Prone positioning does not affect survival in patients with ARDS
- Ognjen Gajic1
© Biomed Central Ltd 2001
Received: 5 September 2001
Published: 5 December 2001
The exact mechanism by which oxygenation is improved in patients ventilated in a prone position (compared to those ventilated in supine position) is not known but may be due to reductions in ventilation/perfusion (VQ) mismatching and chest wall compliance. Improvement in oxygenation is noted in about 60% of patients; significant numbers sustain improvement after being returned to a supine position. Careful positioning usually requires three to five people. Complications are rare, although hemodynamic instability (1.1% per prone cycle), accidental extubation (0.4%), central line dislodgement (0.4%), pressure ulcers (15%) (see Additional information ) have all been reported.
Neither intention to treat nor per-protocol analysis revealed significant differences in the primary outcomes. The prone group had a larger improvement in ratios of partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2/FiO2) at day 10 (63 versus 45 [P = 0.02]) and slightly higher tidal volumes (10.7 versus 10.7 [P = 0.03]). The prone group had a higher incidence of new pressure sores - 2.7% versus 1.9% (P = 0.004). There were no significant differences in endotracheal tube or venous access displacement. However, prone positioning resulted in increased requirements for sedation (55%) and neuromuscular blockade (27.7%), and more episodes of transient airway obstruction (39%) and hypotension (12%). In a post-hoc analysis, a subgroup of patients with the lowest PaO2/FiO2 ratio (<88) in the prone group had a lower 10-day mortality, but this did not persist to discharge from the ICU.
The majority of patients with acute respiratory distress syndrome (ARDS) die not from hypoxemia but from multiple-organ failure (see Additional information ). Thus, improvement in surrogate outcomes (such as PaO2/FiO2 ratio) with prone positioning may be misleading. Minimizing tidal volumes to prevent ventilator associated lung injury at the price of accepting lower physiologic values of PaO2and pH has lead to improved survival (see Additional information ). The patients in this study were ventilated with larger tidal volumes than currently recommended. Although generally safe (in a research setting), routine use of prone positioning cannot be recommended as yet because appropriate timing and duration of prone positioning remain unknown.
A total of 304 patients from 28 ICUs met the criteria (see Additional information ) for ARDS or acute lung injury. They were assigned randomly to either the prone (n = 152) or supine group (n = 152). Patients in the prone group were kept prone for at least six hours per day for 10 days. Physicians used standardized ventilator settings (see Additional information ). Primary endpoints were mortality at 10 days, ICU discharge and six months after randomization; secondary endpoints were oxygenation and organ dysfunction at 10 days.
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Also, see the Editorial in the same issue of N Engl J Med:
Slutsky AS: The Acute Respiratory Distress Syndrome, Mechanical Ventilation, and the Prone Position.
New Engl J Med 2001, 345:610.
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