Shear stress

P>Expanding an atelectatic area from a state where it is completely collapsed to the fully open state requires a much greater change in pressure (pressure amplitude) and higher peak inspiratory pressure than is necessary for inflating a lung area that is already partially held open by an appropriate amount of PEEP. This is similar to inflating a balloon, and follows logically from the the LaPlace law P = 2g/r, where P is the pressure to stabilize the alveolus, g is the surface tension at the air-liquid interface, and r is the radius of the alveolus. The shearing forces generated by repeatedly opening up an atelectatic area and then allowing it to collapse again may lead to structural damage (in particular to the bronchiolar and alveolar epithelia and the capillary endothelium). This disrupted parenchyma may be a source of mediators triggering the continuation of acute respiratory distress syndrome ( LachmaOD 1992).

Further, if the lung is not held open, it may have a deleterious effect on pulmonary surfactant. In health, surfactant molecules are compressed at end-expiration. If the volume of the alveolus is smaller than the surface area of the surfactant, the excess molecules are squeezed out towards the airways. At the next inspiration, the surfactant in this hypophase rejoins the alveolar surface. However, if the lung is forced open with large tidal volumes or high respiratory rates, rather than judicious use of PEEP, the surfactant is lost into the airways and lost completely from the alveolus. To minimize this phenomenon, as much of the lung as possible should be ventilated without either overdistention of the lung or allowing lung collapse.

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