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Volutrauma refers to lung parenchymal damage caused by mechanical ventilation that is similar to ARDS (see Figure 3-1). Volutrauma is VILI manifested by an increase in the permeability of the alveolar capillary membrane, the development of pulmonary edema, the accumulation of neutrophils and proteins, the disruption of surfactant production, the development of hyaline membranes, and a decrease in compliance of the respiratory system (Table 3-2). The term volutrauma is used because the induced injury is the result of alveolar overdistention. Clinically, volume is used as a surrogate for pressure as it is impossible to measure local overdistention at the bedside. The pressure that has been used as a surrogate for local overdistention is Pplat. A Pplat more than 30 cm H2O increases the likelihood of VILI; Pplat should be kept as low as possible.
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Chest Wall Compliance
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Because alveolar distention is determined by the difference between alveolar and pleural pressure, the chest wall has a role in determining the extent of overdistention. When the chest wall is stiff (low compliance), a high Pplat may be associated with less risk of overdistention. That is, a stiff chest wall (eg, abdominal distention, massive fluid resuscitation, chest wall deformity, chest wall burns, morbid obesity) protects the lungs from VILI.
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Active Breathing Efforts
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The alveolar distending pressure can change markedly on a breath-by-breath basis in a spontaneous breathing patient. This most commonly occurs in pressure-targeted ventilation with high inspiratory efforts by the patient. When the airway pressure is constant and the patient forcefully inhales, the alveolar distending pressure may exceed what is expected by the airway pressure setting. For example, if the pressure control is 25 cm H2O and the patient's effort decreases the pleural pressure to –10 cm H2O, alveolar distending pressure is 35 cm H2O, 10 cm H2O greater than expected with an airway pressure setting of 25 cm H2O. During pressure-targeted ventilation, the contribution of patient's effort to alveolar distending pressure must be appreciated.
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Preexisting injury increases the likelihood of VILI. This is called the two-hit process of lung injury. Previous injury predisposes the lungs to a greater likelihood of ventilator-induced injury. The use of lung-protective ventilatory strategies is thus necessary for all patients. The risk of developing ARDS is reduced if lung-protective ventilation strategies are implemented from the onset of mechanical ventilation (eg, volume and pressure limitation).