Fig. 2

Relationship between preload, cardiac output, and central venous pressure (CVP). Relationship between cardiac output and preload (left panel) and between CVP and blood volume (right panel) in a fluid responder (a) and a nonresponder (b). In the fluid responder, the administration of fluids increases blood volume and cardiac preload; the increase in preload is associated with a large increase in cardiac output and a minimal increase in CVP. In the fluid nonresponder, the same increase in blood volume and preload is associated with no change in cardiac output and major changes in CVP. Accordingly, an increase in CVP cannot be used to suggest a positive response to fluids. Volume measurements better evaluate changes in preload in preload-responsive patients while pressure measurements better evaluate changes in preload in preload-nonresponsive patients