Do we have explanations for the improvement of oxygenation and deterioration of outcome by using prone position in acute respiratory failure?

In acute respiratory failure (ARF), in particular acute lung injury (ALI) and respiratory distress syndrome (ARDS), change from supine (SP) to prone position (PP) can improve oxygenation. The efficacy of this intervention can be demonstrated by the course of oxygenation index. Nevertheless prone position ventilation (PPV) showed no improvement in survival so far. Endpoint for the assessment of therapeutic effects of an intervention like PPV is generally the mortality rate. The aim of our study is to attempt to analyze the discrepancy between positive effects of prone position ventilation on oxygenation index in ARF and the comparatively high mortality rates despite of this intervention. We studied 110 consecutive patients with ALI (n = 18) and ARDS (n = 92) at mean age 66 ± 13 [SE] years in a clinical follow-up design at a surgical ICU in a university hospital, who met the criteria of the American European consensus definition. All patients were ventilated intermittent in SP and in PP (135° left/right-side-position) for at least 6 hours/day. Data collection included apart from baseline characteristics individual oxygenation index and underlying diseases of the patients, in particular if of benign or malignant nature. We compared individual oxygenation index (PaO₂/FiO₂) before and after start of prone position (SPSS^® T-test) and the data set of each patient with outcome. PPV was well tolerated in all n = 110 patients and showed an significant increase of PaO₂/FiO₂ in n = 106 within the first 6 hours (SP 149 ± 0.52 vs PP 230 ± 0.73 mmHg [mean ± SEM]). In the remaining four cases there was a positive effect within the first 24 hours. Sixty-seven (61%) of the patients died in the course of intensive care therapy and 43(39%) survived. Seven died with an oxygenation index below 100, another 36 with a ratio below 200, 17 below 300 and seven above 300 mmHg. Patients with a malignant underlying disease as cofactor had a 1.8 times higher and those with sepsis a 3.15 times higher risk to die during their ICU-stay despite of PPV. Despite of positive effects of PPV on oxygenation in our patients a considerable part of them died. To our amazement oxygenation index previous to death was not the main problem for most part of the patients in that phase. Malignant diseases in history and sepsis during the ICU-stay seem to increase the risk to die in the course of ALI or ARDS regardless the use of PPV conspicuously. Our results show that for the assessment of a therapeutic intervention in acute respiratory failure not only mortality as an endpoint seems to be suitable, but also important clinical cofactors.