The hepatic haemodynamic response to acute portal venous blood flow reductions in the dog

Blood flow of the venous system during resuscitative endovascular balloon occlusion of the aorta: Noninvasive evaluation using phase contrast magnetic resonance imaging

BACKGROUND

Resuscitative endovascular balloon occlusion of the aorta (REBOA) is a viable resuscitation approach for a subdiaphragmatic injury that can regulate arterial blood flow. On the other hand, the evaluation of venous or portal venous blood flow during REBOA remains insufficient because invasive cannulation or exposure of the vessel may affect the blood flow, and Doppler echography is highly operator-dependent. However, phase contrast magnetic resonance imaging has enabled accurate evaluation and noninvasive measurement. This study aimed to investigate the change of venous and portal venous blood flow during REBOA in a porcine model.

METHODS

Seven pigs were anesthetized, and a REBOA catheter was placed. The blood flows of the inferior vena cava (IVC), hepatic vein (HV), portal vein (PV), and superior vena cava (SVC) were measured using phase contrast magnetic resonance imaging, in both the balloon deflated (no-REBOA) and fully balloon inflated (REBOA) states. Mean arterial pressure (MAP), central venous pressure, cardiac index, and systemic vascular resistance index were measured.

RESULTS

The blood flows of the suprahepatic, infrahepatic, and distal IVC, HV, and PV in the no-REBOA state were 1.40 ± 0.36 L·min, 0.94 ± 0.16 L·min, 0.50 ± 0.19 L·min, 0.060 ± 0.018 L·min, and 0.32 ± 0.091 L·min, respectively. The blood flow of each section in the REBOA condition was significantly decreased at 0.41 ± 0.078 (33% of baseline), 0.15 ± 0.13 (15%), 0.043 ± 0.034 (9%), 0.029 ± 0.017 (37%), and 0.070 ± 0.034 L·min (21%), respectively. The blood flow of the SVC increased significantly in the REBOA condition (1.4 ± 0.63 L·min vs. 0.53 ± 0.14 L·min [257%]). Mean arterial pressure, central venous pressure, cardiac index, and systemic vascular resistance index were significantly increased after REBOA inflation.

CONCLUSION

Resuscitative endovascular balloon occlusion of the aorta decreased blood flows of the IVC, HV, and PV and increased blood flow of the SVC. This result could be explained by the collateral flow from the lower body to the SVC. A better understanding of the effect of REBOA on the venous and portal venous systems may help control liver injury.

Ultrasonographic evaluation of abdominal organs after cardiac surgery

BACKGROUND

Disturbances of the hepatosplanchnic region may occur after cardiac operations. Experimental studies have implicated impairment of splanchnic blood supply in major abdominal organ dysfunction after cardiopulmonary bypass (CPB). We investigated the impact of the cardiac operation and CPB on liver, kidney, and renal perfusion and function by means of ultrasonography and biochemical indices in a selected group of cardiac surgery patients.

MATERIALS AND METHODS

Seventy five patients scheduled for a major cardiac operation were prospectively included in the study. Criteria for selection were moderate or good left ventricular ejection fraction and absence of previous hepatic or renal impairment. Ultrasound examination of the hepatic and renal vasculature and examination of biochemical parameters were performed on the day preceding the operation (T0), on the first postoperative day (T1), and on the seventh postoperative day (T2).

RESULTS

Portal vein velocity and flow volume increased significantly, whereas hepatic artery velocity and flow volume decreased at T1 in comparison with T0. Hepatic vein indices remained unaffected throughout the observation period. Renal artery velocity and flow decreased, whereas renal pulsatility index and renal resistive index increased at T1 as compared with T0. Aspartate aminotransferase and alanine aminotransferase values were increased as compared with baseline values 24 h postoperatively. All parameters displayed a trend to approach preoperative levels at T2. Strong negative correlations between alanine aminotransferase values at T1 and hepatic artery velocity and flow volume at the same time point were also demonstrated (R = 0.638, P < 0.001 and r = 0.662, P < 0.001, respectively).

CONCLUSIONS

The increase in portal vein flow and velocity and the decrease in hepatic artery flow and velocity in the period after CPB might be attributed to the hypothermic bypass technique and the hepatic arterial buffer response, respectively. The decrease in renal blood flow and velocity and the parallel increase in Doppler renal pulsatility index and renal resistive index could be considered as markers of kidney hypoperfusion and intrarenal vasoconstriction. Maintaining a high index of suspicion for the early diagnosis of noncardiac complications in the period after CPB and institution of supportive care in case of compromised splanchnic perfusion are warranted.

A Hemodynamic Study to Evaluate the Buffer Response in Cirrhotic Patients Undergoing Liver Transplantation

The physiological regulation of the liver blood flow is a result of a reciprocal portal vein and hepatic artery flow relationship. This mechanism is defined as the hepatic arterial buffer response (HABR). This study was addressed to investigate whether HABR is maintained in denervated grafts in liver transplant recipients. Portal blood flow (PBF) and hepatic arterial resistance index (PI) were measured 6 months after transplantation using Doppler. In each patient we consecutively measured the vasodilator (Ensure Plus PO versus placebo) and vasoconstrictor (isosorbide dinitrate 5 mg SL versus placebo) stimuli. The meal ingestion caused a significant increase of both parameters, PBF (from 1495±260 to 2069±250 mL/min, P<0.05) and PI (from 0.7±0.2 to 0.8±0.2, P<0.05). By contrast, isosorbide dinitrate reduced PBF (from 1660±270 to 1397±250 mL/min, P<0.05) and PI (from 0.7±0.2 to 0.5±0.2, P<0.05). We show that PBF and PI are reciprocally modified with the administration of vasoconstrictor and vasodilator stimuli. These results suggest the persistence of the HABR in a denervated human model, suggesting that this mechanism is independent of the regulation from the autonomic nervous system.

