Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

Highlighted lines and questions below provide links to the pertinent description of criteria in The EBM User's Guide, now available at the Canadian Centres for Health Evidence

Article Reviewed:

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Heliox vs Air-Oxygen Mixtures for the Treatment of Patients With Acute Asthma: A Systematic Overview.

Ho AMH, Lee A, Karmakar MK et al.

Chest 2003;123 882-890 [abstract]

Reviewed by Gerhard Wolf, MD, Children's Hospital, Boston

Review posted July 25, 2003

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I. Are the results of the study valid?

A. Primary questions:

1. Did the overview address a focused clinical question?

Yes. The authors state that the primary aim was to assess the efficacy of Heliox on respiratory mechanics and clinical outcomes in patients suffering from an acute asthmatic attack.

2. Were the criteria used to select articles for inclusion appropriate?

Yes. The authors included randomized controlled trials for evaluating their main clinical question. Patients without acute exacerbations or with induced peripheral airway obstruction were excluded. They chose all articles evaluating Heliox in adults and children with acute asthma exacerbation requiring hospital treatment.

B. Secondary questions:

3. Is it unlikely that important, relevant studies were missed?

Yes. The authors searched MEDLINE (1966-2002), EMBASE (1989 to 2002) and Cochrane Trials. Reference lists of articles were also searched for relevant articles. Authors of RCT's were contacted to obtain raw data. 15 articles were included: eight RCT's, one nonrandomized prospective controlled trial, one retrospective case-match controlled trial, four before-after case series, and one case report.

4. Was the validity of the included studies appraised?

Yes. (1) Only RCT's were included in the meta-analysis. (2) The RCTs were ranked to the level of allocation concealment (adequate, unclear, inadequate, double blinding) (3) Only RCT's with a common end point (PEFR%; peak expiratory flow rate as a percent of predicted) were included in the quantitative analysis. Raw data of a crossover-trial was also included after a carryover-effect was proven to be unlikely in this study.

5. Were assessments of studies reproducible?

Not clear. There were minor discrepancies between the authors upon the selection of the articles, but consensus was reported to be easily achieved. No kappa statistic is reported to assess interobserver agreement. The number of authors involved in the selection process is not reported.

6. Were the results similar from study to study?

Yes. Since none of the studies showed a significant difference between Heliox and air-oxygen mixtures in PEFR%, the results were similar in that respect.

On the level of nonsignificant increases in pulmonary function, all the studies that used heliox to drive a nebulizer showed a nonsignificant trend towards an increase in pulmonary function.

Different respiratory parameters were measured with different interventions. One study (Kudukis et al.) showed significant improvements for PP (pulsus paradoxus) and DI (dyspnea index: the DI is one measure of shortness of breath, dyspnea is categorized in 4 levels) These results were not pooled in the meta-analysis, since raw data were not obtained from the authors. All studies but one showed no statistically significant improvement in PEFR%. However, this result was questioned by the authors of the review after analysis of variances and taking baseline FEV into account.

Other studies showed improvement of single respiratory parameters.

II. What are the results?

1. What are the overall results of the review?

Meta-analysis of heliox vs. airO2 on PEFR% in asthmatic patients showed no significant difference between the two interventions (weighted mean difference 3%; 95% CI Ð2 to +8%).

Meta-analysis of trials that used heliox vs air O2 to nebulize albuterol showed no difference in PEFR% outcome (weighted mean difference, -0.03; 95%CI -6.43 to 6.37).

Trials that administered the test gas via standard/rebreathing masks also showed no difference in PEFR% (weighted mean difference 7.36, 95% CI -1.18 to 15.9)

The weighted linear regression model to examine the relationship between baseline PEFR% and treatment effect (weighted mean difference) suggested that there was a relation between baseline PEFR% and the PEFR% readings after treatment. Patients with a PEFR < 43% may benefit more from heliox.

There was a significant difference between interventions for DI from pooling the data from Carter and Kass (crossover double blind RCT/unblinded parallel RCT). The weighted mean difference was 0.60%; 95% CI 0.04 to 1.16 5/8 RCT's, 1/1 nonrandomized clinical trial and 1/1 retrospective case-mached study showed improvements in at least one respiratory parameter (statistically nonsignificant) within 20-30 minutes after initiation of therapy. With one exeption, that showed less subjective dyspnea at 2 hrs, this effect is not sustained beyond the first hour.

2. How precise were the results?

The 95% CI's for effect size crossed zero, suggesting there is no difference between the groups.

III. Will the results help me in caring for my patients?

1. Can the results be applied to my patient care?

Probably. The range of patient included is from 16 months to 70 years. Heliox is applicable to children in similar ways as to adults.

2. Were all clinically important outcomes considered?

No. Clinically important outcomes would include intubation, length of ICU stay, length of hospital stay. This study only evaluated PEFR% as one indicator of pulmonary function in children with asthma.

3. Are the benefits worth the harms and costs?

There is currently insufficient evidence that heliox/oxygen-mixtures significantly improve respiratory parameters during asthma-exacerbations.

Because the studies were underpowered or not targeted at clinically important outcomes, it is unclear whether Heliox actually decreases intubation risk or risk of ICU admission or duration of hospitalization. The safety of Heliox is therefore unclear. However, were no reports of barotrauma or hypothermia. One patient in the studies had to be withdrawn due to hypoxia.

 

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