- Severe dehydration (defined by a central venous pressure < 3 cm H2O due to severe diarrhea or to a strong response to diuretic treatment) at inclusion in the study; the patients with PVC lower than 3 will receive plasma volume expansion with saline and will be reevaluated within 24 h. If the expansion is able to correct PVC (defined as PVC > 3), the patients will be apt to be included in the study.