Morphological and biomechanical remodelling of the hepatic artery in a swine model of portal hypertension

OBJECTIVES

To obtain the biomechanical and morphological remodelling of hepatic arteries in swine with portal hypertension.

METHODS

A number of 20 white pigs was used, of which 14 were subjected to liver cirrhosis and portal hypertension (PHT) induced by carbon tetrachloride and pentobarbital; the rest were used as the control group. The biomechanical remodelling of the hepatic arteries was measured, namely, the incremental elastic modulus (E inc), pressure-strain elastic modulus (E p), volume elastic modulus (E v), the incremental compliance (C), the opening angle and the stained microstructural components of the vessels.

RESULTS

The percentages for the microstructural components and the histologic data significantly changed in the experimental group, three incremental elastic moduli (E inc, E p, and E v) of the experimental group were significantly larger than those of the control group (P < 0.05); the compliance of hepatic arteries decreased greatly (P < 0.05) too. The opening angle (OA) was considerably larger than that of control group (P < 0.05).

CONCLUSIONS

The study suggests that the morphological and biomechanical properties of swine hepatic arteries have changed significantly during the process of portal hypertension and that from biomechanical aspects, the hepatic arteries have also suffered from extensive remodelling, which in turn deteriorates the existing portal hypertension.

Fluid replacement with hypertonic or isotonic solutions guided by mixed venous oxygen saturation in experimental hypodynamic sepsis

BACKGROUND

Splanchnic perfusion is prone to early injury and persists despite normalization of global hemodynamic variables in sepsis. Volume replacement guided by oxygen derived variables has been recommended in the management of septic patients. Our hypothesis was that a hypertonic isoncotic solution would improve the benefits of crystalloids replacement guided by mixed venous oxygen saturation.

METHODS

Seventeen anesthetized and mechanically ventilated mongrel dogs received an intravenous infusion of live E. coli in 30 minutes. They were then randomized into three groups: control group (n = 3) bacterial infusion without treatment; normal saline (n = 7), initial fluid replacement with 32 mL/kg of normal saline during 20 minutes; hypertonic solution (n = 7), initial fluid replacement with 4 mL/kg of hypertonic solution during 5 minutes. After 30 and 60 minutes, additional boluses of normal saline were administered when mixed venous oxygen saturation remained below 70%. Mean arterial pressure, cardiac output; regional blood flows, systemic and regional oxygen-derived variables, and lactate levels were assessed. Animals were observed for 90 minutes and then killed. Hystopathological analysis including apoptosis detection using terminal deoxynucleotidil transferase mediated dUTP-biotin nick end labeling was performed.

RESULTS

A hypodynamic septic shock was observed after bacterial infusion. Both the fluid-treated groups presented similar transient benefits in systemic and regional variables. A greater degree of gut epithelial cells apoptosis was observed in normal saline-treated animals.

CONCLUSIONS

Although normalization of mixed venous oxygen saturation was not associated with restoration of markers of splanchnic or other systemic perfusion variables, the initial fluid savings with hypertonic saline and its latter effect on gut apoptosis may be of interest in sepsis management.

H2S contributes to the hepatic arterial buffer response and mediates vasorelaxation of the hepatic artery via activation of K(ATP) channels

Hepatic blood supply is uniquely regulated by the hepatic arterial buffer response (HABR), counteracting alterations of portal venous blood flow by flow changes of the hepatic artery. Hydrogen sulfide (H(2)S) has been recognized as a novel signaling molecule with vasoactive properties. However, the contribution of H(2)S in mediating the HABR is not yet studied. In pentobarbital-anesthetized and laparotomized rats, flow probes around the portal vein and hepatic artery allowed for assessment of the portal venous (PVBF) and hepatic arterial blood flow (HABF) under baseline conditions and stepwise reduction of PVBF for induction of HABR. Animals received either the H(2)S donor Na(2)S, DL-propargylglycine as inhibitor of the H(2)S synthesizing enzyme cystathionine-gamma-lyase (CSE), or saline alone. Additionally, animals were treated with Na(2)S and the ATP-sensitive potassium channel (K(ATP)) inhibitor glibenclamide or with glibenclamide alone. Na(2)S markedly increased the buffer capacity to 27.4 +/- 3.0% (P < 0.05 vs. controls: 15.5 +/- 1.7%), whereas blockade of H(2)S formation by DL-propargylglycine significantly reduced the buffer capacity (8.5 +/- 1.4%). Glibenclamide completely reversed the H(2)S-induced increase of buffer capacity to the control level. By means of RT-PCR, Western blot analysis, and immunohistochemistry, we observed the expression of both H(2)S synthesizing enzymes (CSE and cystathionine-beta-synthase) in aorta, vena cava, hepatic artery, and portal vein, as well as in hepatic parenchymal tissue. Terminal branches of the hepatic afferent vessels expressed only CSE. We show for the first time that CSE-derived H(2)S contributes to HABR and partly mediates vasorelaxation of the hepatic artery via activation of K(ATP) channels.

