Figure 1.

Graphic representation of the stress index concept. Pao indicates airway pressure. During constant flow, there is volume-cycled mechanical ventilation. For stress index values <1, the Pao curve presents a downward concavity, suggesting a continuous decrease in elastance during constant-flow inflation. For stress index values >1, the curve presents an upward concavity, suggesting a continuous increase in elastance. Finally, for a stress index value equal to 1, the curve is straight, suggesting the absence of tidal variations in elastance. Reprinted with permission from Proc (Bayl Univ Med Cent). 2015;28(2):163–171.