Special Feature

Should I Prone My Patient with ARDS?

By Dean R. Hess, PhD, RRT, Assistant Director, Respiratory Care, Massachusetts General Hospital, Department of Anesthesiology, Harvard Medical School, Boston, is Associate Editor for Critical Care Alert.

Dr. Hess reports no financial relationship relating to this field of study.

In 1976, piehl and brown showed, in a retrospective study, that the prone position improved oxygenation in 5 patients with the acute respiratory distress syndrome (ARDS).1 One year later, Douglas et al demonstrated, in a prospective study, that prone positioning could effectively improve oxygenation in ARDS.2 Starting from these reports, interest in prone positioning in ARDS has increased over the years, culminating in several randomized controlled trials with survival as the primary outcome.

Does Prone Position Improve Oxygenation?

Pappert et al3 evaluated 12 patients with severe ARDS before, during, and after a 2-h period of mechanical ventilation with the patient in the prone position. Prone position resulted in an overall increase of PaO2 after 120 min (98.4 ± 50.3 to 146.2 ± 94.9 mm Hg). Although 8 patients had an improvement of PaO2 > 10 mm Hg after 30 min in the prone position (responders), 4 patients had a deterioration of PaO2 in the prone position (nonresponders).

Fridrich et al4 evaluated 20 patients with ARDS after multiple trauma. Patients were turned to the prone position at noon and were turned back to the supine position at 8:00 am on the next day. Immediately after the first turn from the supine to the prone position, the PaO2 increased from 97 ± 4 to 152 ± 15 mm Hg. Most of this improvement was lost when the patients were turned supine, but could be reproduced when prone positioning was repeated after 4 h in the supine position.

Chatte et al5 studied changes in oxygenation in 32 patients with ARDS who were turned to and from supine to prone position at 1- and 4-h intervals. Seven patients (22%) had no response (non-responders), and 25 (78%) had a positive response (responders). Among the 7 non-responders, 2 did not tolerate the prone position and were returned to supine before the end of the 4-h trial. In 10 of the 23 responders (43%), the PaO2/FIO2 returned to its starting value when patients were repositioned to supine (response, but not persistent). The pattern of response could be categorized as a persistent response, not persistent response, or no response (see Figure 1).

These and other studies6 suggest that arterial oxygenation is improved in many patients with ARDS who are turned from supine to prone position. In some patients, the response is maintained even after the patient is returned to a supine position (persistent responders). However, the response is not universal. About 20%-30% of patients do not improve their PaO2 when turned to the prone position. Unfortunately, there is no reliable way of predicting which patients are likely to respond to prone position with an improvement in PaO2.

What is the Mechanism of Action for Prone Position?

Changes in ventilation-perfusion (V/Q) relationships in the prone position most likely explain improvements in oxygenation when patients are turned to the prone position.7 In ARDS, pulmonary blood flow is minimally affected by body position, and the prone position markedly increases the number of normal V/Q units compared with supine position. More uniform distribution of ventilation in the prone position may be largely responsible for the improvement in V/Q. In the supine position, gravity creates a pleural pressure gradient from the ventral (nondependent) to the dorsal (dependent) regions. The ventral regions are exposed to relatively more negative pleural pressure causing a higher regional transpulmonary pressure. The dorsal lung regions are exposed to a lower, relatively more positive, pleural pressure, so that transpulmonary pressure is less. Changing body position from supine to prone alters the ventral-dorsal pleural pressure gradient and regional transpulmonary pressure (see Figure 2) by altering gravitational forces and by reducing the compressive effects of the abdominal wall and the heart and mediastinal structures.

Other mechanisms may also contribute to an improvement in oxygenation in the prone position. Prone positioning facilitates drainage of secretions for the airways. There may be less compression of the lungs (particularly the left lung) by the heart.8 There are also effects on the chest wall compliance.9 The greatest improvement in PaO2 when turned to prone may be in patients with a more compliant chest wall, and the greatest improvements in PaO2 may in cases where the chest wall compliance decreases by the greatest degree in the prone position.

Does Prone Position Improve Patient Outcomes?

We know that many patients with ARDS have an improvement in their oxygenation when turned from supine to prone, and we have some idea of the mechanism for this observation, but does this translate into improved patient outcomes? This has been explored in 4 randomized controlled trials.

Gattinoni et al10 studied the effect on survival of prone position, compared with conventional treatment in the supine position, of patients with acute lung injury (ALI) or ARDS in a multicenter, randomized trial. There was a predefined strategy of placing patients in a prone position for ≥ 6 hrs/day for 10 days. The study enrolled 304 patients, 152 in each group. The relative risk of death in the prone group as compared with the supine group was 0.84 at the end of the study period (95% confidence interval, 0.56 to 1.27), 1.05 at the time of discharge from the intensive care unit (95% interval, 0.84 to 1.32), and 1.06 at 6 months (95% confidence interval, 0.88 to 1.28). During the study period, the increase in the PaO2/FIO2, measured each morning while patients were supine, was greater in the prone than the supine group. The incidence of complications related to positioning (such as pressure sores and accidental extubation) was similar in the 2 groups. The authors concluded that, although placing patients with acute respiratory failure in a prone position improves their oxygenation, it does not improve survival.

