In particular, the proportion of subjects with sedation failure at the maximum possible dose (defined within the study protocol) will be measured. Additionally, the safety and tolerability (including withdrawal effects) of clonidine compared to midazolam will be evaluated. A pharmacokinetic-pharmacodynamic relationship of clonidine for sedation in PICU will be established. Genetic polymorphisms of clinical relevance affecting pharmacokinetics, pharmacodynamics and metabolism will be also identified.