During ARDS, Ventilator-Induced Lung Injury (VILI) occurs as a means of baro/volutrauma and atelectrauma and is a key factor in determining patients’ outcome. Barotrauma and volutrauma develop as a consequence of the excessive pressure/volume load to the aerated lung, which is markedly reduced by the disease (i.e. the ‘baby lung’). Conversely, atelectrauma represents the cyclic opening and closing of alveoli during tidal ventilation and mainly happens at the edge between normally aerated and collapsed lung zones .
In the 2000s, a ventilation strategy providing low-tidal volumes to avoid baro/volutrauma and higher PEEP to limit atelectrauma, has been suggested to mitigate VILI and protect the lung. Nonetheless, while low tidal volumes convincingly improve clinical outcome and are widely used in the daily practice, the benefit of higher PEEP remains controversial despite some positive results, with its use not so widespread in the clinical setting [2–4].
Cressoni et al. recently reported the results of a physiological study to clarify whether the PEEP levels usually applied with the high-PEEP strategies really avoid atelectrauma and ensure adequate recruitment in ARDS patients.
The authors studied 33 ARDS patients with heterogenous disease severity (5 mild, 10 moderate, 9 severe and 9 severe receiving ECMO and ultra-low), who underwent CT scans at end-expiration and end-inspiration while receiving mechanical ventilation with low-tidal volumes and PEEP set both at 5 and 15 cmH2O. In addition, to quantify patients’ total recruitability (increase in the amount of aerated tissue as airway pressure is raised), CT scans were also conducted with airway pressure = 45 cmH2O. CT scans were used to assess alveolar recruitment, atelectrauma (intra-tidal cyclic recruitment of collapsed areas) and ventilation inhomogeneities, which were the addressed endpoints.
As already reported by the same group, the total recruitability between 5 and 45 cmH2O increases with ARDS severity, being neglectable in mild, modest in moderate and greater in severe ARDS, independently from the use of ECMO [6,7]. Importantly, the recruited lung tissue between 5 and 15 cmH2O is minimal in mild, higher in moderate and maximal in severe ARDS both with and without ECMO.
The risk of intra-tidal recruitment increases with ARDS severity, and is not mitigated by setting PEEP at 15 cmH2O, as compared to PEEP = 5 cmH2O.
As about gas distribution, the extent of inhomogeneities increases, but not significantly, along all the ARDS severity spectrum. Raising PEEP from 5 to 15 cmH2O enhances lung homogeneity in mild and moderate ARDS patients but not in the severe ones. Conversely, pressures far higher than set PEEP (i.e. plateau pressure) seem to improve lung homogeneity in moderate and severe ARDS patients.
It is widely accepted that PEEP setting should aim to a balance between its capability to recruit the collapsed tissue and the unavoidable damage generated in the “baby lung”. Results from the present elegant physiological study confirm that the amount of recruitable lung (and thus the mechanistic benefit from higher PEEP) depends on the severity of the disease.
Accordingly, in mild ARDS, the benefit of PEEP up to 15 cmH2O may be trivial and this approach is likely unnecessary, also considering robust clinical data showing no effect (or harm?) in milder patients treated with more aggressive PEEP-setting protocols .
In moderate to severe ARDS, conversely, raising PEEP up to 15 cmH2O reopens a relevant proportion of lung tissue, possibly contributing to the clinical benefit by higher PEEP protocols documented in this subset of patients . To date, most experts agree that PEEP = 15 cmH2O generating plateau pressure around 30 cmH2O is a good compromise between the need for keeping the lung open and the need for avoiding barotrauma.
The present results robustly question this concept: PEEP = 15 cmH2O cannot fully exert its beneficial effects in moderate to severe ARDS as a significant proportion of the recruitable remains closed, tidal recruitment is not adequately prevented and lung homogeneity is not enhanced.
Indeed, PEEP generating plateau pressure exceeding 30 cmH2O could better serve these needs but would unavoidably damage the already open alveoli as a means of excessive pressure/volume load delivered in the aerated lung.
Whether it could be more convenient to accept atelectrauma due to inadequate recruitment or baro/volutrauma due to higher PEEP remains to be established in further clarifying investigations.
Article review prepared and submitted by EJRC members Gian Marco Anzellotti, MD and Domenico Luca Grieco, MD (Department of Anaesthesiology and Intensive Care Medicine, Catholic University of The Sacred Heart, Rome, Italy)
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