![]() In adult awake human subjects at rest, diversity exists in the breathing pattern not only in terms of tidal volume and inspiratory and expiratory duration and derived variables, but also in the airflow profile. The clinical picture of a normal breathing pattern is, however, much more complex. Since 5–7 ml/kg is considered to constitute a normal physiological range, some authors now consider that 6 ml/kg tidal volume might be recommended for any patient under mechanical ventilation. The lack of a clear protective effect of PEEP was possibly explained because the enrolment criteria were unable to distinguish between recruiters and non-recruiters. New trials testing different levels of PEEP, supposed to minimize the end-expiratory small airway and alveolar closure through alveolar recruitment, and therefore to limit the strain at end-inspiration induced by tidal volume, did not give a uniform answer to this question. The strain forces and shear stress induced by the reopening of repetitively collapsed alveoli or closed terminal airways might play an important role in the poor outcome of patients experiencing VILI. Taking together the results of clinical trials and of retrospective analysis of some of these data, the amplitude of the tidal volume delivered seems to generate a strong signal influencing mortality. Other arguments nourished the debate, such as the distinction between predicted and actual body weight to retrospectively analyze databases, or the lack of relation between tidal volume and mortality observed in some of these studies, as well as the negative results of other randomized controlled trials. Also, a high tidal volume was proposed as a possible culprit for patients free of ALI on admission but who later develop ALI during the course of mechanical ventilation. Other authors argued that only two settings had been rigorously tested, that the end-inspiratory level of distension reached, evaluated by plateau pressure as a surrogate, was a key component of the risk of VILI, and that there may also be risks of having an unnecessarily low tidal volume leading to atelectasis, dysynchrony and discomfort. For some authors, the choice of 6 ml/kg came to be considered as the new standard. The topic is of considerable importance for any clinician, since the mortality difference observed in the 6 ml/kg predicted body weight tidal volume group suggested that 25% of all ARDS deaths in the 12 ml/kg group were caused by this excessive ventilation. ![]() This landmark study made very clear that the choice of 12 ml/kg predicted body weight tidal volume was associated with a poor outcome, presumably through the mechanism of ventilator-induced lung injury (VILI), which was already described decades before. Since the study published by the ARDSNet in 2000, there have been strong debates about the optimal tidal volume to set-and to recommend-for patients under mechanical ventilation for acute lung injury (ALI).
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