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What effects do surgery and anesthesia have on normal liver function?
Anesthesia and surgery affect the liver independently. The liver benefits from a dual blood supply. Most of its perfusion comes from the portal vein, which has little autoregulatory ability. The hepatic artery, however, has significant autoregulatory properties. Changes in blood flow to the liver are of major significance and will be considered in detail. Venous return from the splanchnic organs is via the portal vein. The portal vein delivers about 70% of the total hepatic blood flow, whereas the hepatic artery delivers the remaining 30% of hepatic blood flow. Although the portal vein delivers approximately 70% of the total hepatic blood flow, it only provides 50% of the delivered oxygen since it is venous blood returning from the splanchnic organs. Hepatic arterial blood flow is autoregulated; furthermore, decreases in portal flow can be partially compensated by increases in hepatic arterial blood flow. This increase in arte-rial flow is an attempt to maintain hepatic oxygen delivery.
Splanchnic circulatory disturbances are induced by surgical interventions. During laparotomy, surgical manip-ulation and placement of pads in the abdomen reduce hepatic blood flow. Upper abdominal surgery may limit hepatic blood flow by as much as 40%. Other common perioperative factors that limit hepatic perfusion are intermittent positive pressure ventilation (IPPV), őĪ-adrenergic agonists, and hypocapnia. Laparoscopy or surgery in the prone position impairs hepatic blood flow as well.
Anesthetics have multiple effects on the liver, which can be divided into two main categories: blood flow and cel-lular metabolism. Extensive animal studies have shown that volatile anesthetics can produce marked reductions in total hepatic blood flow. It appears that volatile anesthetic agents have a direct dilatory effect on splanchnic vasculature and thereby reduce portal blood flow. This reduction is further accentuated by decreased cardiac output secondary to potent inhaled anesthetic agents. For example, halothane when administered to dogs at 2 minimum alveolar concentration (MAC) produced a 53% reduction in total hepatic blood flow, while isoflurane at the same MAC produced only a 22% reduction in hepatic blood flow. Compensation by enhanced hepatic artery flow is only partial. It appears that isoflurane is more effective at preserving hepatic blood flow than halothane. In humans, volatile anesthetics produce dose-dependent reductions in systemic and portal blood pressure, as well as flow. However, there is little proof that halothane has a more detrimental effect than isoflurane on liver blood flow in humans. Spinal and epidural anesthesia will produce reductions in hepatic blood flow commensurate with the degree of sympathetic blockade and decrease in blood pres-sure. These changes can be prevented by intravenous ephedrine.
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