Assessment of lung non-homogeneity : clinical consequences

We recently found that the lung damage occurs when the applied strain and stress are in the region of total lung capacity. Nevertheless, VILI may develop at stress and strain far lower than the threshold observed in experimental animals. A possible explanation is that the damaged lung is more “fragile” and injury develops at lower stress and strain thresholds. Either, that in a disomogenous lung the applied force, used to be evenly distributed, is locally concentrated leading to localized increase of stress. Mead et al described a mathematical model in which they simulated the effect, as an example, of letting decrease if lung volume down to one in a region of lung where that volume of gas was equal to 10. Referring to the ratio A1/A0 instead of V1/V0 by taking the V1/V0 to the power of 2/3 the stress, for an applied pressure of 30 cmH2O (transpulmonary) will result 30 * (V1/V0)2/3 i.e. 30*4.64 = 139.25 cmH2O. Thus there is a theoretical potential that location of disomogeneity throught the lung may act as a “pressure multiplier”. Accordingly, a pleural pressure below the threshold observed in “healthy lung”, if multiplied sufficiently, may locally reach a level recognized as surely injurious.