Passive leg raising: five rules, not a drop of fluid!

In acute circulatory failure, passive leg raising (PLR) is a test that predicts whether cardiac output will increase with volume expansion [1]. By transferring a volume of around 300 mL of venous blood [2] from the lower body toward the right heart, PLR mimics a fluid challenge. However, no fluid is infused and the hemodynamic effects are rapidly reversible [1,3], thereby avoiding the risks of fluid overload. This test has the advantage of remaining reliable in conditions in which indices of fluid responsiveness that are based on the respiratory variations of stroke volume cannot be used [1], like spontaneous breathing, arrhythmias, low tidal volume ventilation, and low lung compliance. The method for performing PLR is of the utmost importance because it fundamentally affects its hemodynamic effects and reliability. In practice, five rules should be followed. First, PLR should start from the semi-recumbent and not the supine position (Figure 1). Adding trunk lowering to leg raising should mobilize venous blood from the large splanchnic compartment, thus magnifying the increasing effects of leg elevation on cardiac preload [2] and increasing the test’s sensitivity. A study that did not comply with this rule misleadingly reported a poor reliability of PLR [4]. Second, the PLR effects must be assessed by a direct measurement of cardiac output and not by the simple measurement of blood pressure. Indeed, reliability of PLR is poorer when assessed by using arterial pulse pressure compared with cardiac output [1,5]. Although the peripheral arterial pulse pressure is positively correlated with stroke volume, it also depends on arterial compliance and pulse wave amplification. The latter phenomenon could be altered during PLR, impeding the use of pulse pressure as a surrogate of stroke volume to assess PLR effects.