Systolic pressure variations (SPV) during mechanical ventilation and its single components, related to short apnea, reflect changes of the volemic condition of the patient. To introduce their determination during clinical monitoring for different fluid states and for different tidal volumes, they must be computed on-line without introducing interference with standard activities. A system computing on-line systolic pressure variation during mechanical ventilation, connected to standard monitoring devices, has been proposed. It is based on a notebook PC implemented with graphical software comprising a user panel in the form of a virtual instrument and is able to acquire, process and present signals from different instruments utilized in ICU and during surgery. It can be used as a base to assess the ability of computed parameters helpful in clinical decision. The use of a notebook PC and open software allows operators, even if non-expert in computer science, to test and implement this, as well as other innovative tools in clinical practice.