The aim of circulatory management is to allow the heart to recover from the trauma of surgery while providing adequate oxygen delivery to the rest of the body. The circulation is manipulated to minimize myocardial oxygen demand and maximize myocardial oxygen supply while ensuring sufficient cardiac output and systemic blood pressure to provide adequate tissue perfusion.
Although increased heart rate can increase cardiac output up to a point, rapid rates restrict filling and reduce output while reducing diastolic perfusion of the myocardium and thus its oxygen supply. Optimizing preload is the most efficient method of improving cardiac output, at least as far as oxygen demand is concerned. Once this has been achieved, attention can be directed at increasing contractility and reducing afterload.
Oxygen delivery may be increased by blood transfusion, but high blood viscosity reduces tissue oxygen delivery. The optimal hematocrit in humans is unknown and controversial. Regional differences in perfusion caused by vasoconstriction, which may be fixed (atherosclerosis) or functional (vasospasm), may also reduce myocardial and tissue oxygen delivery. Measures to reduce oxygen consumption also reduce the required oxygen delivery. These include analgesia, sedation, and occasionally paralysis to minimize muscular activity, particularly shivering.
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