Bacterial infections are still associated with a high mortality in intensive care unit patients. Especially patients diagnosed with ventriculitis caused by a bacterial infection have a bad outcome. Therefore, antibiotic therapy is the only causal opportunity to treat those infections. However, there are many chances in pharmacokinetic and -dynamic in critically ill patients with unpredictable antibiotic concentrations. Furthermore, it is important that the concentration in the blood as well as in the effect-compartment "CSF" is sufficient. Less data of the last years describe antibiotic concentrations in the CSF and show subtherapeutic levels, which is endangering for the therapeutic success. The introduced study is a prospective, observational study that analyses antibiotic concentrations in CSF in critically ill patients. The distribution between CSF and blood will be evaluated by comparing these two compartments.