CRD summary

This systematic review of oxygen therapy in myocardial infarction suggested that high-flow oxygen may increase the infarct size and risk of mortality. Based on the available data, the conclusion that infarct size may be increased seems reliable. However, as only two small studies met the inclusion criteria, the validity of these conclusions may be limited.

Authors' objectives

To undertake a systematic review and meta-analysis of randomised placebo-controlled trials of oxygen therapy in myocardial infarction.

Searching

MEDLINE, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials (CENTRAL), EMBASE and CINAHL were searched to March 2007. Reference lists of all relevant studies were examined. Search terms were reported. Studies in languages other than English were included.

Study selection

Included studies had to be randomised placebo-controlled clinical trials of oxygen therapy in myocardial infarction that reported clinical outcome measures. Studies that used hyperbaric oxygen or intracoronary oxygen infusion were excluded as these were not commonly used. The primary outcome was in-hospital mortality. Secondary outcome variables included surrogate measures of infarct size (cardiac enzyme levels such as aspartate aminotransferase), occurrence of ventricular arrhythmia, opiate use and other relevant clinical variables.

Participants were patients with confirmed uncomplicated myocardial infarction in one study and patients with myocardial infarction who had received streptokinase within six hours in the other.

One included study compared oxygen therapy to compressed air through a medium concentration mask at a flow rate of 6L/min for 24 hours and the other compared oxygen to room air at 4L/min via face mask for 24 hours.

Two people examined each paper’s title and abstract and the full paper if necessary. It was unclear how differences were resolved.

Assessment of study quality

The trial quality was assessed using the Jadad score of adequacy of randomisation, blinding and follow-up (maximum score of 5 points).

The authors stated neither how many reviewers performed the validity assessment nor how differences were resolved.

Data extraction

Extraction of data was based on reported summary statistics (counts, means and standard deviations) for the intention-to-treat population.

The authors stated neither how data were extracted for the review nor how many reviewers performed the data extraction.

Methods of synthesis

Point estimates and confidence intervals (CI) were calculated for the main outcome variables for each included study, using relative risk (RR) for categorical variables and a t test for continuous variables. As only two studies met the inclusion criteria and only one reported the number of deaths in each treatment group, no formal meta-analysis was undertaken.

Results of the review

Two studies that randomised 250 participants (200 in a double-blinded trial and 50 in an unblinded trial) met the inclusion criteria. Although the authors stated intention-to-treat data would be extracted, outcome data was reported for 157 and 42 participants from each trial. The Jadad score for both included studies was a maximum 5, but there was no power calculation for either study.

In the double-blind trial of oxygen therapy compared to compressed air risk of death (RR 2.9, 95% CI 0.8 to 10.3, p=0.08), ventricular tachycardia (RR 2.1, 95% CI 0.8 to 5.8, p=0.13) and opiate use (RR 1.1, 95% CI 0.9 to 1.3, p=0.61) were not significantly different. The level of serum aspartate aminotransferase was reported to be higher in the oxygen therapy group (19.2 IU/mL, 95% CI 0 to 38.4, p=0.05) .

In the non-blinded trial of oxygen compared to room air, risk of ventricular tachycardia (RR 0.9, 95% CI 0.3 to 2.7, p=0.86) and opiate use (RR 0.8, 95% CI 0.6 to 1.1, p=0.15) were not significantly different. Oxygen saturation below 80% was significantly lower in the oxygen therapy group (RR 0.1, 95% CI 0.02 to 1.0, p=0.01).

Authors' conclusions

The limited evidence identified suggested that routine use of high-flow oxygen in uncomplicated myocardial infarction may result in a greater infarct size and possibly increase the risk of mortality.

CRD commentary

The review question and inclusion criteria were clear. The search strategy was good as several databases and other sources were searched and efforts to limit language bias were taken. The assessment of study validity used appropriate criteria. The authors reported methods designed to reduce reviewer bias in the selection of studies, but not for assessment of validity or extraction of data. It was unclear how any differences were resolved. Based on the available data, the authors' conclusions about infarct size seem reliable. However, the conclusion that mortality may be increased may not be reliable as there was no significant difference between groups. As only two small studies met the inclusion criteria, the validity of the conclusions may be limited.

Implications of the review for practice and research

Practice: The authors stated that there was insufficient evidence to support routine use of high-flow oxygen in treatment of uncomplicated myocardial infarction.

Research: The authors stated that there was an urgent requirement for randomised controlled trials of the use of oxygen therapy in myocardial infarction. Studies needed to be sufficiently powered to enable the risk of mortality to be assessed.

Funding

One author was a Wellington Hospitals and Health Foundation Research Fellow and one was a Training Fellow of the Health Research Council of New Zealand.

Bibliographic details

Wijesinghe M, Perrin K, Ranchord A, Simmonds M, Weatherall M, Beasley R. Routine use of oxygen in the treatment of myocardial infarction: systematic review. Heart 2009; 95(3): 198-202. [PubMed: 18708420]

Original Paper URL

http://heart.bmj.com/content/95/3/198.abstract

Indexing Status

Subject indexing assigned by NLM

MeSH

Evidence-Based Medicine; Guideline Adherence; Humans; Myocardial Infarction /mortality /therapy; Oxygen Inhalation Therapy /adverse effects; Practice Guidelines as Topic; Randomized Controlled Trials as Topic; Treatment Outcome

AccessionNumber

12009104186

Database entry date

24/03/2010

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.