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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Evaluation of a Ventilation Strategy to Prevent Barotrauma in Patients at High Risk for Acute Respiratory Distress Syndrome

Stewart TE, Meade MO, Cook DJ, et al.

N Engl J Med 1998; 338:355-61. [abstract]

Reviewed by Mona McPherson, MD and Michele Mariscalco, MD, Texas Children's Hospital

Review posted August 18, 1998

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I. What is being studied?:

The study objective:

To determine whether a mechanical ventilation strategy that placed specific limits on peak inspiratory pressure and tidal volume in patients at risk for ARDS would affect in-hospital mortality.

The study design:

A non-blinded randomized clinical trial with controls. Eight tertiary care centers were involved.

The patients included:

120 adult patients from eight tertiary care centers who had been intubated for less than 24 hours and who were at significant risk for ARDS. Other patient inclusion criteria were age >18 years, a PaO2/FiO2 ratio <250, and PEEP 5. The total number of eligible patients was not stated.

The patients excluded:

1. Patients with anticipated intubation time < 48 hours.
2. Patients with little chance of survival.
3. Patients with cardiogenic pulmonary edema, previous congestive heart failure, cor pulmonale, or high risk of arrhythmias.
4. Patients with intracranial abnormalities.
5. Pregnant women.
6. Patients enrolled in another study.

The interventions compared:

An experimental ventilation strategy with peak inspiratory pressure (PIP) limited to 30 cm of water and tidal volume < 8 ml/kg verses a control ventilation strategy with PIP up to 50 cm of water and tidal volumes 10-15 ml/kg. Both groups used PEEP 5-20 and respiratory rates 5-35 as needed.

The outcomes evaluated:

Primary outcome - in-hospital mortality.
Secondary outcomes -

1. Barotrauma as indicated by free air on roentgenographs
2. Total multiple organ dysfunction score
3. Dysfunction of individual organs
4. Clinically significant arrhythmias
5. Need for dialysis
6. The duration of mechanical ventilation, ICU stay, and hospital stay

II. Are the results of the study valid?

Primary questions:

1. Was the assignment of patients to treatments randomized?

Yes. Patients were randomized by computer generated random number tables with stratification according to center.

2. Were all patients who entered the trial properly accounted for and attributed at its conclusion?

Was followup complete?

Yes. Results from all 120 patients were considered in the final analysis including the 14 patients who met criteria for discontinuation from the study.

Were patients analyzed in the groups to which they were randomized?

Yes. All patients were included in the final analysis. Patients who met criteria for discontinuation from the study were included and an intention-to-treat analysis were used.

Secondary questions:

3. Were patients, health workers, and study personnel "blind" to treatment?

No.

4. Were the groups similar at the start of the trial?

Yes. Patients were similar in age, APACHE score, oxygenation index, number of organs with failure, multiple organ dysfunction score, and number of risk factors for ARDS. The control group did have a significantly higher PaO2/FiO2 ratio - 145 vs 123 (p < 0.05).

5. Aside from the experimental intervention, were the groups treated equally?

Probably. Both groups were ventilated with a decelerating wave-form flow pattern with either an assist mode or pressure control depending on the PIP. Both groups were managed with the same range of PEEP and respiratory rates. Other maneuvers such as adjustments to inspiratory flow rates, sedation, and paralyzation were at the discretion of the attending physicians at the eight tertiary care centers. Some variation probably existed between the tertiary care centers and different physicians at those centers.

III. What were the results?

1. How large was the treatment effect?

There was no difference in the primary outcome between the experimental and control groups. In the experimental group, 30 patients died versed 28 patients in the control group (p < 0.72). The experimental ventilation strategy did not reduce the risk of mortality (Relative risk 1.04 - 95% CI 0.48-2.23) even when adjusted for APACHE score.

When secondary outcomes were considered, the only significant finding was the experimental ventilation group underwent more dialysis (13 patients vs 5 control patients, p < 0.04) and received more paralytic drugs (23 vs 13 control patients, p < 0.05).

2. How precise was the estimate of the treatment effect?

The 95% confidence intervals reported above allow us to be confident the true risk of death in the experimental ventilation group is 50% better or 220% worse than with the control strategy.

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

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

Maybe. These were adult patients as opposed to children; but they had similar underlying disease processes, such as sepsis, aspiration, burns, trauma.

2. Were all clinically important outcomes considered?

Yes. The primary outcome was in-hospital mortality but common and important morbidities such as barotrauma, other organ dysfunction, and length of stay were also considered.

3. Are the likely treatment benefits worth the potential harms and costs?

No. Based on the results of this study, the experimental ventilation strategy limiting pressure and tidal volume does not improve mortality and may actually be harmful in that more patients required dialysis under this management.

Comments

The only comment I have is that I feel this is an important study when defining ventilator strategies. This study identified those at risk for ARDS and therefore the use of low tidal volumes as indicated in ARDS were not indicated in the absence of ARDS. Though they evaluated PIP's rather than Ppl, the pressures were not significant and therefore show that higher volumes can be safely used in patients without ARDS.

Ronald Barrow RRT, Arkansas Children's Hospital
September 3, 2000

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