Guerin et al11 determined whether prone positioning improves mortality in patients with acute respiratory failure in a multi-center randomized controlled trial of 791 patients. Patients were randomly assigned to prone position (n = 413), applied as early as possible for at least 8 hrs/day, or to supine position (n = 378). The 28-day mortality rate was 32.4% for the prone group and 31.5% for the supine group (relative risk, 0.97; 95% confidence interval, 0.79 to 1.19). Mortality at 90 days was 43.3% for the prone group and 42.2% for the supine group (relative risk, 0.98; 95% confidence interval, 0.84 to 1.13). The incidence of ventilator-associated pneumonia was 1.66 for the prone group vs 2.14 for the supine group, per 100-patients days of intubation (P = 0.045). Pressure sores, selective intubation, and endotracheal tube obstruction were higher in the prone group. This trial demonstrated no beneficial effect of prone positioning on mortality and some safety concerns associated with prone positioning. Prone position may lower the incidence of ventilator-associated pneumonia.

Curley et al12 evaluated whether infants and children with ALI treated with prone positioning would have more ventilator-free days than those treated with supine positioning in a multi-center randomized controlled trial. Patients were randomized to either supine or prone positioning within 48 hrs of meeting acute lung injury criteria, with those patients in the prone group being positioned within 4 hours of randomization and remaining prone for 20 hrs/day for a maximum of 7 days. There were no differences in the number of ventilator-free days between the 2 groups. There were no differences in the secondary end points, including proportion alive and ventilator-free on day 28, mortality from all causes, the time to recovery of lung injury, organ-failure-free days, and cognitive impairment or overall functional health at hospital discharge or on day 28. The authors concluded that prone positioning does not reduce ventilator-free days or improve other clinical outcomes in pediatric patients with acute lung injury.

In the most recent study to examine patient outcomes with prone position, Mancebo et al13 evaluated whether it is beneficial to administer prone ventilation early in the course of respiratory failure and for longer periods of time than previous studies. The study enrolled 136 patients within 48 hours of tracheal intubation for severe ARDS (60 to supine and 76 to prone). The prone group was targeted to be placed in this position 20 hrs/day. The ICU mortality was 58% in the patients ventilated supine and 43% in the patients ventilated prone (P = 0.12). A total of 718 turning procedures were done and prone position was applied for a mean of 17 hrs/day for a mean of 10 days. The authors concluded that prone ventilation is feasible and safe, and may reduce mortality in patients with severe ARDS when it is initiated early and applied for most of the day.

None of these 4 studies demonstrated a statistically significant reduction in mortality with prone position in patients with ALI and ARDS. There are several possible explanations for this disappointing finding. Perhaps prone position does not, in fact, affect important patient outcomes despite an improvement in oxygenation. Perhaps the timing or duration of prone position was not correct (wrong dose). Perhaps the studies were under-powered, although a meta-analysis pooling the results of the 4 randomized controlled trials demonstrates no benefit for prone position with respect to mortality (see Figure 3).

It might also be that patient outcomes are improved with prone position in selected subgroups of patients. For example, in post-hoc analysis, Gattinoni10,14 has reported a survival benefit in patients with the highest risk (PaO2/FIO2 ≤ 88 and SAPS II score > 49) and in patients who have a decrease in their PaCO2 with prone position. However, the results of these post-hoc analyses need verification in properly designed randomized controlled trials.

Is Prone Positioning Safe?

Turning a critically ill patient from supine to prone is not simple and descriptions for this procedure have been published.7,15 Care must also be exercised to position the patient safely while in the prone position. Patients may have an increased need for sedation and paralysis related to turning to the prone position. Endotracheal tube obstruction has been reported while patients are in the prone position,10,11 most likely related to drainage of airway secretions. Pressure sores (anterior chest wall, ventral shoulders, patellae, forehead) and facial edema are more prevalent when patients are in the prone position. Contraindications of prone positioning include increased intracranial pressure, fractures (facial, spinal, pelvic), open wounds on the ventral surface, and hemodynamic instability.

So What to Do?

Prone position in patients with ALI/ARDS is associated with an increase in PaO2 in the majority of patients who receive this therapy. However, the improvement in oxygenation may not translate into a survival benefit. Based on the available evidence, prone position cannot be considered standard care in patients with ALI/ARDS. It is not wrong to use prone position in selected patients with ARDS, but it is also not wrong if prone is not used.16


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