It is now clear that gas exchange can and does improve in a large fraction of patients with ARDS when they are turned prone, that the improvement is frequently of sufficient magnitude that PEEP and/or the FiO2 can be reduced, and that managing patients in the prone position does not seem to be associated with any undue risks or complications. In addition, recent limited studies suggest the response to a recruitment maneuver may be better when patients are prone.
Although a number of questions remain, of greatest importance is whether prone ventilation reduces the morbidity and mortality of patients with ARDS.
There are two mechanisms by which this might occur and they are not mutually exclusive. First, the improvement in oxygenation seen with prone ventilation frequently allows a reduction in the FiO2. If oxygen toxicity occurs in the setting of ALI, if it contributes to the morbidity or mortality of ARDS, and if it can be reduced by using slightly lower FiO2s, then prone ventilation might save lives.
Second, lung overdistension and/or cyclic airspace opening and closing are thought to cause ventilator-induced lung injury (VILI) and the results of the recently published ARDS Network study on low-stretch ventilation indicates that VILI contributes to the mortality of ARDS . Since prone positioning should reduce lung overdistension in non-dependent regions, and should reduce cyclical airspace opening and closing in dependent regions, it may also reduce VILI .
The first attempt to study the effects of prone ventilation on mortality was published by Gattinoni and colleagues . While they found that the patients randomized to the prone position had better oxygenation, no survival benefit was observed. Unfortunately, the study had numerous methodological problems:
1. Patients randomized to prone ventilation only received it an average of seven hours/day. This is of concern because of the data from a number of experimental studies reporting that an injurious ventilatory strategy could produce VILI in a matter of minutes or hours.
2. The study was too small for mortality to be a valid end-point.
3. Prone ventilation was not instituted early in the course of ALI/ARDS.
4. Standard ventilation and weaning protocols were not used
5. The study only lasted 10 days.
A recently completed study from Spain found that mortality was reduced 25% in patients with ARDS who were randomized to receive prone ventilation . Unfortunately, only 138 patients were enrolled and this difference in mortality was not statistically significant (p = 0.12). Accordingly, the effect of prone ventilation on outcome in patients with ARDS must await additional studies.
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