The physiologic basis for goal-directed hemodynamic and fluid therapy: the pivotal role of the venous circulation.

PURPOSE: Understanding cardiovascular physiology should help clinicians to understand the purpose of fluid and drug management during the perioperative period. The purpose of this narrative review is to describe the pivotal role of the venous circulation in goal-directed hemodynamic and fluid therapy.

SOURCE: We selected relevant literature that examines the appropriateness of fluid therapy and pharmacologic interventions during the perioperative period.

PRINCIPAL FINDINGS: The interaction between the stressed and unstressed intravascular volume (Vs and Vu, respectively) regulates the venous return, which is the main determinant of cardiac output. The lack of hemodynamic response to an intravascular fluid challenge likely results from an unpredictable distribution of infused fluid between the Vs and Vu. Other factors affecting hemodynamic responses include the pharmacodynamics of common vasoactive drugs, which further highlight the complexity of the regulation of venous return during infusion of exogenous catecholamines. The response to even a highly selective agent can result in different hemodynamic effects. Low doses of α-adrenergic agonists constrict veins and may often shift blood from the Vu to the Vs, subsequently increasing the venous return and cardiac output, whereas higher drug doses constrict arteries and usually decrease cardiac output.

CONCLUSIONS: The physiologic basis of goal-directed hemodynamic therapy is complex and not necessarily reflected in the information received from hemodynamic monitors. Understanding the physiologic basis of such therapy is a logical step towards its optimal use.