Monitoring fluid therapy.

Hypovolaemia is a common cause of circulatory failure in the perioperative period. However, only 50% of critically ill patients respond to volume expansion with an adequate increase in cardiac output. Therefore, in daily clinical practice it is still a challenge to assess each subject's individual position on the Starling curve in order to optimize cardiac preload and avoid deleterious fluid overload. Recently, systolic pressure variation, stroke volume variation, and pulse pressure variation have been introduced as dynamic variables of fluid responsiveness which reflect ventilation-induced cyclic changes in left ventricular stroke volume. The concept of fluid responsiveness has been recognized for more than 20 years, and several studies have shown the superiority of these dynamic variables compared to static filling pressures. However, the usefulness of dynamic variables in critically ill patients is limited by several factors that the clinician needs to know for proper interpretation.