Understanding the role of oxygen in acute coronary syndromes.

The routine administration of oxygen to patients presenting with features of acute coronary syndrome (ACS) is a practice that is deeply embedded in the routine of frontline health care professionals, and it has been a primary intervention for persons with ACS for more than 100 years. It is noteworthy that this tradition was supported by the American Heart Association (AHA) from 1975 through 2005 in the form of recommendations for treatment, and it was supported by the American College of Cardiology through 2007 as well. This practice of administering oxygen has been supported by research completed during the past century, which concluded that supplementary oxygen could diminish the size of myocardial ischemic injury resulting from ACS. However, the conclusions derived from the aforementioned research were generalized from animal models. More recently, members of the scientific community have questioned these generalizations and the validity of the studies, noting the vast differences in the 2 species (dogs and humans) in terms of coronary anatomy, collateral circulation, natural disease state, and hemodynamic responses. The routine administration of oxygen for all patients presenting with symptoms suggestive of ACS has become a treatment of tradition that may not be supported by scientific evidence. In fact, the 2010 Advanced Cardiac Life Support guidelines recommend oxygen supplementation for uncomplicated ACS only with an oxyhemoglobin saturation of ≤94% or with signs and symptoms suggestive of respiratory distress. Less familiar to frontline practitioners are the critiques surrounding the 1970s recommendation to routinely administer oxygen to all patients presenting with suspected ACS. In addition to the generalization of the original animal studies to human cases, several studies documented the potentially harmful effects of administering supplementary oxygen in the absence of hypoxemia. In addition, no conclusive study has been performed to indicate that the administration of oxygen is beneficial in the reduction of myocardial ischemic pain. Strangely, this information has not captured much attention in the practice setting in terms of deterring the routine administration of oxygen. The query raised over the years relates to the administration of supplementary oxygen to persons who are normoxic and the potential to induce a state of hyperoxia that could result in potentially negative cardiovascular effects. Hypoxia has been described by some researchers as an oxyhemoglobin saturation of less than 90% and by others as less than 94%. For the purposes of this article, we consider persons with an oxyhemoglobin saturation of 94% or greater to be normoxic. The aim of this article is to review the evidence available and clarify for frontline practitioners the indications for supplementary oxygen and the potential harmful effects of hyperoxia as it relates to ACS.