Cardiac output (CO) is a parameter usually requested to assess hemo dynamic status of patient and efficacy of therapy especially in critically ill patients. This paper, in line with other research activities and new market availability, intends to correlate a parameter derived from data already acquired in standard patient monitoring (systemic arterial pressure--Pas) in order to identify CO trends and, more generally, to obtain information on the efficiency of cardiovascular system of the patient in examination. Attention has been focussed on patients undergoing abdominal aneurysm aortic (AAA) surgery with combined anaesthesia (epidural and light general). Awake correlation was found between maximum value of Pas time derivative, max (dPas/dt), and CO measured with thermodilution method on 56 measurements on 10 patients. To evaluate further diagnostic capability of max (dPas/dt), we compare its trend with other parameters but no statistical significant results have been obtained. Therefore the target parameter can be profitably used in the examined patients to monitor CO trend and, in correlation with other parameters, as a sign of efficiency of the cardio circulatory system.