“The indoctrinated treatment concept that all patients experiencing a heart attack should receive high-concentration oxygen therapy is not supported by available evidence,” Dr. Richard Beasley from the Medical Research Institute of New Zealand in Wellington, who worked on the study, told Reuters Health by email.
“This realisation,” he said, “has led to changes to oxygen guidelines, which now recommend that oxygen should not be administered routinely to patients with chest pain of suspected cardiac origin, but limited to patients who are hypoxic. It is important that health professionals follow these guideline recommendations, as the unrestricted use of high concentration oxygen therapy to patients experiencing heart attacks, has the potential to cause harm,” Dr. Beasley said.
He and his colleagues studied 136 patients presenting with a first STEMI uncomplicated by cardiogenic shock or marked hypoxia. For six hours after presentation, half of the patients received high-concentration oxygen therapy (6 L/min via medium concentration mask) and the other half received titrated oxygen (to achieve oxygen saturation 93% to 96%).
The primary outcome variables were 30-day mortality and infarct size assessed by troponin T level at 72 hours. According to a report in the American Heart Journal for February, one patient in the high-concentration arm and two in the titrated oxygen therapy arm died (relative risk 0.5; p=0.56).
As anticipated, note the authors, the study had insufficient power to determine statistically significant differences in mortality between the two groups so they performed a meta-analysis including these data and data from two previous studies reporting mortality data from similar randomized trials. This analysis suggested about a two-fold increased odds of death with high-flow oxygen therapy compared with room air or titrated oxygen.
The researchers didn’t see any significant between-group differences in troponin T level, infarct mass, or percent infarct mass.
In their report, Dr. Beasley and colleagues emphasize that they excluded patients with a history of previous MI, those with cardiogenic shock or marked hypoxia, and those with severe COPD in whom high-concentration oxygen therapy might result in hypercapnia.
These exclusions resulted in a “low-risk trial population, limiting the generalizability of the findings to those with an initially uncomplicated first myocardial infarction, and consequently reduced the power to determine differences in mortality.”
They further note that the randomized oxygen regimen was administered for six hours in accordance with guideline recommendations at the time of protocol development. Treatment could not be blinded because dose adjustments were required in the titrated group. “Although this should not have resulted in significant bias because the end points were objective and the MRI measurements were made blind to the randomized treatment, an influence on subsequent care from the knowledge of randomized treatment cannot be excluded,” they say.
In summary, the researchers say there is “no substantive evidence of benefit to support the routine administration of oxygen in patients with myocardial infarction, not complicated by cardiogenic shock or marked hypoxia at initial presentation.”
In their opinion, a “strong case exists for multicenter randomized controlled trials, sufficiently powered to determine whether the two strategies have a clinically relevant influence on mortality. Pending the results of such studies, we concur with the recent oxygen guidelines recommendation that supplementary oxygen should not be administered routinely to patients with acute chest pain of suspected cardiac origin but limited to patients in whom hypoxia is present, with oxygen saturation monitored and used to guide its administration,” the authors conclude.
In an email to Reuters Health, Dr. Lawrence Sinoway, from the Penn State Hershey Heart and Vascular Institute in Hershey, Pennsylvania, who was not involved in the study, praised the study team for doing the study. Yet, “sadly the types of heart attack patients in their two groups were not that similar (the high-dose oxygen group had a lot of ‘inferior infarcts’ and the oxygen titration group had a lot of ‘anterior infarcts’). This issue makes evaluation of the result difficult,” Dr. Sinoway commented.
Another limitation, Dr. Sinoway said, is the fact that the randomization (high vs titrated oxygen) was not done on initial ambulance evaluation; “thus all patients got similar therapy for a period of time after developing symptoms. This also makes evaluation tough,” he said.