Does Prone Positioning Improve Survival in Acute Respiratory Failure?
Abstract & Commentary
This paper reports the results of a multi-center, randomized trial of supine vs. prone positioning in patients with acute respiratory failure. Enrolled patients had a PaO2/FIO2 ratio of £ 200 mm Hg with positive end-expiratory pressure (PEEP) £ 5 cm H2O, or a PaO2/FIO2 of £ 300 with PEEP ³ 10 cm H2O, plus bilateral pulmonary infiltrates on chest x-ray and absence of left heart failure. Patients were excluded who were younger than 16 years of age, who had evidence of cardiogenic pulmonary edema, cerebral edema, or intracranial hypertension, or who had conditions that might have contradicted use of the prone position such as spinal fracture or severe hemodynamic instability. Patients randomized to be managed prone were placed into this position for ³ 6 h/d for 10 days. There were 304 patients enrolled, with 152 in each group.
When patients were turned to the prone position, the PaO2/FIO2 increased by at least 10 mm Hg in 73% of patients. The daily PaO2/FIO2, measured each morning while all patients were prone, increased significantly more over time in the prone group than the supine group. Complications such as pressure sores and accidental extubation were similar in the 2 groups. During management in the prone position, however, there was an increased need for sedation (55% of patients); airway obstruction occurred in 39% of patients, facial edema developed in 30% of patients, and there was an increased need for muscle relaxants (28% of patients). Patients spent an average of 7 h/d in the prone position. Most important, the mortality rate did not differ significantly between groups, either at the end of the 10-day study period, at the time of discharge from the ICU, or at 6 months. On post-hoc analysis, a survival benefit for prone (at the end of the 10-day study period) was found for the sickest patients: PaO2/FIO2 £ 88, SAPS score > 49, or tidal volume > 12 mL/kg of predicted body weight (Gattinoni L, et al. Effect of prone positioning on the survival of patients with acute respiratory failure. N Engl J Med. 2001;345:568-573).
COMMENT BY DEAN R. HESS, PhD, RRT
In recent years, there has been increasing interest in the use of prone position for patients with the acute respiratory distress syndrome (ARDS). Clinical studies, such as the one described here, have reported improvements in PaO2 for about two thirds of patients with ARDS. Moreover, animal models of ARDS have suggested that the prone position may attenuate the effects of ventilator-induced lung injury.1,2 Accordingly, there is a sound clinical and experimental rationale for the use of the prone position. However, despite improvements in oxygenation that occurred with prone position in this study, there was no improvement in survival. Therefore, prone position cannot be recommended as a standard therapy in the care of patients with ARDS.
This study once again demonstrates the follies of over-reliance on physiologic outcomes such as improvements in PaO2. Similar findings have been reported for the use of inhaled nitric oxide in the treatment of ARDS3 and use of mask continuous positive airway pressure for treatment of acute hypoxemic respiratory failure.4 Moreover, in the ARDSnet trial, patients with the higher PaO2 were those receiving the higher tidal volume—the group with a worse survival.5
One can only speculate about the reasons for the finding of no improvement in survival with the use of the prone position. Perhaps the study was under-powered. In the ARDSnet trial, for example, more than 800 patients were enrolled. Perhaps the wrong dose was used. On average, proning was only done for 7 h/d, and for no more than 10 days. My personal experience has been to use prone position nearly continuously throughout the day, rather than limiting the duration of the therapy. Finally, it may be that the prone position, unlike a low tidal volume strategy, does not attenuate the effects of ventilator-induced lung injury. Moreover, it is interesting to speculate that prone positioning may be effective in reducing mortality if combined with other strategies such as a low tidal volume, permissive hypercapnia, or inhaled nitric oxide. Although prone positioning with inhaled nitric oxide has been shown to effectively improve PaO2,6,7 there are no published reports of enhanced survival with the use of this strategy.
It is intriguing that this study did find a survival benefit on post-hoc analysis for the sickest patients. Although these findings must be confirmed with a properly designed prospective study, it does suggest a limited role for the prone position in patients with ARDS. It is also interesting to note that the incidence of complications was similar for patients in the prone and supine groups, suggesting that prone positioning is safe in this patient population. However, increased needs for sedation and neuromuscular blocking agents was reported for management of patients in the prone position.
Prone positioning in patients with ARDS is inexpensive, results in an improvement in PaO2 in the majority of patients, and is relatively risk-free. However, it does not improve survival—at least as applied in this study—except possibly in the subgroup of the sickest patients.
References
1. Broccard AF, et al. Influence of prone position on the extent and distribution of lung injury in a tidal volume oleic acid model of acute respiratory stress syndrome. Crit Care Med. 1997;25:16-27.
2. Broccard A, et al. Prone positioning attenuates and redistributes ventilator-induced lung injury dogs. Crit Care Med. 2000;28:295-303.
3. Dellinger RP, et al. Effects of inhaled nitric oxide in patients with acute respiratory distress syndrome. Crit Care Med. 1998;26:15-24.
4. Delclaux C, et al. Treatment of acute hypoxemic nonhypercapnic respiratory insufficiency with continuous positive airway pressure delivered by a face mask. JAMA. 2000;284:2352-2360.
5. The ARDS Network. Ventilation with lower tidal volumes as compared with traditional tidal volume for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301-1308.
6. Borelli M, et al. Hemodynamic and gas exchange response to inhaled nitric oxide and prone positioning in acute respiratory distress syndrome patients. Crit Care Med. 2000;28:2707-2712.
7. Papazian L, et al. Respective and combined effects of prone position and inhaled nitric oxide patients with acute respiratory distress syndrome. Am J Respir Crit Care Med. 1998;157:580-585.
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