icu

Clinical Application

　　What does all this mean from a clinical perspective? The key messages are that one should try to adopt a ventilatory strategy that limits: (1) intraparenchymal stress/strain that occurs at low lung volumes, and (2) excessive stress to the lung parenchyma at end-inspiration. Limiting intraparenchymal stress and strain can be implemented by increasing the PEEP level and/or by ventilating patients in the prone position. A number of studies have demonstrated that the distribution of gas/tissue ratios (which can be considered equivalent to regional lung inflation) is much more uniform when patients are placed in the prone position compared with the supine position. This is likely the explanation for the decreased VILI observed in animal studies examining the effect of the prone position in lung injury models. For example, Broccard and colleagues[7] used an oleic acid lung injury model and ventilated animals in either the supine or the prone position. They found that the prone position produced less severe and more homogeneous distribution of VILI as assessed by wet/dry weight ratios and histologic scores. Unfortunately, the clinical studies that have been performed to date in patients with ARDS have not demonstrated an improvement in mortality, although they have shown that most patients have an improvement in oxygenation.
One method that has been used to assess what PEEP level to use to minimize intraparenchymal stress and strain is to measure the pressure-volume (P-V) curve of the lung. In many patients with ARDS, one can identify a point in the inflation P-V curve where the slope increases substantially -- the lower inflection point. It had been suggested that using PEEP somewhat above this level would prevent intraparenchymal stress/strain, since this represents the point at which the lung is recruited, and thus one could prevent VILI. However, a number of studies dating back to the 1960s have demonstrated that the lower inflection point of the P-V curve represents the start of recruitment as pressure is increased, and hence there is likely limited utility in using the lower inflection point of the P-V curve to set the PEEP level.
　　There have been a number of clinical trials that have addressed the importance of VILI in patients with ARDS by applying so-called protective ventilatory strategies. The trial that has demonstrated the most convincing results is the trial published by a consortium of investigators sponsored by the National Institutes of Health -- the ARDSNet investigators. These investigators found that a ventilatory strategy using a Vt of 6 mL/kg (based on predicted body weight [PBW]) and limiting Pplat as much as possible to less than 30 cm H2O decreased mortality by 22% compared with the use of a Vt of 12 mL/kg (PBW).[8] Of note, although one might think that this approach might be most effective in patients with the stiffest lungs (and highest Pplat), an initial reanalysis of the data suggests that there may be a major mortality benefit in decreasing Vt even in patients with low Pplat (< 30 cm H2O). The practical implications of this finding are unknown but suggest that a Pplat limit of 30 cm H2O (in patients who do not have stiff chest walls) may in fact be too high.