Targeted fluid therapy has received increasing attention in the management of patients showing acute circulatory failure in both intensive care unit (ICU) and operating room (OR), aiming at preventing both inadequate tissue blood flow and fluid overload [1]. In fact, unnecessary fluid administration can increase morbidity and mortality and length of hospital stay of critically ill and surgical patients [2-10].~Since the only physiological reason to give a fluid challenge (FC) is to increase the stroke volume (SV) [11-13] and this effect is obtained only in about 50% of ICU and OR patients [14, 15], a vast literature investigated the possibility of predict this effect before FC administration, but the issue remains extremely challenging [1, 13, 16-18]. Bedside clinical signs and pressure and static volumetric static variables, do not predict fluid responsiveness [17]. Moreover, several physiological factors affect the reliability of the ventilator-induced dynamic changes in pulse pressure and stroke volume [pulse pressure variation (PPV) and stroke volume (SV) variation (SVV), respectively], and their echographic surrogates, in a significant number of ICU and OR patients [19-22].~To overcome these limitations, the functional hemodynamic assessment (i.e. the assessment of the dynamic interactions of hemodynamic variables in response to a defined perturbation), of fluid responsiveness has gained in popularity [17, 18, 23]. A functional hemodynamic test (FHT) consist in a manoeuvre determining a sudden change in cardiac function and/or heart lung interaction, affecting the hemodynamics of fluid responders and non-responders to a different extent [17, 18, 23].~The FHT called passive leg raising (PLR) has been successfully used for assessing the fluid responsiveness in ICU patients since 2009 [24] and its reliability has been confirmed by three large meta-analyses [25-27]. However, the PLR is not usually practicable in the OR.~A lot of different FHTs have been proposed, as alternative to the PLR, in ICU and, more recently, OR. These tests could be basically subdivided in two groups. A subgroup of FHTs is based on sudden and brief variations of the mechanical ventilation to induce a change in right ventricle preload and/or after load and, as consequence, of left ventricle SV [24, 28]. Among these tests, the end-expiratory occlusion test (EEOT) has been previously successfully tested in surgical patients, consisting of the interruption of the mechanical ventilation for 30 seconds, and in the evaluation of the changes in the SV. A second subgroup aims at testing the increase in SV after the rapid administration of a small aliquot of the predefined FC [29, 30]. This test is called mini fluid challenge (mFC). Both these test have been previously evaluated in small-sized studies and never compared each other.~The primary aim of the present study is to compare the reliability of EEOT and mFC in predicting fluid responsiveness in patients undergoing general surgery.
End-expiratory occlusion test (EEOT) has been previously successfully tested in surgical patients, consisting of the interruption of the mechanical ventilation for 30 seconds, and in the evaluation of the changes in the SV.~The mini fluid challenge test (mFC) aims at testing the increase in SV after the rapid administration of a small aliquot of the predefined FC.~Both these test have been previously evaluated in small-sized studies and never compared each other.