Dr Schmidt2 argues for the stress index as a better method of determining PEEP. Our previously stated concerns about the stress index include limited data (15 human subjects),5 demonstrated benefit only for surrogate outcomes,5 requirement for heavy sedation and/or paralysis, and potential confounding of chest wall pressure-volume relationships.6 Additionally, we find visual interpretation of concavity or convexity, frequently such as with pressure-time curves, difficult and occasionally confusing. What if the interpretation is clear once and then not the next time? How often need the interpretation be made? Dichotomizing around a stress index of one may be overly simple also. Specialized recording equipment is presently necessary, and patients must be on volume-cycled ventilation with square-wave inspiratory flow. This manner of ventilation is uncommon, as most patients on volume-cycled modes receive decelerating ramp inspiratory flow. As to whether PEEP is a relevant target at all or whether an approach like the stress index might direct treatment in some other way, we defer judgment until more data are available. Forced oscillation techniques7 suffer these same limitations and lack confirmation in humans.