Mechanical Power and Ventilator-induced Lung Injury: What Does Physics Have to Say?

Ventilator-induced lung injury (VILI) is a potential threat to anyone receiving supportive mechanical ventilation for acute respiratory failure.Despite decades of research, however, the safest way to ventilate any given patient remains controversial.This makes fertile ground for novel concepts, and one that has arisen recently concerns the idea that a ventilator imparts potentially damaging mechanical energy to the lungs (1, 2).The motivation for this concept is clear: Energy transfer is involved when any structure becomes physically damaged.It may be intuitive, then, that the rate at which energy is delivered to the lungs by a ventilator-namely, mechanical powershould be associated with VILI.Nevertheless, understanding the relationship between mechanical power and VILI requires clarity on the difference between stored versus dissipated energy, regardless of whether ventilation is caused by positive pressure at the airway opening or negative pressure in the pleural space (3).When a ventilator inflates the lung, some of the applied work is stored in the lung tissues as elastic potential energy.This type of energy is what physicists call conservative, meaning it is completely recoverable.This can only be the case if those tissues that become distorted during inspiration are returned precisely to their preinflation configurations on expiration.The net result to the tissues is equivalent to the energy never having been delivered in the first place.