Hepatic Hemodynamics and Cell Functions in Human and Experimental Sepsis

The liver plays a major role in modulating the systemic response in severe sepsis because it contains most of the macrophages of the body (Kupffer cells) able to clear the endotoxin and bacteria that may stimulate the systemic inflammatory response [1-3]. Hepatocytes synthesize the acute phase proteins and the enzymes required to modulate the inflammatory response [4-5]. Additionally, during bacterial translocation from the gut, the liver limits the access of proinflammatory substances into the systemic circulation [6]. Modifications of hepatic blood flow have been extensively studied in experimental sepsis, but these findings are difficult to extrapolate to humans. In human sepsis, hepatic dysfunction is usually mild and is defined by modifications of biochemical tests. However, there is no consensus on the most appropriate criteria to define and quantify hepatic dysfunction, and these criteria do not reflect the full spectrum of alterations in hepatic cell functions encountered in experimental sepsis. Although several articles have reviewed the numerous alterations of hepatic cell functions encountered in these experimental models, none has addressed the modifications of hepatic blood flow and hepatic function observed in humans sepsis. We review these alterations as reported in both clinical and experimental studies. Although all published clinical studies are included in the text, because of the large number of experimental studies, only the most illustrative experimental data are discussed. We hope that this text will contribute to a better understanding of the role of the liver in human sepsis. Modifications of Hepatic Hemodynamics in Sepsis Hepatic Vessels and Cells The liver has a dual blood supply. Approximately two thirds of the blood perfusing the liver is venous and is supplied by the portal vein draining the splanchnic vascular bed, which collects the blood coming from the digestive tract below the diaphragm, the spleen, and the pancreas. One third of the blood perfusing the liver is arterial and is provided through the hepatic artery. Within the liver, the portal vein and the hepatic artery branch in parallel. After a number of divisions, terminal branches of these vessels supply blood to the hepatic capillaries or sinusoids, which are organized in a dense network. In sinusoids, several types of cells have been identified: endothelial cells, Kupffer cells, and stellate cells. Endothelial cells are perforated by large fenestrae and are not surrounded by a basal lamina. Thus, the porosity of the sinusoids enable the dispersion of plasma into the space of Disse (which separates sinusoids and hepatocytes). Endothelial cells have a high endocytic activity and produce various mediators, such as thromboxane and prostaglandins. Stellate cells store intracytoplasmic fat droplets containing vitamin A and synthesize collagen and other constituents of the extracellular matrix. After acute or chronic hepatic injury, stellate cells undergo activation, a process characterized by the conversion to a myofibroblastic phenotype with de novo expression of the cytoskeletal protein smooth muscle alpha-actin. Thus, during sepsis, stellate cells undergo contractile properties and, similar to endothelial cells, participate to the modifications of hepatic blood flow. Kupffer cells are hepatic macrophages and represent 80%-90% of all resident macrophages of the body. They play a major role in the uptake and destruction of bacteria and endotoxin. After activation by endotoxin, they secrete cytokines, lipid mediators such as leukotrienesand prostaglandins, O2-derived radicals, and lysosomal enzymes. Hepatocytes, which represents 60% of hepatic cells, have numerous metabolic functions, including gluconeogenesis and glycogenolysis, protein synthesis (albumin, fibrinogen), urea synthesis, bile formation, and drug biotransformation by cytochrome P-450 enzymes. Estimation of Hepatic Blood Flow in Human Studies Hepatic blood flow in humans has been assessed in septic patients or healthy volunteers injected with endotoxin by clearance methods using indicators injected into the blood and cleared by the liver. The disappearance rate of these indicators is usually used to estimate hepatic blood flow. These methods require the insertion of a catheter into the right hepatic vein under fluoroscopic visualization. Indocyanine green (ICG) is the most commonly used indicator and is injected as a single bolus or infused over a longer period. The continuous ICG infusion method was first described by Bradley et al. [7] and is based on the Fick principle. Under conditions of constant flow, the volume of blood moving through an organ per time (Q) can be calculated by determining the amount of ICG extracted (R) over that time and the difference between ICG concentrations entering (Ci) and leaving (C0) the organ [8]. The Equation forthis relationship is Q = R/(Ci - C0). Under steady-state conditions, the IV infusion rate (I) of ICG removed exclusively by the liver equals the rate of hepatic removal (R = I). Ci is determined from blood collected from a peripheral vein or artery (because the ICG concentrations are similar), and C0 is measured from blood collected in the hepatic vein. Because ICG is distributed only in the plasma, the calculated plasma flow must be converted to blood flow by knowing the hematocrit (Hct): Q (blood flow) = Q (plasma flow) x 1/(1 - Hct). ICG is the