Opening pressures in ARDS
We consider ourselves privileged to have been asked to compose an editorial about the latest research by a renowned and extremely creative group of investigators dealing with the critical lung opening pressures in ARDS patients .
The recommended practice of mechanical ventilation in ARDS is to provide lung protection and in particular to set tidal volume between 4 and 8 ml/kg predicted body weight and to limit airway plateau pressure below 28–30 cm H2O .
Recent insights in four directions are highly relevant for a better use of the ventilator in ARDS patients. First, the homogeneity in the distribution of lung aeration loss/tissue gain is better understood through the definition of patterns of lung recruitability over a 5- to 45-cm H2O airway pressure range  and the determination of opening and closing pressures that drive intra-tidal recruitment/derecruitment over a much narrower range of pressure than the above . Second, three ARDS stages of severity based on hypoxemia intensity have been defined , with the greater the hypoxemia, the higher the lung weight and the potential of recruitment. Henceforth, ventilator strategies have been proposed to match each ARDS stage , in particular, lowering tidal volume at every ARDS stage, setting low PEEP (5–10 cm H2O) in mild ARDS, middle PEEP (10–15 cm H2O) in moderate ARDS and high PEEP (>15 cm H2O) in severe ARDS. Third, in some ARDS patients, the precise assessment of lung mechanics requires the measurement of trans-pulmonary pressure . Fourth, an alveolar pressure difference of 15 cm H2O between inspiration and expiration (driving pressure) may be a safety threshold, better related to patient outcome than plateau pressure .
We know that the overall stress and strain (leading to baro-and volutrauma, respectively) must be minimized, and this can be done by lowering tidal volume. We know that cyclic opening and closing of the terminal respiratory units (atelectrauma) should be minimized, and that this can be done by using a sufficient PEEP level in order to maintain open the recruited lung mass gained at the end of inspiration. Evidence based on clinical trials suggests that atelectrauma, which is a very attractive concept, is likely less harmful than volutrauma. We know that lung homogenization needs to recruit either not or poorly aerated lung mass and that this process requires the delivery to the lung of pressures in the range of 45 cm H2O or even more , well above the recommended safety threshold. Furthermore, the response to a 5- to 45-cm H2O airway pressure range is not homogenous among ARDS patients nor through the entire lung in a dedicated ARDS patient . However, whether this holds true over the whole spectrum of ARDS severity is not clear.
Taking previous considerations together, the ultimate goal of lung protective ventilation is both to preserve the baby lung and to increase the homogeneity in the distribution in lung aeration.
Our attention was also drawn towards two insights and two limitations of the present study. The first unexpected result was that lung hyperinflation appeared really marginal. Indeed, it has been shown that 33% of patients with ARDS receiving lung protective settings (tidal volume of 6 ml/kg and plateau pressure lower than 30 cm H2O) exhibited significant hyperinflation at the end of tidal inspiration . This illustrates the fact that CT scans cannot determine hyperinflation accurately because the lung aeration loss is so important that a gas/tissue ratio of 9/1 (or 10/1 depending on the thresholds used to define hyperinflation from Hounsfield units) cannot be reached. The second new insight was that lung properties were similar in severe ARDS treated or not treated with ECMO. This result could have implications for mechanical ventilation settings in ARDS patients under ECMO, However, these latter findings need to be confirmed. Indeed, the sample of patients may not be representative of the usual ARDS population, considering the rate of mortality, which was 80% in the severe group not treated with ECMO. This can be explained by the fact that the number of patients was reduced by tstudy design. Second, the tidal volume used was in the same range as that reported in the Lung Safe study , i.e. well above the 6 ml/kg predicted body weight, in particular in the severe ARDS group, and it was as high as 4.8 ml/kg in the ECMO group.
These points should be investigated in the future. Nevertheless, this elegant study  is an outstanding contribution in our understanding of the complex interplay between ARDS patients and mechanical ventilation.
Compliance with ethical standards
Conflicts of interest
The authors declare no conflict of interest regarding this editorial.
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