Jan 202018

image: Wikipedia

In case you missed it, major professional societies in critical care now strongly recommend prone positioning for patients with severe acute respiratory distress syndrome (ARDS), with a PaO2-to-FiO2 (P/F) ratio of ≤ 100. The recommendation marks a major shift in advised care for ARDS.

Prone positioning improves ventilation-perfusion matching (transferring delivered oxygen into the bloodstream more efficiently) and keeps alveolar units open and evenly distributed at end-expiration (improving gas exchange and preventing ventilator-induced lung injury). Through one or more of these mechanisms, prone positioning is believed to improve survival for some patients with ARDS.

In making their recommendation, the guideline authors relied on the PROSEVA study, which found a somewhat incredible 50% reduction in death from ARDS (16% vs 33%), and post hoc analyses of previous ARDS trials testing prone positioning. Those trials did not show mortality benefit individually, but pooled pre-specified analyses revealed that severe ARDS patients assigned to prone positioning did survive more often (with about a 25% relative reduction in risk of death).

P/F ratios are easy to calculate, especially in severe ARDS: look at the FiO2 as a percentage (e.g., 50 for 0.50). If the PaO2 is not higher than that (≥51), and other ARDS criteria are present, your patient has severe ARDS. If the PaO2 is less than 2x the FiO2 as a percentage, she has moderate ARDS. The guideline authors made no recommendation regarding prone positioning for mild or moderate severity ARDS.

Prone positioning is to be used in addition to usual low tidal volume ventilation for ARDS (4-8 ml/kg predicted body weight).

How Much PEEP to Use In Prone Position for ARDS?

PROSEVA used PEEP levels from the low-PEEP arm of the ARDS trials, in which PEEP was 5 to 8 cm H2O for FiO2 ≤ 0.5, and only exceeded 12 cm H2O for patients at FiO2 ≥ 0.8. Other prone positioning trials for ARDS patients generally used PEEP levels 8-12 cm H2O. Higher PEEP levels have not been well-tested in ARDS prone positioning trials.

How Long to Prone-Position Patients With ARDS?

There is no known ideal timing or duration for prone positioning for ARDS. Some studies used alternating cycles of four hours prone, two hours supine throughout the day; others kept patients prone for 20 continuous hours per day with a four hour supine epoch for intensive nursing care. There has been no clear signal as to superiority between strategies. In PROSEVA, patients were kept prone for about 17 hours a day in roughly four-hour cycles. Some experts advise longer epochs of proning (i.e., fewer turns) to minimize the risk of complications during patient turning.

It’s also unclear when it is ideal to stop prone positioning. Patients are generally returned to the supine position when there is clear improvement. In PROSEVA, prone positioning was maintained for up to 28 days; patients were returned to a supine position without additional prone positioning when oxygenation remained adequate for at least four hours in the supine position. This was defined as PaO2:FiO2 ≥150 mmHg, on no more than 60% FiO2 with a PEEP no greater than 10 cm H2O.

Proning must also be interrupted for necessary unscheduled nursing care, emergencies, travel off the ICU, or interventional or surgical procedures.

Risks of Prone Positioning for ARDS

There’s a natural resistance to prone positioning for patients in ARDS among physicians, nurses, and administrators. Moving severely hypoxemic, ventilator dependent patients is inherently risky. Studies testing prone positioning showed an increased incidence of endotracheal tube obstruction among those proned. Pressure sores were also increased. In one study, arrhythmias or hypotension, vomiting, lost IV access, and endotracheal tube displacement were all significantly more likely in proned patients.

The PROSEVA study was conducted at French centers with several years’ prior experience in prone positioning for patients with ARDS. Less-experienced centers would be correct to ignore the low observed complication rate in PROSEVA, and to use extreme caution with each patient.

To mitigate the risk of pressure sores and endotracheal tube dislodgement during frequent patient turning, many U.S. hospitals have turned to the use of rotating beds like the Roto-Prone.

Authors included a standard caveat--

No guideline or recommendations can take into account all the compelling and unique clinical features of individual patients, and therefore clinicians, patients, policy makers, and other stakeholders should not regard these recommendations as mandatory."

But make no mistake: with their strong recommendation, the three signatory specialty societies have effectively made prone positioning for severe ARDS the new standard of care in the U.S. and Europe.

Source: Society guideline

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Prone positioning for severe ARDS advised by major societies