According to Guyton's model of venous return, the fluids that effectively increase cardiac output are those that, once administered, increase the vascular stressed volume, thereby increasing the mean systemic filling pressure (Pms) without increasing the central venous pressure (CVP). In this way, since the gradient between Pms and CVP increases, venous return-and consequently cardiac output-also increases.
In cases where physiologically ineffective fluids are administered, the situation arises in which, in addition to increasing the stressed volume and thus Pms, CVP also increases. As a result, the gradient between Pms and CVP remains unchanged, and cardiac output does not increase.
The hypothesis is that only a portion of the fluids administered during volume expansion are actually effective in increasing the gradient between Pms and CVP.
Critically ill patients admitted to the intensive care unit
Invasive or minimally invasive hemodynamic monitoring
Clinical indication for volume expansion with 500 mL of crystalloids
Suspected or confirmed pregnancy
Severe aortic stenosis or regurgitation
Severe mitral stenosis or regurgitation
History of peripheral arterial disease