Continuous vs dynamic strain

When lung deformation is only caused by cyclic inflation of tidal volume, strain is entirely dynamic. When positive endexpiratory pressure is applied, as in most mechanically ventilated patients, lungs are also kept tonically inflated above their functional residual capacity and thus exposed to an additional static strain. Any end-inspiratory lung volume can then be achieved using different combinations of dynamic and static strains, always resulting in the same global strain. Whether dynamic and static strains play an identical role in the pathogenesis of ventilator-induced lung injury remains controversial. Some evidence suggests that overall lung inflation is the major determinant of damage, regardless of its components. Accordingly, lungs inflated up to their maximum physiological limits should always fail, no matter whether strain is mainly dynamic or static. Some other data indicate that, for a given global deformation, static may be less injurious than dynamic strain, possibly because of increased mean airway pressure, diminished pulmonary perfusion, and thus extravascular fluid filtration. Large static strains would then be better tolerated than equivalent, but dynamic, ones. Lung edema forms (possibly as an all-or-none response) depending not only on global strain but also on its components. Large static are less harmful than large dynamic strains, but not because the former merely counteracts fluid extravasatio