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Oxygen has been widely used in acute coronary syndromes (ACS) but it is not certain if it does any good. Or, indeed, any harm. Yesterday, a Hot Line presentation from Professor Ralph Stewart (Auckland City Hospital, Auckland, New Zealand) reported results from the large NZOTACS study showing that a liberal oxygen strategy was no better or worse than a conservative oxygen strategy in terms of mortality.
“Given the size of New Zealand and its integrated healthcare system, we were able to conduct a country-wide, randomised, cross-over study comparing two oxygen-delivery protocols used as standard in the treatment of patients with suspected ACS,” explains Prof. Stewart. Around 40,000 patients with suspected ACS were identified from the All NZ Acute Coronary Syndromes Quality Improvement registry (ANZACS-QI) and the St John’s Ambulance ACS registry. A liberal strategy of high-flow oxygen (6–8 L/min) was recommended for suspected or confirmed ischaemic symptoms, whatever the oxygen saturation (SpO2) level. According to the conservative protocol, oxygen was recommended only if SpO2 was <90%. Over the two years of the study, each of the four New Zealand regions were randomised to either the liberal or the conservative strategy, with cross-over to the alternative strategy such that each strategy was administered for about one year in each region. This resulted in a good match of baseline characteristics between the treatment groups.
“The data from this huge study showed that, at least overall, the oxygen-delivery strategy used did not make a difference to outcome in patients with ACS,” says Prof. Stewart.
At two years, the 30-day mortality was almost exactly the same with the liberal and the conservative oxygen-delivery protocols (3.0% vs 3.1%, odds ratio 0.96, 95% confidence interval 0.86–1.08). However, the results may have been influenced by a number of factors. According to Prof. Stewart, “Nearly 60% of the study population did not have a final diagnosis of ACS. In these patients, the mortality rate was a little under 2%, and any effect of oxygen was likely to be small. Another challenge was that adherence to the protocol was only around 80% overall. It was lower in the liberal compared with the conservative strategy group, probably because this was further away from guideline recommendations. Also, many patients did not receive oxygen because they did not have ischaemic symptoms at the time of assessment. Even though we had a very large study, these issues reduced its statistical power.”
However, the study did suggest that some groups of patients could benefit from the liberal oxygen strategy, although the numbers are too small to allow definite conclusions to be drawn. “There was a signal that patients with ST-elevation myocardial infarction, who had a mortality rate of around 10%, may do better with the higher oxygen delivery approach. Also, patients with SpO2 <95%—not considered to be hypoxaemic according to current guidelines—had mortality rates about five-times those of patients with higher oxygen saturation, and a ~1% lower mortality rate with the liberal oxygen-delivery protocol.”
Whether oxygen-delivery strategies will be investigated in more depth will depend on the importance placed on possible small effects on mortality. “In my opinion,” says Prof. Stewart, “even a small benefit or harm from oxygen is clinically relevant. I think the possibility of benefit from oxygen in patients with mild decreases in oxygen saturation levels should be studied further, maybe with an international collaboration because of the large numbers needed.”
In addition to the insights the study provided about oxygen-delivery strategies, Prof. Stewart thinks it has some messages for research as a whole. “Our study shows that randomised studies conducted as a part of healthcare delivery, despite some limitations, have great potential to identify which treatments benefit patients, as well as those which don’t.”
Meet the Trialist – NZOTACS Today, 10:00 – 10:30; ESC TV Stage – ESC Plaza
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