Rate of Fluid Challenge Administration and Fluid Responsiveness: an Open-label, Multicentric, Randomized Clinical Trial.

Dedicated algorithms and protocols of anaesthetic care regarding fluid therapy are key factors to prevent perioperative hypovolaemia and/or hypervolemia, which are known to increase morbidity and length of hospital stay.

Fluid therapy is commonly used in critically ill and surgical patients to restore hemodynamics. The aim of volume expansion is to increase cardiac index and oxygen delivery and to improve tissue oxygenation. However, this occurs only in a situation of preload dependency (i.e. when the ventricle operates on the steep part of the Frank-Starling's curve). Moreover, giving fluids to a non-volume-responsive patient (preload independency) can result in detrimental pulmonary and interstitial oedema. Fluid responsiveness (i.e., increase in stroke volume, SV, after fluid challenge, FC, administration) can be detected in 35-50% of both critically ill and surgical patients. The FC consists in assessing the hemodynamic effects of giving a small amount of fluid in a short period of time. The FC allows restoring fluid depletion when indicated, while minimizing the risk of overloading, which makes it the gold standard for assessing fluid depletion in patients undergoing surgery.

Then, FC administration should be based on predictors of fluid responsiveness. Static indexes, such as the central venous pressure, do not seem appropriate, whereas dynamic indexes, such as pulse pressure variation (PPV) and stroke volume variation (SVV), reliably predict the effect of FC administration during controlled mechanical ventilation only in case of a tidal volume (VT) of at least 8 mL/kg, which unfortunately are rarely found in both critically ill and surgical patients.

To overcome this VT-related limitation of PPV and SVV, the prediction of fluid responsiveness can be also achieved by applying functional hemodynamic tests aiming at increasing venous return and enhancing right ventricle preload dependence.

For example, when a FC is performed using a rapid infusion rate and a relatively "small" dose, its effect is sufficient to test whether the patient is on the ascending part of the cardiac function curve, hence showing an increase in cardiac output (CO).