Web site designed and maintained by
Chris Cairns
© SICS EBM Group 2004
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Setting PEEP in ALI / ARDS
Three-part Clinical Question: Patients: adults receiving mechanical ventilation for hypoxaemic respiratory failure caused by ALI or ARDS. Intervention: comparison of a minimal distension strategy ie. moderate PEEP 5-9 cm H20 against an increased recruitment strategy ie. level of PEEP set to reach a plateau pressure of 28 to 30 cm H20. Tidal volume of 6 ml/kg used in both groups. Outcomes: Primary - 28 day mortality; Secondary - 60 day mortality, ventilator-free days, organ failure free days at 28 days, incidence of pneumothorax during treatment
The Study: Single-blinded, concealed randomised controlled trial with
intention-to-treat. Exclusions: Pneumothorax, do not resuscitate order, chronic liver failure, lack of consent, expected duration of ventilation <48 hours, participation in another trial, bone marrow transplant or chemotherapy induced neutropenia, long-term oxygen therapy, intracranial hypertension, morbid obesity. “Standard strategies”: volume-assist control mode used with targets of: TV <6ml/kg, plateau pressure limit <30 cmH20, RR <35, pH 7.30-7.45, PaO2 55-80mmHg , SpO2 88-95%. These oxygenation goals were achieved by adjusting FiO2. Recruitment manoeuvres were allowed but not recommended, adjunctive therapies ie. inhaled NO, prone positioning, almitrine bismesylate were allowed when oxygenation goal not met despite FiO2 > 0.8 From day 4 onwards if Pa02:Fi02 >150mmHg a standardised weaning PEEP trial performed, if tolerated a standardised spontaneous breathing test undertaken. Based on this the attending physician decided whether to extubated or not.
Experimental group – Increased recruitment group: (N = 385; 385 analysed): Standard strategies plus: PEEP adjusted based on airway pressure, kept as high as possible without increasing maximal inspiratory pressure above 28-30cmH20 regardless of oxygenation.
The Evidence:
Sub-group analysis:
In patients with ARDS the increased distension protocol was associated with an earlier extubation and a trend towards improved mortality. The opposite was observed with patients with ALI but not ARDS.
EBM questions:
1) Do the methods allow adequate testing of the hypothesis? Yes. However, the study is potentially underpowered. The study was stopped early after the 18th scheduled interim analyses by the monitoring board due to failure to demonstrate a 10% absolute reduction in mortality. (Target recruitment was 400 patients in each arm with a 40% mortality.) The sub-group analysis of ARDS vs non-ARDS ALI relies on very small patient numbers.
2) Do the statistical tests correctly test the results to allow differentiation of statistically significant results? Yes
3) Are the conclusions valid in light of results? Perhaps. The distension protocol was associated with less ventilation time and less need for adjunctive therapies.
4) Did results get omitted and why? Yes. One patient was lost to follow-up. However, this patient was included in the intention-to-treat analysis.
5) Did they suggest areas of further research? Yes. They suggested further studies to determine the ideal balance between lower plateau pressures and higher PEEP.
6) Did they make recommendations based on results and were they appropriate? The authors make no general recommendations about what level of PEEP to use. Based on subgroup analysis they do suggest a strategy of high level of PEEP and low tidal volume should be used in caution in ALI.
7) Is this study relevant to my clinical practice? No. This study did look at similar patients with regard to age, sex and severity of illness as present to general ITUs in the UK. However the study really used two “non-standard” ventilation strategies when considering current “normal” practise. One could argue that the gold standard for the control arm of any ARDS study should be the interventional strategy used in the ARDSnet study.
There are two main concerns over the study protocol: a. The minimal distension group used significantly lower PEEP levels compared with conventional modern ventilatory management. b. The PEEP in the recruitment arm is really determined by lung compliance, in that, once the inspiratory pressure has been set to achieve 6ml/kg tidal volume the remainder of the 30cmH2O plateau pressure will be used for PEEP. In relatively compliant lungs this could lead to unnecessarily high PEEP levels which could be detrimental. This may account for the trends towards poorer outcome in ALI only patients and the greater need for fluid boli in this group. In other words many of these patients may not require such a plateau pressure.
8) What level of evidence does this study represent? 1+
9) What grade of recommendation can I make on this result alone? B
10) What grade of recommendation can I make when this study is considered along with other available evidence? No other studies compare these ventilation strategies. However, most recent studies agree that low tidal volume / plateau pressure strategies should be employed.
11) Should I change my practice because of these results? If you currently use ARDSnet type ventilation strategies then there is little evidence here to change your practise.
12) Should I audit my current practice because of these results? Yes.
Appraised by: Dr SC Griffiths* and Dr Chris Cairns†
*SpR Anaesthetics, Royal Liverpool University Hospital, Prescot Road, Liverpool
; †Consultant, Intensive care, Stirling Royal Infirmary. Email: Chris.Cairns2@nhs.net
©SICS EBMG
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