This hospitalized patient with worsening hypoxemia and bilateral lung infiltrates mostly likely has acute respiratory distress syndrome (ARDS), a potential complication of acute pancreatitis.  ARDS involves an inflammatory response in the lungs that leads to alveolar capillary leakage and diffuse pulmonary edema; the edema prevents ventilation of affected alveoli and facilitates alveolar collapse, leading to increased intrapulmonary shunting (perfusion without ventilation).  In addition, functional residual capacity (FRC) (ie, the volume of air in the lungs at end-tidal expiration) is reduced.

Mechanical ventilation is typically needed for the management of ARDS because it allows for the application of positive end-expiratory pressure (PEEP).  PEEP helps treat ARDS by opening collapsed alveoli to reduce intrapulmonary shunting (Choice D) and increase FRC back to near-normal levels.  The increase in FRC has the following beneficial effects:

• There is a critical lung capacity (ie, closing capacity) at which some of the lung's small airways collapse during expiration (due to decreased radial traction at lower lung volumes).  When FRC drops below this critical capacity, alveoli supplied by the collapsed airways are without ventilation for part of the respiratory cycle, which contributes to intrapulmonary shunting and increases ventilation-perfusion mismatching.  Increasing FRC minimizes the time spent below closing capacity and in doing so decreases ventilation-perfusion mismatching.

• Because the FRC air volume remains in the lungs throughout the respiratory cycle, it acts as a store of oxygen that the body can pull from during brief periods of increased need.  Therefore, increasing FRC increases the oxygen reserves in the lungs.

(Choice A)  PEEP increases, rather than decreases, alveolar pressure, which has the negative effect of increasing the risk of pulmonary barotrauma.

(Choice B)  Intrapleural pressure is normally negative throughout the respiratory cycle due to the opposing elasticity of the lungs (tend to collapse) and chest wall (tends to expand).  Increased end-expiratory airway pressure during PEEP reduces the collapsing force of the lungs, causing the intrapleural pressure to increase.

(Choice E)  Minute ventilation is determined by the product of tidal volume and respiratory rate.  PEEP causes breathing to occur at a higher baseline lung volume (ie, higher FRC), but tidal volume and respiratory rate are not directly affected, and minute ventilation is unchanged.

Educational objective:
Positive end-expiratory pressure helps treat acute respiratory distress syndrome by opening collapsed alveoli to reduce intrapulmonary shunting and increase functional residual capacity (FRC).  The increased FRC decreases ventilation-perfusion mismatching and increases the oxygen reserves in the lungs.