when hypovolemia is strongly suspected , prediction of the fluid challenge response remains difficult for the intensivist or the anesthesiologist , because of complex interactions . in addition to the clinical art , each physician wants to use numerical parameters to make the decision to give fluid and to ensure an adequate response to the fluid given . apart from the debate on the type of fluid , the best way to evaluate the fluid challenge response has motivated a lot of clinical research aiming to demonstrate the specificity and sensitivity of several parameters , considering the invasiveness , the accuracy , and the cost of the methods . in the past decade , many devices and parameters have been proposed to dynamically evaluate the response to fluid challenge . if fluid has to be given , the goal remains multifactorial , oscillating between hypotension correction , improvement in cardiac function , increase in cardiac output and oxygen delivery , or vascular recruitment . the effectiveness of this fluid challenge has been assessed by different methods : ( 1 ) echocardiography showing a better function of the right and left ventricles or better filled inferior or superior vena cava ; ( 2 ) changes in oxygen delivery , which is simplified when arterial oxygen saturation is normal in changes in cardiac output , whatever the techniques for cardiac output measurements used ; ( 3 ) changes in pulse pressure amplitude based on determinants of systolic and diastolic arterial blood pressure ; or ( 4 ) cardiac filling pressure variations in association with cardiac output , according to the frank - starling law , but with invasive methods . the study from trof and colleagues examines the interest in measuring cardiac filling pressures or the diastolic cardiac volume index ( global end - diastolic volume index ) in cardiac or vascular surgery in the post - operative period , in the presence or absence of systolic left ventricular dysfunction . the later parameter the global ejection fraction ( gef ) was obtained from the ratio between the stroke volume and global end - diastolic volume ( gedv ) , being > 20% when left ventricular function is normal and < 20% when abnormal . these parameters have been obtained using the precursor technique of the pulse contour method ( cold z-021 ; pulsion medical systems , munich , germany ) with a double indicator . the carefully performed study from trof and colleagues , with great caution for statistical analysis , concluded that regardless of the gef the central venous pressure may be useful for predicting fluid responsiveness in these patients when the positive end - expiratory pressure ( peep ) is low . when the gef is low ( < 20% ) , the pulmonary artery occlusion pressure is more useful than the global end - diastolic volume index for predicting fluid responsiveness ; but when the gef is near normal ( 20% ) , the global end - diastolic volume index is more useful than the pulmonary artery occlusion pressure . fluid responsiveness in terms of hemodynamic improvements deals with pressure , volume , cardiac pumps ( right and left ) , vascular compliance and resistance , and circulatory time . prediction for fluid responsiveness is difficult because of all these interactive factors , but some logical aspects can be taken on the basis of the results presented . first , all of these conclusions are based on three major conditions : the presence of hypovolemia , a low peep level , and relatively adequate right ventricular function three restrictive conditions that are not frequently observed in intensive care unit patients . peep is frequently higher , as is the intrathoracic pressure a situation that may alter the right ventricular function , with important modifications in pulmonary blood volume during the respiratory cycle ( figure 1 ) . during inspiration on mechanical ventilation , the right ventricular afterload and reduced venous return both alter the right ventricular stroke volume towards the pulmonary vessels , which are compressed by intra - alveolar pressure . this lung squeezing as a sponge pushes the blood towards the left ventricle , being better filled and then ejecting a higher stroke volume . as a consequence , the pulmonary blood volume is reduced , with an inverse effect during expiration . ( a ) during insufflation , the right ventricular afterload increases with venous return impairment leading to a reduced stroke volume ( sv ) . simultaneously , lung squeezing pushes blood towards the left ventricle , creating a better left ventricular preload and then a better sv . as a consequence , the pulmonary blood volume decreases . ( b ) during exsufflation , the right ventricle works with a reduced afterload and a better preload , ejecting a higher sv . in addition , the lung is engorged according to pulmonary vessel compliance , storing blood and reducing preload of the left ventricle and its sv . as a consequence , the pulmonary blood volume increases . globally the amplitude of the variations between inspiration and expiration depends on the central blood volume , the pulmonary vascular capacitance , and pump function . the better the pulmonary vessel capacitance , the more the pulmonary vasculature may store blood , which will be pushed during mechanical ventilation insufflation to the left ventricle . this pulmonary blood volume is a part of the gedv , which also includes the right and left heart volumes in diastole . the gedv value comes from the product of cardiac output and the mean transit time for blood to flow from the right to the left side of the circulation ( co  mtt ) . if cardiac output can be considered adequately measured , the mean transit time might be more complex , since it is based on time determination for the geometric mean value of the intravascular indicator to flow ( time of injection to the mean time ) , with potential errors occurring when ventricles have limited functions or valve regurgitations . second , the conclusion from this study supposes that the right ventricle is pumping blood normally towards the left ventricle , which leads to a predominant role for left ventricular function on central blood volume size and on the filling pressure level . when systolic function is normal , therefore , the left ventricular volume after systole is small , suggesting that diastolic filling works on the flat part of the heart pressure / volume curve ( figure 2 ) . as shown by the study , in this condition the filling volume would better predict fluid responsiveness since the volume is not limited ( figure 2 ) and pressure variations are small . when systolic function is limited , the end - systolic volume is higher with an end - diastolic volume located on the steep part of the pressure / volume curve ( figure 2 ) . as a consequence , the filling pressure being higher , diastolic pressure better predicted the fluid responsiveness than did volume . when the change in volume / pressure ( v/p ) is normal , the increase in volume is larger than the increase in pressure when the preload is reduced during hypovolemia . conversely , when compliance is reduced during hypovolemia , the increase in volume generates a larger change in pressure than in volume . p , pressure ; v , volume . to conclude , the results from this nice study fit well with the pathophysiology when the peep is low , right ventricular function is adequate , and the patients are hypovolemic . these conditions are not typically those encountered in complex intensive care unit patients , and can not be generalized . in addition , the preload index gedv is more complex than the heart volumes in diastole , since it also contains the pulmonary blood volume , which may vary largely during mechanical ventilation and when pulmonary vascular compliance changes . gedv : global end - diastolic volume ; gef : global ejection fraction ; peep : positive end - expiratory pressure . the present work was supported in part by a grant from university paris 7 ( plan quadrennial , ea 3509 ) .
fluid resuscitation is a cornerstone of intensive care unit patient care , but prediction of the cardiovascular response remains difficult , despite many efforts in clinical research . 
 the concept of responders and nonresponders illustrates such a difficulty . 
 many techniques have been tested , from strictly non - invasive to invasive , delivering various parameters related to the fluid challenge response . 
 considering the physical parameters available , such as pressure , volume and flow generated by right and left pumps circulating in elastic or compliant tubes , it sounds useful to go back to the basic knowledge to discuss the results of the present article . 
 this published study tested in the postoperative period of cardiovascular surgery the prediction obtained with filling pressures and the diastolic volume . when left ventricular function ( global ejection fraction ) is adequate , the volume before fluid administration seems to predict well the fluid challenge response ; whereas when the global ejection fraction is poor , the filling pressure seems more suitable . 
 the present commentary discusses the main physiological issues related to these findings , with some methodological aspects .