<qa>  What is the primary clinical goal of the study?Recombinant human deoxyribonuclease I rhDNase, dornase alpha, administered by inhalation, is currently used as a mucolytic agent in the treatment of cystic fibrosis. This prospective, randomized, placebo-controlled, double-blind clinical study should carry out whether the therapeutic use of rhDNase can be extended to adults who are undergoing long term ventilation at the intensive care. It is hypothesized that mechanically ventilated patients show elevated concentrations of DNA in airway secretions, and that these pathological DNA levels are reduced by rhDNase. The primary clinical goal of the study is to establish wether ventilation times are shortend under inhalation of rhDNAse during the ventilation time. hDNase verum is compared with equal amounts of 0.9 saline solution placebo. 