What is Stress Index?
Linearity on the pressure over-time curve on volume control mechanical ventilation indicates that the ventilation occurs between the lower inflection point and the upper inflection point of the PV curve, where the elastance is the lowest and remains constant. The stress index here is 1 and no need to adjust the PEEP.
If the stress index is >1, or the pressure-time curve is curved up (concave), that means that the elastance is increasing over time and represents ventilation above the upper inflection point or over-distention and the need to decrease the volume.
On the other hand, if the stress index is <1, or the pressure-time curve is curved down (convex), that means that the elastance is decreasing before it is increased on the PV curve, and indicates de-recruitment and the need to increase the PEEP.