated simultaneously) chambers are pressurized with air, and patients breathe pure oxygen through a tight-fitting face mask, a head tent, or an endotracheal tube as clinically indicated.

Mechanisms of Action

During treatment, the arterial oxygen tension typically exceeds, 1,500 mm Hg and achieves tissue oxygen tensions of 200 to 400 mm Hg—more than fivefold higher than when breathing air.214 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.

Pharmacokinetics

Oxygen inhaled at hyperbaric pressure is rapidly absorbed. 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. 28). Animal models of focal ischemia suggest that HBO rapidly distributes to target organs to improve penumbral oxygenation.210 Hyperbaric oxygen increases tissue oxygen concentration and wound oxygen delivery in humans.182 Regarding pharmaceutical interactions, hyperbaric hyperoxia does not appear to produce appreciable alterations in the pharmacokinetics of pentobarbital, salicylate, or theophylline in canine models117-119 or gentamicin in healthy human volunteers.151

Pharmacodynamics

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