preferred dye indicator because it typically has a high hepatic extraction ratio, no extrahepatic uptake, and low toxicity, and it is easily measured by spectrophotometry. The procedure requires that the catheter be positioned correctly in the hepatic vein, deep enough but not wedged. Otherwise, blood from the inferior vena cava can backflow during sampling; such dilution of the hepatic venous sample introduces error. During experimental endotoxemia, a back-diffusion of ICG from hepatocytes into the hepatic vein has also been described [9]. Although the hepatic extraction ratio (ER) estimated by (Ci - C0)/Ci is usually >90% in control patients [8], it is as low as 30% in septic patients [10]. Low extraction and back-diffusion narrow the difference between ICG concentrations in arterial and hepatic vein samples, leading to an overestimation of hepatic blood flow. Moreover, although extrahepatic ICG extraction is small in normal conditions, it is not known whether it is modified in sepsis. Clearance of a single bolus of ICG is also used, allowing repeated tests. Hepatic blood flow obtained by using this technique correlates well with values obtained with the continuous-injection ICG method in patients with liver disease [11]. Estimation of Hepatic Blood Flow in Human Septic Shock Several studies have attempted to determine the consequences of sepsis on hepatosplanchnic blood flow. In volunteers injected with endotoxin (20 U/kg over 5 min), hepatosplanchnic blood flow increases within 1 h, peaks by 3 h, and finally returns to baseline by 6 h [12]. In intensive care units, septic patients are usually ventilated mechanically. To prevent feeding from modifying hepatic blood flow, enteral feeding is withdrawn and septic patients are infused with various electrolyte solutions. These patients have various sources of infection, including intraperitoneal infections. Ruokonen et al. [13] found that hepatosplanchnic blood flow is higher in hyperdynamic septic patients than in patients after uncomplicated cardiac surgery. Because the cardiac index is also higher in these patients, the ratio between hepatosplanchnic blood flow and cardiac output remains constant (approximately 25%). Dahn et al. [14] compared the hepatosplanchnic blood flow in critically ill patients with or without sepsis and found similar results. These studies suggest that hepatosplanchnic blood flow increases proportionately to the cardiac index in patients with sepsis and that the fractional hepatosplanchnic blood flow remains constant. Whether the relationship between hepatosplanchnic blood flow and cardiac output remains constant throughout the evolution of the disease remains to be determined. O2 Delivery and O2 Consumption in the Hepatosplanchnic Region Hepatosplanchnic O2 delivery (Do2) and O2 consumption (Vo2) were also measured in clinical studies. However, the contribution of the liver to the whole hepatosplanchnic Vo2 is impossible to determine because no sample can be collected from the portal vein [15]. Hepatosplanchnic Vo2 is significantly higher in septic patients (1.9 +/- 0.5 mL [middle dot] min-1 [middle dot] kg-1) than in nonseptic patients (1.3 +/- 0.4 mL [middle dot] min-1 [middle dot] kg-1), whereas systemic Vo2 is similar in both groups [14]. Both hepatosplanchnic Vo2 and systemic Vo2 are significantly increased in septic patients and postoperative patients [13]. In healthy volunteers, hepatosplanchnic Vo2 is 66 +/- 5 mL/min before endotoxin administration and increases 120 min (100 +/- 13 mL/min) and 240 min (90 +/- 12 mL/min) after endotoxin administration. Hepatosplanchnic Vo2 returns to baseline by 360 min [12]. Moreover, Do2 is higher in septic shock patients treated with norepinephrine than in patients with severe sepsis who do not receive noradrenaline [16]. This finding contrasts with the common knowledge that norepinephrine infusion decreases hepatosplanchnic blood flow [15]. It is likely that the vascular hyporesponsiveness to norepinephrine observed in the mesenteric circulation during experimental sepsis is an explanation for this finding [17]. Thus, hepatosplanchnic Vo2 is increased during sepsis, but the contribution of the liver to the hepatosplanchnic Vo2 remains unknown. Hepatic Do2 and Vo2 have been measured in large animals. In anesthetized pigs continuously infused with endotoxin, hepatic Do2 and Vo2 did not change over a 24-h experimental period [18]. In a similar experimental model, hepatic Vo2 was maintained over 6 h, whereas Do2 decreased [19]. However, these experimental findings cannot be extrapolated to clinical studies. In septic patients, most of the increased hepatosplanchnic Vo2 is attributed to an increased hepatic glucose production resulting from increased substrate delivery [12,20]. Hepatic amino acid uptake [20] and other hepatic pathways, such as tumor necrosis factor-alpha (TNF-alpha) and free fatty acid production, are also increased in volunteers injected with endotoxin [12]. Relationship Between Vo2 and Do2 in the Hepatosplanchnic Region In critically ill patients, the relationship between Vo2 and Do (2) is used to assess the O2 deficiency leading to organ dysfunction [21]. Over a wide range of Do2 values, Vo2 remains constant as Do2 varies. Organs extract only as much O2 from the blood as needed to maintain cellular metabolism, and Vo2 and Do2 are independent. However, when Do2 decreases below a critical threshold value, Vo2 decreases in direct proportion to Do2, and Vo2 becomes dependent on Do2. This