Oxygen Therapy and Toxicity
Oxygen was discovered in the late 18th century simultaneously by several investigators. Joseph Priestley, a minister in England, made his discovery after attempting to melt mercury oxide using a magnifying glass and the sun's rays. The vapor that was produced by this melting allowed a candle to burn brighter, and Priestley later discovered the vapor could be used in place of air to keep a mouse alive. He first published his findings in 1774. At about the same time, and in a similar way (i.e., by burning mercury oxide), Carl Scheele, a pharmacist in Sweden, described a gas he called “fire air.” He may have done his experiments before Priestley, but he did not publish his findings until 1777. Finally, after being told about this new vapor during a 1774 visit by Joseph Priestley, the chemist, Antoine Lavoisier, devised quantitative experiments which he used to prove the existence of this element and its role in combustion. He named the element “oxygen” and published his research in 1777.
Soon after its discovery, oxygen was being utilized as a medicine and cure-all for many respiratory diseases, including “consumption” and asthma. However, suspicions about the safety of oxygen therapy were raised almost as soon as it was discovered and even before the element was named. In 1775, Joseph Priestley wrote of oxygen (which, at that time, he called “dephlogisticated air”): “A moralist, at least, may say that the air which nature has provided for us is as good as we deserve.”1
Rigorous scientific evaluation of the potential toxicities of oxygen did not occur until the work of James Lorrain Smith, an Edinburgh pathologist, who first published the pulmonary pathologic alterations associated with oxygen exposure in 1899. It was also recognized around this time that under ambient air, the arterial blood was already at near-maximal oxygen-carrying capacity, putting into doubt the utility of further increasing the fraction of inspired oxygen.
Following these observations, the use of therapeutic oxygen fell into some disrepute.
In the early 1920s, supplemental oxygen therapy was reevaluated yet again. This was occurring as the detrimental effects of tissue hypoxia were being recognized, along with their reversibility with supplemental oxygen. Since the mid-20th century, especially with the advent of improved oxygen delivery systems, mechanical ventilation, and the modern intensive care unit, the use of oxygen has become a standard prescribed therapy for a multitude of cardiac and respiratory diseases. Oxygen therapy is now common in the outpatient setting. As of 2005 in the United States, approximately 1 million people receive long-term oxygen therapy (LTOT) from the Medicare Program, with total reimbursement costs related to LTOT exceeding 2 billion dollars per year.2
In this chapter, the physiology of hypoxia at the tissue level and mechanisms of arterial hypoxemia are discussed. In addition, various clinical assessments of hypoxemia, indications ...