Tonometry revisited: perfusion-related, metabolic, and respiratory components of gastric mucosal acidosis in acute cardiorespiratory failure

Mucosal pH (pHi) is influenced by local perfusion and metabolism (mucosal-arterial pCO2 gradient, DeltapCO2), systemic metabolic acidosis (arterial bicarbonate), and respiration (arterial pCO2). We determined these components of pHi and their relation to outcome during the first 24 h of intensive care. We studied 103 patients with acute respiratory or circulatory failure (age, 63+/-2 [mean+/-SEM]; Acute Physiology and Chronic Health Evaluation II score, 20+/-1; Sequential Organ Failure Assessment score, 8+/-0). pHi, and the effects of bicarbonate and arterial and mucosal pCO2 on pHi, were assessed at admission, 6, and 24 h. pHi was reduced (at admission, 7.27+/-0.01) due to low arterial bicarbonate and increased DeltapCO2. Low pHi (<7.32) at admission (n=58; mortality, 29% vs. 13% in those with pHi>or=7.32 at admission; P=0.061) was associated with an increased DeltapCO2 in 59% of patients (mortality, 47% vs. 4% for patients with low pHi and normal DeltapCO2; P=0.0003). An increased versus normal DeltapCO2, regardless of pHi, was associated with increased mortality at admission (51% vs. 5%; P<0.0001; n=39) and at 6 h (34% vs. 13%; P=0.016; n=45). A delayed normalization or persistently low pHi (n=47) or high DeltapCO2 (n=25) was associated with high mortality (low pHi [34%] vs. high DeltapCO2 [60%]; P=0.046). In nonsurvivors, hypocapnia increased pHi at baseline, 6, and 24 h (all P<or=0.001). In patients with initially normal pHi or DeltapCO2, outcome was not related to subsequent changes in pHi or DeltapCO2. Increased DeltapCO2 during early resuscitation suggests poor tissue perfusion and is associated with high mortality. Arterial bicarbonate contributes more to pHi than the DeltapCO2 but is not associated with mortality. Hyperventilation partly masks mucosal acidosis. Inadequate tissue perfusion may persist despite stable hemodynamics and contributes to poor outcome.

Spontaneous breathing during airway pressure release ventilation in experimental lung injury: effects on hepatic blood flow

OBJECTIVE

Positive pressure ventilation can affect systemic haemodynamics and regional blood flow distribution with negative effects on hepatic blood flow. We hypothesized that spontaneous breathing (SB) with airway pressure release ventilation (APRV) provides better systemic and hepatic blood flow than APRV without SB.

DESIGN

Animal study with a randomized cross-over design.

SETTING

Animal laboratory of Bonn University Hospital.

SUBJECTS

Twelve pigs with oleic-acid-induced lung injury.

INTERVENTIONS

APRV with or without SB in random order. Without SB, either the upper airway pressure limit or the ventilator rate was increased to maintain constant pH and PaCO2.

MEASUREMENTS AND RESULTS

Systemic haemodynamics were determined by double-indicator dilution, organ blood flow by coloured microspheres. Systemic blood flow was best during APRV with SB. During APRV with SB blood flow (ml g(-1) min(-1)) was 0.91+/-0.26 (hepatic arterial), 0.29+/-0.05 (stomach), 0.64+/-0.08 (duodenum), 0.62+/-0.10 (jejunum), 0.53+/-0.07 (ileum), 0.53+/-0.07 (colon), 0.46+/-0.09 (pancreas) and 3.59+/-0.55 (spleen). During APRV without SB applying high P(aw) it decreased to 0.13+/-0.01 (stomach), 0.37+/-0.03 (duodenum), 0.29+/-0.03 (jejunum), 0.31+/-0.05 (ileum), 0.32+/-0.03 (colon) and 0.23+/-0.04 (pancreas) p<0.01, respectively. During APRV without SB applying same Paw limits it decreased to 0.18+/-0.03 (stomach, p<0.01), 0.47+/-0.06 (duodenum, p<0.05), 0.38+/-0.05 (jejunum, p<0.01), 0.36+/-0.03 (ileum, p<0.05), 0.39+/-0.05 (colon, p<0.05), and 0.27+/-0.04 (pancreas, p<0.01). Arterial liver blood flow did not change significantly when SB was abolished (0.55+/-0.11 and 0.63+/-0.11, respectively).

CONCLUSIONS

Maintaining SB during APRV was associated with better systemic and pre-portal organ blood flow. Improvement in hepatic arterial blood flow was not significant.

Dynamic computed tomographic quantitation of hepatic perfusion in dogs with and without portal vascular anomalies

OBJECTIVE

To compare hepatic, pancreatic, and gastric perfusion on dynamic computed tomography (CT) scans of clinically normal dogs with those of dogs with portal vascular anomalies.

SAMPLE POPULATION

Dynamic computed tomography (CT) scans of 10 clinically normal dogs and 21 dogs with portal vascular anomalies.

PROCEDURES

Retrospective analysis of dynamic CT scans. Hepatic arterial perfusion, hepatic portal perfusion, total hepatic perfusion, hepatic perfusion index, gastric perfusion, and pancreatic perfusion were calculated from time attenuation curves.

