PISA is an ancillary study to the NIH funded clinical RESTORE Trial (U01 HL086622). This project will use sophisticated modeling and simulation techniques to evaluate the impact of genetics and other variables such as degree of illness, age, weight and organ dysfunction on the pharmacokinetics and pharmacodynamics of morphine and midazolam in children who are mechanically ventilated for respiratory failure, and require sedation. This proposed work will allow the design of a pharmacologic model that can be used to individualize therapy in children requiring mechanical ventilation with the goal of optimizing sedation while minimizing the duration of mechanical ventilation. This study will provide data that may allow for improved dosing recommendations in this critically ill population of children.