Effect of mechanical ventilation in the prone position on clinical outcomes in patients with acute hypoxemic respiratory failure: a systematic review and meta-analysis

Abstract

Background: Mechanical ventilation in the prone position is used to improve oxygenation in patients with acute hypoxemic respiratory failure. We sought to determine the effect of mechanical ventilation in the prone position on mortality, oxygenation, duration of ventilation and adverse events in patients with acute hypoxemic respiratory failure.

Methods: In this systematic review we searched MEDLINE, EMBASE, the Cochrane Central Register of Controlled Trials and Science Citation Index Expanded for articles published from database inception to February 2008. We also conducted extensive manual searches and contacted experts. We extracted physiologic data and clinically relevant outcomes.

Results: Thirteen trials that enrolled a total of 1559 patients met our inclusion criteria. Overall methodologic quality was good. In 10 of the trials (n = 1486) reporting this outcome, we found that prone positioning did not reduce mortality among hypoxemic patients (risk ratio [RR] 0.96, 95% confidence interval [CI] 0.84-1.09; p = 0.52). The lack of effect of ventilation in the prone position on mortality was similar in trials of prolonged prone positioning and in patients with acute lung injury. In 8 of the trials (n = 633), the ratio of partial pressure of oxygen to inspired fraction of oxygen on day 1 was 34% higher among patients in the prone position than among those who remained supine (p < 0.001); these results were similar in 4 trials on day 2 and in 5 trials on day 3. In 9 trials (n = 1206), the ratio in patients assigned to the prone group remained 6% higher the morning after they returned to the supine position compared with patients assigned to the supine group (p = 0.07). Results were quantitatively similar but statistically significant in 7 trials on day 2 and in 6 trials on day 3 (p = 0.001). In 5 trials (n = 1004), prone positioning was associated with a reduced risk of ventilator-associated pneumonia (RR 0.81, 95% CI 0.66-0.99; p = 0.04) but not with a reduced duration of ventilation. In 6 trials (n = 504), prone positioning was associated with an increased risk of pressure ulcers (RR 1.36, 95% CI 1.07-1.71; p = 0.01). Most analyses found no to moderate between-trial heterogeneity.

Interpretation: Mechanical ventilation in the prone position does not reduce mortality or duration of ventilation despite improved oxygenation and a decreased risk of pneumonia. Therefore, it should not be used routinely for acute hypoxemic respiratory failure. However, a sustained improvement in oxygenation may support the use of prone positioning in patients with very severe hypoxemia, who have not been well-studied to date.

Figure 1: Flow of studies in the systematic review. *Records were identified in electronic database search. †The records retrieved for more detailed evaluation came from the electronic databases and other sources. ‡We included 13 primary trials and 8 references with duplicate or additional data. Of the studies retained for analysis, 12 trials contributed oxygenation data, 10 contributed data on adverse events, and 10 were included in our primary mortality analysis.

Figure 2: Effect of ventilation in the prone position on mortality. We used a random-effects model in our analysis. The duration of prone positioning was up to 24 hours for 1–2 days in the short-term trials and up to 24 hours daily for more than 2 days in the prolonged-duration trials. One trial included data only for patients with acute hypoxemic respiratory failure. Including all patients from this trial (7/25 deaths in the prone group and 14/28 deaths in the supine group) did not change the result (RR 0.95, 95% CI 0.83 to 1.08; p = 0.41). I² = percentage of total variation across studies owing to between-study heterogeneity rather than chance. CI = confidence interval, RR = risk ratio.

Figure 3: Effect of ventilation in the prone position on daily ratio of partial pressure of oxygen to inspired fraction of oxygen. We used a random-effects model in our analysis. Values were recorded at the end of the period of prone positioning (prone group) and simultaneously in the supine group. Ratio of means = mean ratio of partial pressure of oxygen to inspired fraction of oxygen in the prone group divided by that in the supine group. I² = percentage of total variation across studies owing to between-study heterogeneity rather than chance. CI = confidence interval.

Figure 4: Effect of ventilation in the prone position on risk of ventilator-associated pneumonia. We used a random-effects model in our analysis. One trial included data only for patients with acute hypoxemic respiratory failure. I² = percentage of total variation across studies owing to between-study heterogeneity rather than chance. CI = confidence interval, NA = not applicable, RR = risk ratio.

Figure 5: Effect of ventilation in the prone position on risk of pressure ulcers. We used a random-effects model in our analysis. One trial included data only for patients with acute hypoxemic respiratory failure. I² = percentage of total variation across studies owing to between-study heterogeneity rather than chance. CI = confidence interval, RR = risk ratio.