RESULTS

Mean +/- hepatic arterial perfusion was significantly higher in affected dogs (0.57 +/- 0.27 mL/min x mL(-1)) than in clinically normal dogs (0.23 +/- 0.11 mL/min x mL(-1)), and hepatic portal perfusion was significantly lower in affected dogs (0.52 +/- 0.47 mL/min x mL(-1)) than in clinically normal dogs (1.08 +/- 0.45 mL/min x mL(-1)). This was reflected in the hepatic perfusion index, which was significantly higher in affected dogs (0.59 +/- 0.34), compared with clinically normal dogs (0.19 +/- 0.07). Gastric perfusion was significantly higher in dogs with portal vascular anomalies (0.72 +/- 0.44 mL/min x mL(-1)) than in clinically normal dogs (0.41 +/- 0.21 mL/min x mL(-1)), but total hepatic perfusion and pancreatic perfusion were not significantly different. Among subgroups, dogs with congenital intrahepatic portosystemic shunts and dogs with arterioportal fistulae had higher hepatic arterial perfusion than did clinically normal dogs. Dogs with congenital intrahepatic portosystemic shunts also had an increase in gastric perfusion and hepatic perfusion index.

CONCLUSIONS AND CLINICAL RELEVANCE

Hepatic perfusion variables measured on CT scans revealed differences in hemodynamics between clinically normal dogs and those with portal vascular anomalies.

Liver Regeneration and Hemodynamics in Pigs With Mesocaval Shunt

BACKGROUND

A surgical technique using a mesocaval shunt and downstream ligation of the superior mesenteric vein has been recently proposed to overcome the size limitations that restrict the use of partial liver grafts. We designed an experimental study in pigs to evaluate the capacities of liver regeneration and hemodynamic changes after completion of this procedure.

MATERIAL AND METHODS

Liver regeneration after left hepatectomy was compared between two groups of five pigs, with or without mesocaval shunt, sacrificed 11 to 14 days after surgery. A third group of five animals was used for hemodynamic studies.

RESULTS

Liver regeneration in study animals was 45.3% of controls. This was obtained despite a reduction of the venous inflow to 15.6% of the control, resulting in a net decrease of the total blood inflow to 56% of the control, despite a compensatory increase in the arterial inflow. There was no significant difference in mitotic index, hepatocellular size, and glycogen content between study and control animals.

CONCLUSION

Our experimental study confirms that the regenerative capacities of the pig liver are largely preserved despite the dramatic reduction of the venous blood inflow, reduced to its gastroduodenosplenopancreatic component. This lends further support to the hypothesis that the gastroduodenosplenopancreatic blood is enriched in hepatotrophic factors, likely to originate from the pancreas and duodenum.

Prediction of HELLP syndrome with assessment of maternal dual hepatic blood supply by using Doppler ultrasound

OBJECTIVE

Early structural and functional changes in the systemic vasculature have been proposed to play a major pathogenetic role in HELLP (hemolysis, elevated liver enzymes, and low platelet count) syndrome. Our objective was to assess whether the evaluation of maternal hepatic blood supply is instructive to the prediction of onset of HELLP syndrome.

DESIGN

Prospective observation study.

POPULATION

Fifty-eight women with severe preeclampsia and 60 healthy pregnant controls at 25-36 weeks gestation.

METHODS

Angle-corrected time-averaged flow velocity and the cross-sectional area of common hepatic artery and portal vein were measured by using Doppler ultrasonography in 58 women with severe preeclampsia and in 60 healthy pregnant controls at 25-36 weeks gestation. Intravascular flow volumes were calculated from the product of the time-averaged velocity and the cross-sectional area. The total liver blood flow was taken as the sum of flow volumes in the hepatic artery and portal vein.

RESULTS

The total liver blood flow decreased significantly to about 40% of control in 9 women with severe preeclampsia who developed HELLP syndrome within 4 days after the examination, but not in 49 women with severe preeclampsia without HELLP syndrome.

CONCLUSION

The results indicated that the decrease in dual hepatic blood supply preceded the onset of HELLP syndrome.

Hepatosplanchnic blood flow control and oxygen extraction are modified by the underlying mechanism of impaired perfusion

OBJECTIVE

To assess the effects of low hepatosplanchnic blood flow on regional blood flow control and oxygenation.

DESIGN

Three randomized, controlled animal experiments.

SETTING

Two university experimental research laboratories.

SUBJECTS

Pigs of either gender.

INTERVENTIONS

Isolated abdominal blood flow reduction: An extracorporeal shunt with reservoir and roller pump was inserted between proximal and distal aorta in 11 pigs. Abdominal aortic blood flow was reduced by 50% by activating the shunt. Mesenteric ischemia: In seven pigs, superior mesenteric arterial flow was reduced to 4 mL.kg.min for 4 hrs. Cardiac tamponade: In 12 pigs, aortic blood flow was reduced by cardiac tamponade to 50 mL (moderate tamponade) and further to 30 mL.kg.min (severe tamponade) for 1 hr each. In each experimental condition, the same number of control animals was used.

MEASUREMENTS AND MAIN RESULTS

Abdominal blood flow reduction, acute mesenteric ischemia, and moderate tamponade resulted in a portal venous flow (QPV) reduction to 51 +/- 23%, 52 +/- 18%, and 61 +/- 25% (mean +/- sd) of baseline flow, respectively. During abdominal blood flow reduction, QPV and hepatic arterial flow (QHA) decreased proportionally, whereas in moderate tamponade and acute mesenteric ischemia QPV reduction was associated with an increase in QHA of 30 +/- 39% and 102 +/- 108%, respectively (p = .001 and .018). Prolonged mesenteric ischemia restored total hepatic blood flow (Qliver) completely. During all conditions, decreasing mesenteric oxygen consumption was partly prevented by increased mesenteric oxygen extraction (p < .001 for all conditions). In contrast, decreasing hepatic oxygen delivery was associated with increased oxygen extraction in tamponade (p = .009) but not in abdominal blood flow reduction.

