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Fluid Challenge

The most intuitive way to test fluid responsiveness is to administer a fluid challenge and observe its effects on cardiac output. The disadvantage of the “common” fluid challenge is that it consists of administering 250-500 mL of fluid. This volume of fluid may seem negligible at first; however, performing fluid challenges repeatedly, as can occur in the early phase of shock, may contribute to significant fluid overload. A “mini fluid challenge” has been described as an alternative [61, 62]. Nevertheless, small amounts of fluid can only induce small changes in stroke volume and cardiac output. Thus, this test requires a very precise technique for measuring cardiac output. Whether the “mini fluid challenge” is effective for clinical decision-making requires further investigation.

The Passive Leg-Raising Maneuver

An alternative to the fluid bolus for assessing fluid responsiveness is the passive legraising (PLR) maneuver. The PLR maneuver is performed by lifting the patient’s legs passively from the horizontal position, and it is associated with the gravitational transfer of blood from the lower extremities to the intrathoracic compartment [63]. Beyond its ease of use, this method has the advantage of reversing its effects once the legs are returned to the horizontal position. Therefore, the PLR maneuver may be considered a reversible “self-volume challenge” [64]. Since the maximal hemodynamic effects of PLR occur within the first minute of leg elevation, it is important to assess these effects with a method able to track real-time changes in SV. The change in blood pressure following a PLR or fluid challenge is a poor guide to fluid responsiveness; SV may increase without a significant change in blood pressure. The reliability of PLR as a test of preload responsiveness has been confirmed in multiple studies performed in critically ill patients [65].

EVLW as a Therapeutic Target

EVLW is a sensitive marker of the development of pulmonary edema correlating with markers of lung injury, including the PaO2/FiO2 oxygenation ratio, lung compliance, chest radiograph, and lung injury score. Patients with ARDS may therefore benefit from management guided by EVLW. An early diagnosis of the pathological accumulation of EVLW during resuscitation may allow for earlier interventions and changes in the therapeutic plan [12]. In addition, monitoring of EVLW may identify the point when de-resuscitation should be started, namely, the institution of an aggressive negative fluid balance once the hemodynamic status stabilizes. While this approach seems promising, further clinical studies are required to demonstrate the benefit of such a strategy.

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