Tomographic study of the inflection points of the pressure-volume curve in acute lung injury

The inflection points of the pressure-volume curve have been used for setting mechanical ventilation in patients with acute lung injury. However, the lung status at these points has never been specifically addressed. In 12 patients with early lung injury we traced both limbs of the pressure-volume curve by means of a stepwise change in airway pressure, and a computed tomography (CT) scan slice was obtained for every pressure level. Although aeration (increase in normally aerated lung) and recruitment (decrease in nonaerated lung) were parallel and continuous along the pressure axis during inflation, loss of aeration and derecruitment were only significant at pressures below the point of maximum curvature on the deflation limb of the pressure-volume curve. This point was related to a higher amount of normally aerated tissue and a lower amount of nonaerated tissue when compared with the lower inflection point on both limbs of the curve. Aeration at the inflection points was similar in lung injury from pulmonary or extrapulmonary origin. There were no significant changes in hyperinflated lung tissue. These results support the use of the deflation limb of the pressure-volume curve for positive end-expiratory pressure setting in patients with acute lung injury.