CONCLUSIONS

Blood flow redistribution can restore Qliver totally when mesenteric blood flow is reduced selectively, partially when cardiac output is reduced, and not at all during abdominal blood flow reduction. Since hepatic oxygen extraction does not increase in abdominal blood flow reduction, hepatic oxygenation is at risk in this condition.

HELICAL COMPUTED TOMOGRAPHIC ANGIOGRAPHY OF CANINE PORTOSYSTEMIC SHUNTS

Helical computed tomographic (CT) angiography was performed in 16 dogs with known or suspected portosystemic shunts. Fifteen portosystemic shunts were detected including five single intrahepatic shunts, five single extrahepatic shunts, and five multiple extrahepatic shunts. One dog had a normal CT examination. All diagnoses were confirmed by one or several alternate methods including ultrasound, surgery, necropsy, angiography, and liver biopsy. CT detected the origin of 13 of 15 portosystemic shunts and insertion of 13 of 15 shunts. Limitations included inability to resolve two vessels originating very close to each other, and identification of vessels that traveled parallel to the axial image plane. CT angiography is a promising, minimally invasive method of diagnosing a variety of portosystemic shunts in dogs.

The role of ATP and adenosine in the control of hepatic blood flow in the rabbit liver in vivo

BACKGROUND: The role of adenosine and ATP in the regulation of hepatic arterial blood flow in the "buffer response" was studied in vitro and in a new in vivo model in the rabbit. The model achieves portal-systemic diversion by insertion of a silicone rubber prosthesis between the portal vein and inferior vena cava and avoids alterations in systemic haemodynamics. RESULTS: Hepatic arterial (HA) blood flow increased in response to reduced portal venous (PV) blood flow, the "buffer response", from 19.4 (3.3) ml min-1 100 g-1 to 25.6 (4.3) ml min-1 100 g-1 (mean (SE), p < 0.05, Student's paired t-test). This represented a buffering capacity of 18.7 (5.2) %. Intra-portal injections of ATP or adenosine (1 micrograms kg-1-0.5 mg kg-1) elicited immediate increases in HA blood flow to give -log ED50 values of 2.0 and 1.7 mg kg-1 for ATP and adenosine respectively. Injection of ATP and adenosine had no measurable effect on PV flow. In vitro, using an isolated dual-perfused rabbit liver preparation, the addition of 8-phenyltheophylline (10 MicroMolar) to the HA and PV perfusate significantly inhibited the HA response to intra-arterial adenosine and to mid-range doses of intra-portal or intra-arterial ATP (p < 0.001). CONCLUSIONS: It is suggested that HA vasodilatation elicited by ATP may be partially mediated through activation of P1-purinoceptors following catabolism of ATP to adenosine.

Use of contrast harmonic ultrasound for the diagnosis of congenital portosystemic shunts in three dogs

Contrast harmonic ultrasound was used to determine macrovascular and perfusion patterns in three dogs with congenital extrahepatic solitary portosystemic shunts (PSS). With coded harmonic angiographic ultrasound, the size and tortuosity of the hepatic arteries were subjectively increased. Single pulse intermittent low-amplitude harmonic perfusion imaging provided contrast enhancement time-intensity curves from regions of interest in the liver. Mean (+/- standard deviation) peak perfusion times of dogs with PSS were significantly shorter (p = 0.01; 7.0 +/- 2.0 s) than reported in normal dogs (22.8 +/- 6.8 s). The contrast inflow slope for the dogs with PSS (14.6 +/- 3.7 pixel intensity units [PIU] was significantly (p = 0.05) larger than reported for normal dogs (3.6 +/- 1.4 PIU/s). These results indicate that combined coded harmonic angiographic and contrast harmonic perfusion sonography can be used to detect increased hepatic arterial blood flow as an indicator of PSS in dogs.

Splanchnic blood flow in low-flow states

IMPLICATIONS

Insufficient splanchnic blood flow in critically ill patients is the result of a multitude of different diseases, treatment modalities and their interplay, and is associated with increased morbidity and mortality. A combination of diminished and heterogeneous mesenteric blood flow, impaired or exhausted regulatory mechanisms and adverse drug effects may coexist with normal systemic hemodynamics.

A model of dual circulation in liver acini with hypoxia regulated adenosine secretion

