With one of the major limitations to stroke volume and pulse pressure variation as dynamic measurements for volume status functionally requiring mechanically ventilated and sedated (often paralyzed) patients, clinicians and researchers have looked to other means to dynamically test for fluid responsiveness. One method whose popularity in the research community is currently growing is the passive leg raise where a patients legs are lifted above the level of their heart. Lifting the legs of a patient lying horizontally supine will cause a standing volume of fluid to be differentially subjected to gravitational forces than the rest of the body, generating a hydostatic pressure (in simple cases found by the product of gravity, fluid density, and fluid height). Once manifest, the gravitational transfer of blood proceeds from the low pressure environments of the raised lower limbs (from the venous reservoir) to the central circulatory compartments (in the intrathoracic cavity). The raised limb increases the mean circulatory pressure, the driving pressure for venous return, and will tend to increase right cardiac preload. This type of maneuver has been referred to as an auto-fluid challenge since it performs the function of a fluid challenge (a transient increase in preload leading to an increase in cardiac output via the Frank-Starling mechanism) with- out the potential drawbacks of introducing a fluid into the patients system. By dynamically loading the heart in this way, various hemodynamic parameters including pulmonary artery occlusion pressure and left ventricular end-diastolic area can be altered through a passive leg raise.
By itself, a passive leg raise does nothing. It must be tied to some measure of fluid responsiveness. The end measures of choice are usually cardiac output or some sort of arterial pulse pressure monitoring. Computing the differences between baseline operation and the pressure loaded leg-raised condition of these metrics can give a sense of the fluid status of the patient. Measures tied to passive leg raises have proven to reliably predict fluid responsiveness [73, 74, 75, 76, 77], with one meta-analysis suggesting the area under the receiver operating characteristics curve equal to 0.95  regardless of ventilation mode or cardiac rhythm. As such, the passive leg raise as a perturbation to test the cardiac system of patients has grown as a simple yet effective means of measuring fluid responsiveness.
But not without a few major caveats. The first and most obvious caveat is that patients must have sufficient lower limbs to perform the maneuver. In cases of severe trauma or amputation, lower limbs may not be available for lifting. To my knowledge no study exists reporting the efficacy of this technique in such populations. One must not only have sufficient lower limbs, but the patient must be able to receive rapid postural changes without harm. This excludes patients with spinal injuries and the obese.
One major caveat not generally acknowledged by the literature is the difficulty in performing such a maneuver in daily practice. To be most effective, with the patient lying supine the upper half of a patient must first be raised to a semirecumbent position while the lower half is kept horizontal. From this position, the upper half of the patient is lowered to the horizontal and the lower limbs are elevated. The upper half of the patient must be raised before the legs in order for the passive leg raise maneuver to include blood volume from the abdominal compartment to ensure a significant elevation in cardiac preload. Starting the procedure with a patient already in the horizontal position does not effectively load the heart , and reduces the effectiveness of the technique. In many cases, therefore, the procedure would begin with a patient resting horizontally, the upper half of the patient would be raised, the patient would sit for at least a minute, then the upper half would be lowered and the lower half raised, and from this the volume responsiveness measure of choice could be measured (but only for about a minute ).
Automatic beds and wedge pillows may help this process, but more often than not a caregiver of some sort would be required to lift the legs of the patient themselves, possibly with additional assistance if the patient is particularly large (a growing concern among many). Here inconsistencies can rise such as in the pre-procedure of first sitting the patient up before laying them down or ensuring that the legs are lifted to an appropriate height or angle.
As a way to dynamically test fluid responsiveness non-invasively, the passive leg raise is an effective technique. Its popularity among researchers and a subset of clinicians has risen with good reason. However, the practical limitations of the clinic stand in the path of its full adoption.