 2. STUDY OF MINIMAL VOLUME Once the participant is admitted to the ICU, and the monitors are in place, the fluid challenge will be administered. Each fluid challenge will be administered with a syringe 50 ml in five minutes. In order to observe the effect of the volume on Pmsf-arm, the total sample of patients will be divided in 4 groups: in group one Pmsf-arm will be measured at baseline and after 1 mlkg of body weight; in group 2 after 2 mlKg; in group 3 after 3 mlkg and finally in group 4 after 4 mlKg. The fluid challenge will be completed in everybody after the measurement of Pmsf-arm, in order to respect clinical practice. Pmsa will be observed also. The total duration of the fluid challenge will be 5 minutes. The fluid challenge will be repeated in accordance with the clinical protocol or with the clinician advice. 