in this issue of critical care , thiel and colleagues   present a new method for tracking the changes in cardiac output in response to passive leg raising ( plr ) , one of the tests recently proposed to predict volume responsiveness in critically ill patients . recent review articles have emphasised the relevance of using dynamic indices such as pulse pressure variation and stroke volume variation for that purpose [ 3 - 5 ] . nevertheless , the respiratory variation of stroke volume can not be used in cases of spontaneous breathing   or low tidal volume ventilation . in such problematic cases , plr , by acting as an endogenous volume challenge , represents a helpful tool for predicting fluid responsiveness . compared with the classical fluid challenge , it has the advantage of being rapidly and totally reversible . the potential risks of fluid infusion are thus expected to be minimised , which is important to consider in critically ill patients in whom multiple challenges are often necessary . confirming previous reports , thiel and colleagues   have shown that plr is a reliable test for predicting volume responsiveness in mechanically ventilated patients , even in those with spontaneous breathing activity . for tracking the changes in cardiac output during the postural manoeuvre , they used a transcutaneous continuous - wave doppler ultrasound device able to measure blood flow across the aortic or the pulmonary valve . this totally non - invasive method is assumed to be less user - dependent than the classical doppler echocardiography . unlike the oesophageal doppler technique , this device can be applied in non - intubated patients . as predicting volume responsiveness using such a simple method is very attractive , further confirmation studies are necessary . another interesting finding of the study by thiel and colleagues   was the high rate ( 54% ) of patients who did not respond to fluid administration , confirming recent reports . this issue must be discussed in line with the recent evolution of ideas and policies in terms of fluid administration in critically ill patients . the concept of increasing cardiac output to correct an occult oxygen debt in critically ill patients was developed during the ' 90s . although it did not lead to improved outcome of intensive care unit ( icu ) patients enrolled in randomised studies , this concept promoted the idea that critically ill patients are often under - resuscitated , even in the absence of hypotension or of any sign of blood volume deficit . the study by rivers and colleagues   emphasised the importance of increasing cardiac output by using aggressive fluid administration in the early phase of severe sepsis . this concept has been well adopted by pre - hospital , emergency care and critical care physicians as witnessed by the fact that more than 50% of icu patients are volume - unresponsive in recent studies . however , volume unresponsiveness is an abnormal state since it indicates that the patient 's heart operates on the flat part of the frank - starling curve , as does a failing heart . in this condition , further fluid administration should dramatically increase cardiac filling pressures with inherent high risks of pulmonary oedema development , in particular in cases of altered pulmonary vascular permeability . in this regard , there is now increasing evidence that fluid overload negatively impacts the outcome of critically ill patients . we can schematically distinguish between two opposite situations that are frequently encountered in the icu . the first one is represented by the management of patients in the early phase of sepsis . the surviving sepsis campaign   recommends that fluid be administered until the central venous pressure ( cvp ) reaches 8 to 12 mm hg ( or more in mechanically ventilated patients ) provided that the central venous oxygen saturation is less than 70% . as the cvp can not identify volume - unresponsive patients , such an attitude could result in fluid overload of most of those patients . a test capable of reliably detecting volume unresponsiveness at any moment of fluid resuscitation would help to better assess the benefit / risk ratio of continuing such a strategy . the second situation is represented by the management of patients with lung injury after the early stage has passed . however , an uncontrolled fluid restriction attitude ( diuretics or ultrafiltration ) could result in marked volume depletion and subsequent organ hypoperfusion . a test capable of reliably detecting when the degree of volume responsiveness at any moment of fluid restriction is too high would help to assess the benefit / risk ratio of continuing such a strategy . fortunately , the tests developed to detect volume responsiveness can also serve to detect volume unresponsiveness . among different tests , plr is probably one of the most valuable since it can be used in icu patients with spontaneous breathing activity . cvp : central venous pressure ; icu : intensive care unit ; plr : passive leg raising . j - lt and xm are members of the medical advisory board of pulsion medical systems ag ( munich , germany ) .