It was postulated by W.W. Lautt that the hepatic artery flow compensation for changes in portal vein flow (the 'hepatic arterial buffer response') is regulated through the portal blood washout of adenosine from the small fluid compartment that surrounds the hepatic arterial resistance vessels. It is presumed that the adenosine secretion there is constant and independent of oxygen supply or liver demand. It was reported by others that liver secretes variable quantities of adenosine and that secretion is related to the level of liver hypoxia. This paper is an attempt to describe a model of acinar circulation without sources of constant adenosine secretion. The presented model is based on the fact that portal blood enters acinar space near the vascular stalk in the zone 1, while most of the arterial branches empty one-third from the interlobular septa, at the beginning of the zone 2, just downstream from the zone 1. Another important characteristic of liver architecture is that near 5/9 of lobular volume is in the zone 1. Liver cells in zone 1 are well oxygenated by the portal blood and they have low adenosine secretion that might seem almost constant. Since most arterial branches empty more peripherally, the zone 1 normally does not depend on the arterial circuit and most of arterial branches are governed by the adenosine secretion from the upstream zone 1. Low portal flow, would increase adenosine secretion from the zone 1 and thus dilate numerous downstream arterial resistance vessels. An increased flow from these arterial vessels would compensate any decrease in the portal flow. Zones 2 and 3 probably have higher adenosine secretion rates since the oxygenation depends on the amount of added arterial blood and on the liver cell metabolism. Some of the arterial branches in those zones are probably open all the time, preserving them zones from hypoxic injury. Since the main point for arterial inflow is concentrated downstream from the zone 1, in cases of low portal pressures, or elevated upstream resistance, some of the arterial blood might leave the acinus in retrograde direction via the portal branch and enter some other acinus as a part of portal blood. These arterio-portal communications might be important in cases of low or none portal flow when zone 1 is in hypoxia. In the 3D liver space with tightly packed acini, very complex and ever-changing patterns of combined antegrade and retrograde flows can be expected.

Clinical review: splanchnic ischaemia

Inadequate splanchnic perfusion is associated with increased morbidity and mortality, particularly if liver dysfunction coexists. Heart failure, increased intra-abdominal pressure, haemodialysis and the presence of obstructive sleep apnoea are among the multiple clinical conditions that are associated with impaired splanchnic perfusion in critically ill patients. Total liver blood flow is believed to be relatively protected when gut blood flow decreases, because hepatic arterial flow increases when portal venous flow decreases (the hepatic arterial buffer response [HABR]). However, there is evidence that the HABR is diminished or even abolished during endotoxaemia and when gut blood flow becomes very low. Unfortunately, no drugs are yet available that increase total hepato-splanchnic blood flow selectively and to a clinically relevant extent. The present review discusses old and new concepts of splanchnic vasoregulation from both experimental and clinical viewpoints. Recently published trials in this field are discussed.

Rat auxiliary liver transplantation without portal vein reconstruction: comparison with the portal vein-arterialized model

Auxiliary liver transplantation (ALT) has been reintroduced in clinical cases recently and is now believed to be a viable alternative to orthotopic liver transplantation. To provide a simple rat ALT model for studying the physiological and immunological aspects of the ALT graft, a new ALT was performed, and the comparison between this new model and the portal arterialized one that was reported by other investigators was carried out. At first, we confirmed that liver could tolerate the deprivation of its portal flow well, using a portosystemic shunted rat model. The new rat ALT model, in which the ALT graft obtained its blood inflow only from the hepatic artery, was then performed. Our results demonstrated that 50% of the hepatic artery-alone ALT graft showed almost normal structure histologically at 1 month after grafting, with bile secretion preserved. By contrast, only 8% 1-month graft survival was noted in the portal arterialized group, and all grafts stopped bile secretion 1 week after operation. In conclusion, with arterial blood supply alone, the ALT graft survived and demonstrated normal bile secretion function for more than 1 month. Portal vein arterialization is not an appropriate way to establish the graft's blood supply if no pressure adjustment measures were taken in advance.

A model of hydraulic interactions in liver parenchyma as forces behind the intrahepatic bile flow

The small diameters of bile canaliculi and interlobular bile ducts make it hard to attribute the bile flow solely to the process of secretion. In the model liver within its capsule is considered a limited space in which volume expansions of one part are possible only through the shrinking of other parts. The liver capsule allows only very slow volume changes. The rate of blood flow through the sinusoides is governed by the Poisseuill-Hagen law. The model is based on a concept of circulatory liver units. A unit would contain a group of acini sharing the same conditions of arterial flow. We can imagine them as an acinar group behind the last pressure reducer on one arterial branch. Acini from neighboring units compose liver lobules and drain through the same central venule. One lobule can contain acini from several neighboring circulatory units. The perfusion cycle in one unit begins with a transient tide in the arterial flow, governed by local mediators. Corresponding acini expand, grabbing the space by compressing their neighbors in the same lobules. Vascular resistance is reduced in dilated and increased in compressed acini. Portal blood flows through the dilated acini, bypassing the compressed neighbors. The cycle ends when the portal tide slowly diminishes and acinar volume is back on the interphase value until the new perfusion cycle is started in another circulatory unit. Each cycle probably takes minutes to complete. Increased pressures both in dilated and in compressed acini force the bile to move from acinar canalicules. Both up and down changes in acinar volume might force the acinar biliary flow. In cases of arterial vasoconstriction, increased activity of vasoactive substances would keep most of the circulatory units in the interphase and increased liver resistance can be expected. Liver fibrosis makes all acini to be of fixed volume and result in increased resistance. Because of that, low pressure portal flow would be more compromised, as reported. In livers without arterial blood flow, although some slow changes in the portal flows can be expected, acinar volume changes should be reduced. In acute liver injury, enlarged hepatocytes would diminish sinusoidal diameter and increase acinar resistance. In liver tumors, areas of neovascularization with reduced resistance would divert the arterial flow from the normal tissue, while in the compressed perifocal areas, increased vascular resistance should diminish mainly the portal flow.

