Fluid therapy is an important part of the management of patients with hemodynamic instability in the intensive care unit. By increasing cardiac preload, it aims at elevating cardiac output (CO) and thus restoring hemodynamic conditions in patients who are preload responsive. By contrast, volume administration can be deleterious in terms of pulmonary edema formation or other manifestations or fluid overload, especially in patients who are not preload responsive. Functional dynamic parameters that use heart-lung interactions, such as pulse pressure variation (PPV) and stroke volume variation (SVV) are considered accurate predictors of preload responsiveness in patients receiving fully controlled mechanical ventilation. However, in cases of spontaneous breathing activity where heart-lung interaction indices fail to predict fluid responsiveness, one needs parameters able to reliably predict the hemodynamic response of fluid administration.

A new index that could indicate fluid responsiveness, so-called the fluid responsiveness index (FRI), has been elaborated. The advantage is that it could be used in patients who are not in control mechanical ventilation as well as in patients who are fully adapted to mechanical ventilation. The FRI is based upon the analysis of continuous arterial and continuous central venous pressure. The FRI is determined by the relation of cardiac and respiratory activity; both are evaluated by means of power spectrum analysis of the pressures recorded. The aim of this study is to test the value of the FRI to predict the hemodynamic response to fluid infusion in patients with hemodynamic instability not receiving fully controlled mechanical ventilation.