In ARDS, we know that the lungs are so diffusely injured ...

In ARDS, we know that the lungs are so diffusely injured that the remaining total area of relatively spared lung may be reduced to the size of a baby’s lungs, hence the “Baby Lung” concept. Despite this, it is also important to realize that in ARDS, lung injury may be diffuse, but not necessarily uniformly distributed, and we often see heterogenous patterns of lung injury mixed with regions of relative lung sparing. In that sense, both lungs may have multiple “Baby Lungs” with different baseline volumes (FRC) that depend on the area of lung spared in those regions. This is where the concept of Lung Strain and Lung Stress come into play. 

Lung Strain is Tidal Volume/FRC. Consider a patient in ARDS where 6 ml/kg corresponds to 400 ml tidal volume. If a spared region has an FRC of 400 ml, strain is calculated as 1; however, if a neighboring region has an FRC of only 100 ml, then lung strain is 4! 4 times the amount of strain! Likewise, stress and strain are directly correlated to one another. Therefore, even at low tidal volumes and plateau pressures <30, we may be providing lung protective ventilation to one region of spared lung while causing significant lung strain and lung stress to another.

Keep this in mind, and always shoot for the minimum in terms of low tidal volumes. 6 ml/kg is what has been classically taught, but if you can get away with 3-4 ml/kg, the risk of lung strain is even further minimized. Same with lung stress; we are taught to keep plateau pressures < 30; but shooting for the lowest possible plateau pressure is likely the best way to minimize lung stress. Ultimately, the goal is to minimize the risk of Ventilator-Induced Lung Injury (Barotrauma, Volutrauma) and promote Lung rest and recovery.

#pathophysiology #diagnosis #management #ards #foamed #criticalcare #treatment #respiratory #clinical

Contributed by

Obiajulu Anozie
Intensivist, Educator, Applied Physiologist
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