An additional shortcoming of the present study is the lack of separate analyses by more specific ICU patient cohorts. Unfortunately in most of the trials, it was difficult or impossible to distinguish further the type of subjects admitted or the attempt to categorization would have compelled an excessive subgrouping with loss of reliability of results. Moreover, the secondary outcome is variable between the present review and the previous ones, but this may be because of subgroup analysis which may throw unexpected results based on differing inclusion criteria and study selection.