Vo2/Do2 dependency may be responsible for organ dysfunction. To determine whether the increased hepatosplanchnic Vo2 is limited by the regional Do2 in septic patients, both hepatosplanchnic Vo2 and Do2 were measured before and after therapeutic interventions, such as the infusion of packed red blood cells [22] or dobutamine infusion [10,23][13], to study the evolution of the hepatosplanchnic Vo2 while increasing the regional Do2. Ruokonen et al. [13] first reported an increase in hepatosplanchnic Vo2 after the administration of dopamine, dobutamine, or norepinephrine. After the infusion of 2 U of packed red blood cells, the hepatosplanchnic Vo2 moderately increased by 9%, whereas hepatosplanchnic Do2 increased by 26% [22]. These two studies demonstrate a flow-dependent O2 consumption in hepatosplanchnic organs. In contrast, when hepatosplanchnic Vo2 and Do2 were measured before and after the infusion of dobutamine, hepatosplanchnic Vo2 remained constant, whereas hepatosplanchnic Do2 increased by 29% [10]. These contradictory results may be explained by a recent study [23] showing that, in septic patients with a gradient between mixed venous and hepatic vein saturation >10%, both hepatosplanchnic Do2 and Vo2 increased during dobutamine perfusion, demonstrating Vo2/Do2 dependency. In contrast, in patients with a gradient <10%, hepatosplanchnic Vo2 remained constant and the regional Do2 increased with the dobutamine perfusion [23]. Consequently, hepatosplanchnic Do2 seems sufficient to fulfill the regional O2 demand in most septic patients, but not in those with a high gradient between mixed venous and hepatic vein saturation. Lactate Flux in the Hepatosplanchnic Region The evolution of hepatosplanchnic lactate flux has also been determined while the regional Do2 is increased. In a study by Steffes et al. [22], regional lactate uptake did not change in response to the increased hepatosplanchnic Do2. In contrast, lactate uptake increased by 38% in septic patients infused with dobutamine, and the increased lactate uptake was similar for patients who had a Vo2/Do2 dependency and for those in whom Vo2 did not increase in response to the dobutamine perfusion [23]. However, interpreting the evolution of lactate flux while hepatosplanchnic Do2 increases is difficult. During Vo2/Do2 dependency, an increase in Vo2 after Do2 increase is not always associated with an increased lactate uptake because lactate uptake is influenced not only by the amount of O2 available for the conversion of lactate to pyruvate, but also by the total amount of lactate delivered to the liver. Consequently, lactate uptake can increase when blood flow is increased, even in the absence of hypoxia. Moreover, hepatosplanchnic lactate uptake represents the net lactate flux through both the gut and the liver. Because lactate is produced by the gut and taken up by the liver, and because lactate concentrations in portal vein are not available in clinical studies, it is difficult to interpret hepatosplanchnic lactate flux in humans. In summary, sepsis increases hepatosplanchnic blood flow and Vo2. Besides an increased production of TNF-alpha and free fatty acids, hepatic glucose production is likely responsible for the increased hepatosplanchnic Vo2. The increased hepatosplanchnic blood flow, however, may not be sufficient to meet the hepatosplanchnic Vo2 in patients with a high gradient between mixed venous and hepatic vein O2 saturation. Hepatic Blood Flow in Experimental Studies In contrast to human studies, both hepatic artery and portal vein blood flows can be measured in animals. Various models of septic shock have been described, including the injection of endotoxin or bacteria (IV or intraperitoneal injection or perfusion) and cecal ligation and puncture (CLP). These experimental models of sepsis induce either a hypodynamic syndrome (defined mainly by a low cardiac output) or a hyperdynamic syndrome (associated with an increased cardiac output). The type of animals used (for example, pigs do not often develop hyperdynamic syndrome), the treatment (fluid challenge), and the model of sepsis determine whether the syndrome will be hypodynamic or hyperdynamic [24-26]. In septic models associated with hypotension and low cardiac output, hepatic perfusion is compromised by a decrease in both portal vein and hepatic artery flows [27-28]. In rabbits anesthetized with pentobarbital after endotoxin injection, portal vein blood flow decreased, whereas hepatic artery blood flow increased [29]. However, the type of shock in these experimental models from the described hyperdynamic shock observed in humans. In experimental models in which the increased cardiac output is similar to that observed in patients, both hepatic flows increased by h Moreover, the two hepatic flows also to maintain a constant blood flow to the liver. The arterial is defined as the in hepatic artery blood flow in response to the in portal flow. example, an increase in hepatic artery flow for a decrease in portal vein flow, a constant blood flow to the liver. This hepatic arterial response is in shock but is during sepsis During endotoxemia, a decrease of portal blood flow is not by an increased of hepatic artery blood flow, and the increased production observed in this model is not the for the of the hepatic arterial response The liver is also an model for the direct of endotoxin administration without the of cardiac output and other organs. 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