Mechanical ventilation may cause injury to the ventilated lung. This Critical Care Medicine article reviews the probable causes of such injury and ways to prevent it.
There has been a renewed focus on the worsening injury that mechanical ventilation can cause in previously damaged lungs and the damage it can initiate in normal lungs. This damage is characterized pathologically by inflammatory-cell infiltrates, hyaline membranes, increased vascular permeability, and pulmonary edema. The constellation of pulmonary consequences of mechanical ventilation has been termed ventilator-induced lung injury.
• Why do the authors suggest that the term ‘barotrauma’ is misleading?
Ventilator-induced lung injury can occur because of ventilation at high (absolute) lung volumes, leading to alveolar rupture, air leaks, and gross barotrauma (e.g., pneumothorax, pneumomediastinum, and subcutaneous emphysema). The term barotrauma can be misleading, because the critical variable leading to the air leaks is regional lung overdistention, not high airway pressure per se. Dreyfuss et al. found that pulmonary edema developed in animals undergoing ventilation with high tidal volumes, whereas such edema did not develop in animals undergoing ventilation with similar airway pressures but with straps around their abdomens and chests that reduced the tidal volumes. Thus, their experiments showed that volume (i.e., lung stretching), not airway pressure, was the most important factor in determining injury, a finding that led them to coin the term “volutrauma.”
• How is a low-tidal-volume strategy helpful in patients with ARDS?
Patients with ARDS often have relatively nonaerated dependent lung regions and relatively normally aerated nondependent lung regions. Because there is smaller volume available for ventilation, this condition has led to the term “baby lung.” The implication is that a decreased tidal volume (i.e., one that might be normal for a baby) should be used to prevent overinflation of the relatively small, normally aerated regions. In a seminal study that built on previous studies, the ARDS Network investigators compared a control strategy that used a tidal volume of 12 ml per kilogram of predicted body weight with a low-tidal-volume strategy that used 6 ml per kilogram of predicted body weight. The low-tidal-volume strategy was associated with an absolute reduction of 9 percentage points in the rate of death.
Morning Report Questions
Q: When is a relatively high positive end-expiratory pressure (PEEP) considered helpful in patients with ARDS?
A: Pulmonary edema and end-expiratory alveolar collapse characterize several forms of respiratory failure. In these situations, a low PEEP may not stabilize alveoli and keep them open, thereby increasing the likelihood of ventilator-induced lung injury from atelectrauma.
Conversely, a higher PEEP has potentially adverse effects, including impairment of venous return and pulmonary overdistention. A recent individual-patient meta-analysis of randomized trials addressed these tradeoffs in patients with ARDS and concluded that a higher PEEP was associated with an absolute reduction of 5 percentage points in the rate of death among patients who had increased impairment in oxygenation, which was defined as a ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2:FiO2) of 200 mm Hg or less.
Q: Is prone positioning in patients with ARDS and hypoxemia helpful?
A: About 70% of patients with ARDS and hypoxemia have improved oxygenation when they are placed in a prone position. Possible mechanisms for this effect include increased end-expiratory lung volume, better ventilation-perfusion matching, less effect of the mass of the heart on the lower lobes, and improved regional ventilation. Most important, as has been shown in studies in animals, the prone position should minimize lung injury by increasing homogeneity of ventilation. A recent meta-analysis of seven trials involving a total of 1724 patients showed that prone positioning lowered mortality by approximately 10 percentage points in the subgroup of patients with ARDS and severe hypoxemia (PaO2:FiO2 ratio, <100 mm Hg). Patients who were treated with prone positioning had an increased number of potentially preventable complications, including pressure ulcers, endotracheal-tube obstruction, and chest-tube dislodgement.