The role of the vasculature in regulating venous return and cardiac output: historical and graphical approach.

PURPOSE To review the physiology of cardiac output regulation by the peripheral vasculature. This will enable the clinician to understand and manage the complex circulatory changes in various forms of shock, and in other common altered circulatory states encountered in anaesthetic practice. SOURCE Articles were obtained from a Medline review (1966 to present; search terms: shock, venous return, cardiac output) and a hand search (Index Medicus). Other sources include review articles, personal files, and textbooks. PRINCIPAL FINDINGS At steady state, cardiac output is equal to venous return (VR). Venous return depends on mean systemic pressure (PMS), which is the pressure in the peripheral vasculature driving blood flow to the heart, right atrial pressure (PRA), and the resistance to venous return (RV). When considering VR, PRA is the downstream pressure to VR, and not simply an indirect measure of the volume status. The pressure gradient for VR is, therefore, PMS-PRA, and in a system obeying Ohm's Law, [formula: see text] Shock and other altered circulatory states cause changes in both VR and cardiac function. The circulation can be conveniently described by a venous return and a cardiac output curve. By drawing these curves for each clinical situation, a clear understanding of the altered circulatory state is obtained, and treatment options can be clearly defined. CONCLUSION The peripheral circulation controls cardiac output in many clinical conditions. Manipulation of the peripheral circulation is as important to the successful treatment of shock and other altered circulatory states, as is the manipulation of cardiac output.