Effects of systemic arterial hypoperfusion on splanchnic hemodynamics and hepatic arterial buffer response in pigs

The hepatic arterial buffer response (HABR) tends to maintain liver blood flow under conditions of low mesenteric perfusion. We hypothesized that systemic hypoperfusion impairs the HABR. In 12 pigs, aortic blood flow was reduced by cardiac tamponade to 50 ml. kg(-1). min(-1) for 1 h (short-term tamponade) and further to 30 ml. kg(-1). min(-1) for another hour (prolonged tamponade). Twelve pigs without tamponade served as controls. Portal venous blood flow decreased from 17 +/- 3 (baseline) to 6 +/- 4 ml. kg(-1). min(-1) (prolonged tamponade; P = 0.012) and did not change in controls, whereas hepatic arterial blood flow decreased from 2 +/- 1 (baseline) to 1 +/- 1 ml. kg(-1). min(-1) (prolonged tamponade; P = 0.050) and increased from 2 +/- 1 to 4 +/- 2 ml. kg(-1). min(-1) in controls (P = 0.002). The change in hepatic arterial conductance (DeltaC(ha)) during acute portal vein occlusion decreased from 0.1 +/- 0.05 (baseline) to 0 +/- 0.01 ml. kg(-1). min(-1). mmHg(-1) (prolonged tamponade; P = 0.043). In controls, DeltaC(ha) did not change. Hepatic lactate extraction decreased, but hepatic release of glutathione S-transferase A did not change during cardiac tamponade. In conclusion, during low systemic perfusion, the HABR is exhausted and hepatic function is impaired without signs of cellular damage.

Hepatic arteriolo-portal venular shunting guarantees maintenance of nutritional microvascular supply in hepatic arterial buffer response of rat livers

To elucidate the hepatic microvascular response upon the hepatic arterial buffer response (HABR), we analysed blood flow (ultrasonic flowprobes) of the hepatic artery (HA) and portal vein (PV), microcirculation (intravital microscopy), and tissue oxygenation (polarography) in anaesthetized Sprague-Dawley rats and re-evaluated the role of adenosine in mediating the HABR by using 8-phenyltheophylline as a competitive antagonist. 2. Upon restriction of PV blood flow to 11 +/- 3 % of baseline values, HA blood flow increased by a factor of 1.77 (P < 0.05), thus confirming HABR. Strikingly, red blood cell velocity and volumetric blood flow in terminal hepatic arterioles (THAs) did not increase but were even found to be slightly decreased, by 8 and 13 %, respectively. In contrast, red blood cell velocity and volumetric blood flow in terminal portal venules (TPVs) decreased to only 66 % (P < 0.05), indicating upstream hepatic arteriolo-portal venular shunting. As a consequence, red blood cell velocity and volumetric blood flow in sinusoids were found to be reduced to only 66-68 % compared with baseline (P < 0.05). Diameters of neither of those microvessels changed, thus excluding THA-, TPV-, and sinusoid-associated mechanisms of vasomotor control in HABR. 3. Tissue PO2 and hepatocellular NADH fluorescence remained unchanged, indicating HABR-mediated maintenance of adequate oxygen delivery, despite the marked reduction of total liver blood flow. Further, hepatic arteriolo-portal venular shunting guaranteed homogeneity of nutritive blood flow upon HABR, as given by an unchanged intra-acinar coefficient of variance of sinusoidal perfusion. 4. Pretreatment of animals with the adenosine antagonist 8-phenyltheophylline completely blocked the hepatic arterial buffer response with the consequence of decreased tissue oxygenation and increased heterogeneity of sinusoidal perfusion. 5. In conclusion, hepatic microhaemodynamics, in particular unchanged diameters of THAs, TPVs and sinusoids, during HABR indicate that reduction in resistance to HA flow is located upstream and functions via hepatic arteriolo-portal venular shunts resulting in equal distribution of microvascular blood flow and oxygen delivery under conditions of restricted PV blood supply.

Impact of intrinsic blood flow regulation in cirrhosis: maintenance of hepatic arterial buffer response

The hepatic arterial buffer response (HABR) effectively controls total blood perfusion in normal livers, but little is known about blood flow regulation in cirrhosis. We therefore studied the impact of HABR on blood perfusion of cirrhotic livers in vivo. After 8-wk CCl(4) treatment to induce cirrhosis, 18 anesthetized rats (and 18 noncirrhotic controls) were used to simultaneously assess portal venous and hepatic arterial inflow with miniaturized ultrasonic flow probes. Stepwise hepatic arterial blood flow (HAF) or portal venous blood flow (PVF) reduction was performed. Cirrhotic livers revealed a significantly reduced total hepatic blood flow (12.3 +/- 0.9 ml/min) due to markedly diminished PVF (7.3 +/- 0.8 ml/min) but slightly increased HAF (5.0 +/- 0.6 ml/min) compared with noncirrhotic controls (19.0 +/- 1.6, 15.2 +/- 1.3, and 3.8 +/- 0.4 ml/min). PVF reduction caused a significant HABR, i.e., increase of HAF, in both normal and cirrhotic livers; however, buffer capacity of cirrhotic livers exceeded that of normal livers (P < 0.05) by 1. 7- to 4.5-fold (PVF 80% and 20% of baseline). Persistent PVF reduction for 1, 2, and 6 h demonstrated constant HABR in both groups. Furthermore, HABR could be repetitively provoked, as analyzed by intermittent PVF reduction. HAF reduction did not induce changes of portal flow in either group. Because PVF is reduced in cirrhosis, the maintenance of HAF and the preserved HABR must be considered as a protective effect on overall hepatic circulation, counteracting impaired nutritive blood supply via the portal vein.

