The concept of "baby lung"

Background: The "baby lung" concept originated as an offspring of computed tomography examinations which showed in most patients with acute lung injury/ acute respiratory distress syndrome that the normally aerated tissue has the dimensions of the lung of a 5- to 6-year- old child ( 300 - 500 g aerated tissue). Discussion: The respiratory system compliance is linearly related to the " baby lung" dimensions, suggesting that the acute respiratory distress syndrome lung is not " stiff" but instead small, with nearly normal intrinsic elasticity. Initially we taught that the " baby lung" is a distinct anatomical structure, in the nondependent lung regions. However, the density redistribution in prone position shows that the " baby lung" is a functional and not an anatomical concept. This provides a rational for "gentle lung treatment" and a background to explain concepts such as baro- and volutrauma. Conclusions: From a physiological perspective the "baby lung" helps to understand ventilator-induced lung injury. In this context, what appears dangerous is not the VT/kg ratio but instead the VT/" baby lung" ratio. The practical message is straightforward: the smaller the "baby lung," the greater is the potential for unsafe mechanical ventilation.