Decrease in PaCO2 With Prone Position is Predictive
Objective: To determine whether gas exchange improvement in response to the prone position is associated with an improved outcome in acute lung injury (ALI)/acute respiratory distress syndrome (ARDS).
Design: Retrospective analysis of patients in the pronation arm of a controlled randomized trial on prone positioning and patients enrolled in a previous pilot study of the prone position.
Setting: Twenty-eight Italian and two Swiss intensive care units.
Patients: We studied 225 patients meeting the criteria for ALI or ARDS.
Interventions: Patients were in prone position for 10 days for 6 hrs/day if they met ALI/ARDS criteria when assessed each morning. Respiratory variables were recorded before and after 6 hrs of pronation with unchanged ventilatory settings.
Measurements and Main Results: We measured arterial blood gas alterations to the first pronation and the 28-day mortality rate. The independent risk factors for death in the general population were the Pao₂/Fio₂ ratio (odds ratio, 0.992; confidence interval, 0.986-0.998), the minute ventilation/Paco₂ ratio (odds ratio, 1.003; confidence interval, 1.000-1.006), and the concentration of plasma creatinine (odds ratio, 1.385; confidence interval, 1.116-1.720). Pao₂ responders (defined as the patients who increased their Pao₂/Fio₂ by ≥20 mm Hg, 150 patients, mean increase of 100.6 ± 61.6 mm Hg [13.4 ± 8.2 kPa]) had an outcome similar to the nonresponders (59 patients, mean decrease -6.3 ± 23.7 mm Hg [-0.8 ± 3.2 kPa]; mortality rate 44% and 46%, respectively; relative risk, 1.04; confidence interval, 0.74-1.45, p = .65). The Paco₂ responders (defined as patients whose Paco₂ decreased by ≥1 mm Hg, 94 patients, mean decrease -6.0 ± 6 mm Hg [-0.8 ± 0.8 kPa]) had an improved survival when compared with nonresponders (115 patients, mean increase 6 ± 6 mm Hg [0.8 ± 0.8 kPa]; mortality rate 35.1% and 52.2%, respectively; relative risk, 1.48; confidence interval, 1.07-2.05, p = .01).
Conclusion: ALI/ARDS patients who respond to prone positioning with reduction of their Paco₂ show an increased survival at 28 days. Improved efficiency of alveolar ventilation (decreased physiologic deadspace ratio) is an important marker of patients who will survive acute respiratory failure.

The prone position is widely used in the treatment of patients with severe acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), although an initial outcome study failed to show any enhancing advantage in survival rates. The main indication for prone positioning is severe hypoxemia: Indeed, prone positioning is associated with a significant and lasting improvement in oxygenation in 60-80% of patients. However, we do not yet know whether the improvements in gas exchange in the prone position are associated with an enhanced outcome and, if so, which are the possible salutary mechanisms.

We began to examine these questions by retrospectively analyzing the database of the prone/supine study, in which 152 ALI/ARDS patients were in prone position for ≤10 days. To these patients, we added a group of 73 patients (unpublished data) enrolled in the pilot study performed before we began the randomized trial. We wish to report our findings, focusing on the association between improved respiratory gas exchange in the prone position and outcome.