The quality of evidence is downgraded by study limitations (selective outcome reporting), and by imprecise results (few outcome events and wide confidence intervals).
A Cochrane review [Abstract] 1 included 5 studies with a total of 1 173 subjects with confirmed (2 studies) or suspected (3 studies) acute myocardial infarction (AMI). The intervention was inhaled oxygen at 4 to 8 L/min given by mask in 4 studies and by a nasal cannula in one study. The comparator was air in 4 studies and titrated oxygen delivered by nasal prongs or mask adjusting the flow-rate to achieve an oxygen saturation of 93% - 96% in 1 study.
There was no difference in all-cause mortality at hospital discharge in patients with confirmed AMI or in an intention-to-treat analysis including all participants, also those without confirmed AMI (table T1). In the meta-analysis, the same number of people died (n = 16) in each group. Only one study reported all-cause mortality at 6 months: 9 participants out 318 died in oxygen group versus 13 out 320 in the air group (RR 0.39, 95% IC 0.14 to 1.07; 1 study, n=628). One study measured pain directly, and 2 others measured it by opiate usage. There was no effect for oxygen on pain relief when pain was directly measured nor when studies measured opiate use as a surrogate for pain. Recurrent ischaemia tended to be higher in the oxygen group compared to the air group but the difference was not statistically significant. There was no clear effect for oxygen on infarct size.
| Outcome | Relative effect (95%) | Assumed risk - Air or titrated oxygen | Correasponding risk - Oxygen (95% CI) | Participants (studies) |
|---|---|---|---|---|
| * including those without confirmed AMI; **statistical heterogeneity I2 =80% | ||||
| All-cause mortality (participants with AMI) | RR 1.02 (0.52 to 1.98) | 36 per 1000 | 37 per 1000(19 to 71) | 871(4 studies) |
| All-cause mortality (all participants*) | RR 0.99 (0.50 to 1.95) | 28 per 1000 | 28 per 1000(14 to 55) | 1 123(4 studies) |
| All-cause mortality (all participants*) in studies done in the revascularisation era | RR 0.58 (0.24 to 1.39) | 27 per 1000 | 16 per 1000(7 to 38) | 923(3 studies) |
| Opiate use as a proxy for pain (all participants*) | RR 0.97 (0.78 to 1.20) | 583 per 1000 | 566 per 1000(455 to 700) | 250(2 studies) |
| Recurrent myocardial infarction (or ischaemia) | RR 1.67 (0.94 to 2.99)** | 64 per 1000 | 87 per 1000(50 to 152) | 578(2 studies) |
Another meta-analysis 2 included 8 RCTs with a total of 7 998 subjects comparing the use of supplemental O2 therapy with room air. Oxygen did not reduce the risk of in-hospital (OR 1.11, 95% CI 0.69 to 1.77) or 30-day mortality (OR 1.09, 95% CI 0.80 to 1.50) in patients with suspected AMI, and the results were similar in the subgroup of patients with confirmed AMI. O2 therapy reduced the risk of hypoxaemia (OR 0.29, 95% CI 0.17 to 0.47). The infarct size, based on cardiac MRI, was not different between groups with and without O2 therapy.
A registry-based RCT 3 randomly assigned patients (n=6 629) with suspected myocardial infarction (MI) and an oxygen saturation of 90% or higher to receive either supplemental oxygen or ambient air.Hypoxemia developed in 1.9% of patients in the oxygen group and 7.7% in the ambient-air group. There was no difference in death from any cause within 1 year after randomization (HR 0.97, 95% CI 0.79 to 1.21) or rehospitalization with MI within 1 year (HR 1.13, 95% CI 0.88 to 1.46).
The results of this review do not mean that oxygen should be withheld from patients with obvious hypoxia.
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