Fluid Responsiveness and Dynamic Tests: Physiological Background

Fluid administration is often the first therapeutic step in a patient with acute circulatory failure. Its goal is to increase cardiac preload, cardiac output and, ultimately, arterial oxygen delivery. However, the effects of volume expansion are variable. If it is of sufficient volume, it increases the mean systemic pressure, the upstream pressure of the systemic venous return. In the event of preload responsiveness of the two ventricles, it leads to an increase in systemic venous return and therefore of cardiac output. If haemodilution caused by the infusion of fluid is not too large and if the microvascular and mitochondrial functions are not excessively impaired, it may result in improved tissue oxygenation. In contrast, in the absence of preload responsiveness, fluid administration does not increase cardiac output and can only have deleterious effects. These harmful effects of the administration of fluid, which results in an increased fluid balance, have been clearly demonstrated. Fluids should therefore be regarded as drugs, the effect of which is inconstant and the deleterious side effects numerous. Basically, this justifies predicting before administering a volume expansion whether or not it will have the desired effect on cardiac output.