人工呼吸管理と肺損傷

Mechanical ventilation is essential to take care of critically ill patients, and it has been saving a lot of lives. On the other hand, it is apparent that mechanical ventilation promotes lung injury. NIH trial proved that low tidal volume ventilation to avoid ventilator-induced lung injury （VILI） improved the mortality of acute lung injury （ALI） and acute respiratory distress syndrome （ARDS）. When we take care of mechanically ventilated patients, we should be careful to minimize VILI. To achieve it, it is prudent to understand the mechanisms of VILI.
In humans, the extent to which mechanical ventilation exacerbates antecedent lung injury in ALI/ARDS is hard to evaluate, and the lung injury related to mechanical ventilation is called ventilator-associated lung injury （VALI）. The clinical value of ventilatory strategy to avoid alveolar overdistension was demonstrated in NIH trial. Low tidal volume ventilation reduced mortality of ALI/ARDS by ２０ percent. The strength of this finding has changed practice, and low tidal volume ventilation has become the standard of care in ALI/ARDS. No doubt the benefit is related to a decrease in VALI.
Many animal studies supported a variety of methods to minimize VILI. In ARDS widespread atelectasis is common, and shear forces are created to distend the adjacent lung units that remain open. A rabbit model demonstrated that cyclical atelectasis promoted VILI, and that PEEP prevented the development of VILI. The ideal level of PEEP in ARDS is not known. Body position also influenced the development of VILI in animal studies. Prone position was protective against VILI. High inspiratory flow also injured the lungs. Unfortunately these modalities have not been proved to be effective in humans.
The mortality of ALI/ARDS decreased significantly in the last decade, it is still high. When we take care of mechanically ventilated patients, lung protective strategies are essential. We should search for safer ventilatory strategies to improve the mortality of ALI/ARDS.

四国医学雑誌 : http://www.tokushima-u.ac.jp/med/research/shikoku_acta_medica.html