Oxygen in Suspected ACS May Benefit Those With Low Levels

September 03, 2019

PARIS — In patients with suspected acute coronary syndrome (ACS), there was no difference in overall 30-day mortality between a high- and low-usage oxygen protocol in a new study involving the whole of New Zealand. However, patients with reduced oxygen saturation levels (below 95%) did appear to benefit from oxygen treatment.

"Our results suggest that patients with suspected ACS with normal oxygen saturation levels are very unlikely to benefit from routine high-flow oxygen treatment, although no harm was seen," said lead investigator Ralph Stewart, MD, University of Auckland, New Zealand.

"It is possible, however, that high-flow oxygen improves outcomes for patients who have a lower than normal oxygen saturation level, less than 95%, but above the current recommended guideline for use of oxygen, 90% saturation," he concluded.

"After these results, I would suggest that oxygen levels be monitored and oxygen be given if saturation levels fall below 95%," Stewart told theheart.org | Medscape Cardiology.

The New Zealand Oxygen Therapy in Acute Coronary Syndromes (NZOTACS) trial was presented this week at ESC Congress 2019.

Routine Care

Stewart explained that oxygen has been used as part of routine care for patients with suspected ACS for many years, but no study has shown benefits of oxygen in this situation and in recent years some studies have suggested harm.

Current ESC guidelines recommend that oxygen be given when oxygen saturation levels are below 90%, but not above.

The aim of this study was to compare 30-day mortality associated with two different oxygen protocols (high and low usage) as part of routine care in patients with suspected ACS in New Zealand.

The high-oxygen protocol consisted of oxygen delivered by face mask at 6 to 8 L per minute given to all patients with suspected ACS, irrespective of oxygen saturation levels, with oxygen stopped when clinical evidence indicated the ischemia had resolved.

The low-oxygen protocol recommended that oxygen be given only when saturation fell below 90%. Oxygen was then given to a level of 90% to 94% and was stopped when it was not needed to maintain levels above 90%.

The study had a randomized cluster design and involved the whole of New Zealand, with all regions using the two protocols separately for about a year each.

A total of 40,872 patients were included from two New Zealand registries of ACS patients, which were linked to administrative datasets to determine socioeconomic and demographic characteristics of the patients, their final diagnosis, and 30-day mortality.

Final diagnoses showed that 10% had ST-segment elevation MI (STEMI), 25% non-ST-segment elevation MI (NSTEMI), and 8% unstable angina. Almost half the patients did not have ACS as their final diagnosis.

Primary outcome showed very similar 30-day mortality rates between the two groups — 3.02% in the high-oxygen cohort and 3.12% in the low-oxygen group — which is a nonsignificant difference.

In the prespecified group of patients with STEMI (n = 4000), 30-day mortality rates were much greater, as would be expected, and there was a suggested benefit in the high-oxygen group (8.8% vs 10.6%; odds ratio, 0.81), although this did not reach significance.  

Results in NSTEMI-ACS and no-ACS patients showed no differences in 30-day mortality between the high- and low-oxygen protocols.

Noting that it is possible that the effects of oxygen may be determined on whether the patient has a normal or a reduced oxygen saturation level, Stewart said the study evaluated this for the majority of patients cared for in the ambulance registry by looking at the first oxygen level when the ambulance arrived.

These results showed that in those with a normal oxygen saturation level (above 95%), who accounted for almost 90% of the population, mortality rates were similar between the high- and low-oxygen protocols (2.1% vs 1.9%).

However, in the patients with lower oxygen saturation level (less than 95%), who accounted for 12% of patients, 30-day mortality was substantially higher (10.1% in the high-oxygen group vs 11.1% in the low-oxygen group).

"There appeared to be a trend for benefit with high use of oxygen across all the diagnostic groups in these patients with lower oxygen saturation levels," Stewart said. "While this didn't reach statistical significance, they would be clinically significant, with about a 2% absolute difference in 30-day mortality rates between the high versus low use of oxygen in patents with ACS."

Demanding Setting

Discussant of the study, Robin Hofmann, MD, Karolinska Institute, Stockholm, congratulated the NZOTACS investigators on their study. "To perform a clinical trial in the acute prehospital/hospital setting is very demanding. To do this in a whole nation is extraordinary," he said.

"For a very long time we have been giving oxygen routinely to all patients with suspected ACS irrespective of baseline saturation levels to prevent hypoxemia, but this may cause hyperoxemia inadvertently, which may involve risks," Hoffman commented.

He noted that the AVOID trial in 2015 showed excessive MI infarct size in STEMI patents treated with oxygen, but the subsequent DETO2X-AMI trial (led by Hofmann) showed neutral results, which was the basis for the most recent ESC guidelines, which recommend oxygen for patients with saturation levels below 90% only.

He said the latest New Zealand trial involved an impressive 41,000 patients, representing suspected ACS patients close to the underlying population, and results therefore are generalizable. But he added that the pragmatic design comes at a price, with a low adherence to the protocol (for example, only 40% of patients in the high-oxygen group actually received oxygen), and at this stage there are limited baseline and procedural data on secondary outcomes.

Hofmann concluded that in patients with suspected ACS who do not have hypoxemia, routine oxygen therapy provides no benefit and should therefore not be given.

"With the new data from this trial, I believe this recommendation can now be upgraded to level of evidence A," he said. "But concerning subgroups and the ideal cutoff to initiate oxygen therapy, more data are need to draw robust conclusion."

European Society of Cardiology Congress 2019. Presented September 1, 2019.

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