Can passive leg raising be used to guide fluid administration?

In many instances, hemodynamic optimization requires the use of fluids. However, the response to fluids may be quite variable and cannot be adequately predicted from the measurements of intravascular pressures (central venous pressure or pulmonary artery pressure) [1] or volumes. Indeed, the relationship between stroke volume and preload varies considerably between the patients. Accordingly, extreme values only can predict fluid responsiveness. Dynamic indices reflecting respiratory-induced variations in stroke volume have been developed [2], but these cannot be used in patients with cardiac arrhythmias or in patients with spontaneous respiratory movements [3] or ventilated with a low tidal volume [4]. Recently, the so-called passive leg raising (PLR) test has been proposed. This test is based on the principle that PLR induces an abrupt increase in preload due to autotransfusion of blood contained in capacitance veins of the legs. This abrupt increase in preload leads to an increase in cardiac output in preload-dependent patients but not in other patients. However, the test requires the determination of cardiac output with a fast-response device, because the hemodynamic changes may be transient. In a previous issue of Critical Care, Lafanechère and colleagues [1] used esophageal Doppler to monitor cardiac output and reported that a PLR-induced increase in cardiac output higher than 8% can predict fluid responsiveness in critically ill patients. The predictive value of the PLR test was similar to that of respiratory-induced variations in pulse pressure. Although this study basically confirms the results of Monnet and colleagues [5], it brings some new pieces of information to the field, but also raises important questions.Indeed, the 22 patients investigated by Lafanechère and colleagues [1] were all in acute circulatory failure and treated with high doses of epinephrine or norepinephrine. However, the use of vasopressor agents may be of paramount importance in deter