My favorite unconfirmed ideas for future ICU practice : part 2

One possible explanation of the discrepancy between clinics (where VT of 12 mL/kg IBW have been found harmful) and experimental VILI, is the presence, in the diseased lung, of “pressure multipliers” or “stress risers”. In general, when a force is applied to a material with dishomogeneities (as for example holes or inclusions), the lines of force crossing the material are more concentrated around them. In other words, the stress/strain load is borne by the structures lying close to dishomogeneities. In the ARDS lung, the consolidated /collapsed regions may be considered as dishomogeneities, which act as “stress risers” towards open healthy regions connected to them. This hypothesis is usually inferred from CT scan and, more recently, by respiratory input impedance. If a “stress riser” is able to multiply the applied pressure 2-3 times, an “innocent” transpulmonary pressure applied to the whole lung may locally generate a lethal pressure (the one at which TLC is nearly reached). Mead et al., in their theoretical analysis of this phenomenon, computed that the pressure multiplication could be as high as 4.5, if completely expanded and a completely collapsed regions were connected. To reach this conclusion, they assumed that the volume ratio of a fully expanded to a fully collapsed region would be 10 to1. This, in turn, would correspond to an area ratio (area expanded: area collapsed) of (10:1)2/3, which is equal to 4.57. The shift from volume to area is necessary, as stress is expressed per unit of area. Mead concluded that, for an applied PL of 30 cmH2O, the fully expanded area would experience a stress of 30 x 4.57=137 cmH2O. It is worth noting that this finding, although often quoted as experimental, is purely theoretical. However, Rausch et al. recently showed, employing synchrotron-based x-ray tomographic microscopy on isolated rat lungs, that the local strain developing in alveolar walls carried a multiple of the value of global strain up to 4 times. This value is impressively similar to the value inferred from Mead’s computations. Techniques as higher PEEP or prone position, decrease lung dishomogeneity and, consequently, reduce the extent of stress risers by keeping open previously collapsed regions. This in turn reduces the harmfulness of MV and may potentially improve survival of ARDS patients. It is however conceivable that, in a subgroup of severe patients, the diffuse presence of stress risers may simply not allow safe MV.