Oxygen therapy

Ensuring that tissues are adequately oxygenated, with the help of supplemental oxygen where necessary, is a prime objective of critical/acute medical care. By measurement of the amount of oxygen dissolved in blood (pO₂(a)) and that bound to hemoglobin (sO₂(a)), arterial blood gas analysis provides the most accurate method for diagnosing hypoxemia and identifying those patients who require supplemental oxygen therapy. 

It also provides the means for monitoring such therapy. An interesting and wide-ranging review article recently published in the journal Respiratory Care provides an account of the current evidence base that supplemental oxygen is a safe and effective therapy. 

A take-home message of the article is that oxygen therapy is commonly used in a range of clinical contexts for which the evidence of efficacy is either non-existent or weak. Indeed, for some conditions, oxygen therapy, particularly high-dose oxygen therapy, may be more harmful than breathing room air. 

The article implies the need for more considered use of this widely prescribed drug. The need for more research directed at establishing the safety and efficacy of oxygen is stated explicitly.

The article begins with a brief history of oxygen discovery in the 18th century and its early medical use; apparently oxygen was first used medically in 1783 by a French physician for the treatment of a patient with tuberculosis. There follows a short general discussion of the use of oxygen in modern medical care, which includes some reference to current expert guidelines on the use of supplemental oxygen. 

For example, US guidelines recommend that documented hypoxemia (pO₂(a) <60 mmHg or sO₂(a) <90 %) should provoke the use of oxygen; other indications include: suspected hypoxemia, severe trauma, acute myocardial infarction and recovery from anesthesia.

The rest of the article, by far the greatest part, is devoted to detailing conditions for which oxygen is often used therapeutically, and discussing the available evidence that either supports or refutes its use in each of these. 

The conditions discussed are: chronic obstructive airways disease (COPD), trauma, acute respiratory distress syndrome (ARDS), myocardial infarction (MI), cardiac arrest, heart failure, stroke, wound infection, postoperative nausea and vomiting, cluster headache, carbon monoxide poisoning and breathlessness.  There is also discussion of the use of oxygen in perinatal and neonatal care.

Conditions for which the author found no clear evidence of benefit (in the absence of hypoxemia) include: myocardial infarction, heart failure, stroke and breathlessness. 

Most trauma patients are given supplemental oxygen but evidence suggests they gain no benefit from it unless they are hypoxemic or have suffered brain injury (there is evidence that mild to moderate hyperoxemia induced by oxygen administration increases survival following traumatic brain injury, although severe hyperoxemia reduces survival). The evidence considered by the author led him to conclude that “many trauma patients need little or no oxygen”.

It remains unclear if supplemental oxygen is beneficial in terms of survival following cardiac arrest. Limited study has demonstrated decreased survival among hyperoxemic patients, militating against the use of oxygen during cardiac arrest.

Cluster headache and carbon monoxide poisoning represent two conditions for which there is clear unequivocal evidence that oxygen therapy is beneficial.

There is strong undisputed evidence that oxygen should be used sparingly in patients with COPD (due to the risk of hypercapnia) and for all premature neonates (due to the risk of retinopathy of prematurity). In the case of premature neonates the actual target oxygen saturation remains controversial but is in the range 88-94 %.