Hepatic artery buffer response following left portal vein ligation: its role in liver tissue homeostasis

Occlusion of a lobar portal vein is known to induce atrophy of downstream liver lobes and hypertrophy of contralateral lobes. Changes in portal flow are known to be compensated by changes in hepatic arterial flow, thus defining the hepatic artery buffer response (HABR). To understand the role of liver flow in liver atrophy, we measured portal flow and hepatic artery flow after different degrees of left portal vein stenosis (LPVS). Surgery was performed to obtain 0, 43, 48, 59, 68, 72, 78, and 100% LPVS. Systemic and splanchnic blood flows were measured at 4 h or 7 days after surgery using radiolabeled microspheres. At 4 h, LPVS produced no changes in systemic hemodynamics. Increasing degrees of LPVS produced a significant decrease in left portal flow (P < 0.0001) and a fully compensatory increase in right portal flow (P < 0.0001) without significantly affecting total portal flow. Left hepatic artery flow increased by 210% (P = 0.002), and right hepatic artery flow decreased by 67% (P = 0.05) after full LPVS. There was a significant inverse correlation between portal and arterial flow changes induced by different degrees of LPVS in the left (r(2) = 0. 61) and right (r(2) = 0.41) lobes. Despite this HABR, we observed a reduction in left liver flow (-45%; P = 0.01) and an increase in right liver flow (+230%; P = 0.01) with 100% LPVS. At 7 days, a significant decrease in the weight of left liver lobes (-75%; P < 0. 0001) and a compensatory increase in the weight of the right lobes (+210%; P < 0.0001) were observed with 100% LPVS. Left and right liver flows were similar to results measured at 4 h, and HABR was still present. However, when expressed per gram of liver, liver flows were identical to results obtained with sham animals. Reduction in lobar portal flow is accompanied by an increase in ipsilateral hepatic artery flow and a compensatory increase in portal flow to the rest of the liver. In a given lobe, when compensatory HABR is overcome, liver weight changes occur so that at the end total liver flow per gram of liver tissue is restored. This suggests that in normal conditions liver flow is a major regulator of liver volume.

Changes in tissue oxygenation of the porcine liver measured by near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a novel method for the measurement of tissue oxygenation and may have a role in monitoring liver oxygenation and viability. The aim of this study is to validate the application of NIRS for monitoring hepatic tissue oxygenation. Large Landrace pigs (n = 12) underwent laparotomy and liver exposure. Total hepatic blood flow (THBF) was measured by the Transonic Medical Flowmeter system. NIRS probes were placed on the liver surface to continuously record changes in hepatic tissue oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and the reduction-oxidation state of cytochrome oxidase (Cyt Ox). Reduction of hepatic tissue oxygenation was achieved by hepatic vascular inflow occlusion (n = 6) or reduction of inspired oxygen (FIO2; n = 6). The THBF changes correlated significantly with hepatic HbO2 (r = 0.84; P <.001) and Cyt Ox (r = 0.88; P <.001). With reduction of FIO2, a significant correlation was found between arterial oxygen saturation and hepatic HbO2 and Hb (r = 0.99 and r = -0.99, respectively; P <.0001). NIRS measurement of liver parenchymal oxygenation correlates well with changes in liver blood flow and arterial oxygenation.

The 1995 Ciba-Geigy Award Lecture. Intrinsic regulation of hepatic blood flow

Intrinsic regulation of hepatic blood flow is mediated only through the hepatic artery because the liver is not able to directly regulate portal vein blood flow. Hepatic metabolic activity does not affect hepatic artery flow. Although the hepatic artery is affected by sympathetic nerves and blood-borne agents, the intrinsic regulation of the hepatic artery can be demonstrated if these factors are controlled. The primary intrinsic regulator of the hepatic artery is the hepatic arterial buffer response, which is the inverse response of the hepatic artery to changes in portal vein flow. The hepatic arterial buffer response is sufficiently powerful that doubling portal vein flow leads to maximal constriction in the hepatic artery, while low portal vein flow can result in maximal dilation. The mechanism of the hepatic arterial buffer response is based on adenosine washout, whereby adenosine is produced at a constant rate, independent of oxygen supply or demand, and secreted into a small fluid compartment that surrounds the hepatic arterial resistance vessels. If portal vein flow decreases, less adenosine is washed away into the portal blood and the accumulated adenosine leads to hepatic arterial dilation. Similarly, hepatic arterial autoregulation operates by the same mechanism, whereby a decrease in arterial pressure leads to a decrease in hepatic arterial flow, thus resulting in less adenosine washout into the hepatic artery blood. The accumulated adenosine leads to hepatic artery dilation. These intrinsic regulatory mechanisms tend to maintain total hepatic blood flow at a constant level, thus stabilizing hepatic clearance of hormones, venous return, and cardiac output.

Diminished hyperaemic response of the hepatic artery to portal venous occlusion (the buffer response) in Asian hybrid minipigs: a comparison of the response to that observed in dogs

The hyperaemic response of the hepatic artery to portal vein occlusion (the buffer response) and the action of exogenous adenosine upon hepatic artery blood flow was studied in Asian hybrid minipigs as a potential alternative experimental model to that previously developed in dogs. Adenosine produced a dose-dependent hepatic artery vasodilatation, but of lesser extent than that observed in dogs. A greatly diminished buffer response was observed in the pigs compared to that seen in dogs, and could not be replicated consistently. The adenosine uptake inhibitor dipyridamole did not potentiate responses to adenosine or the buffer response. It is concluded that the minipig is an unsuitable alternative model for the study of the hepatic artery buffer response.

Characterization of P2X- and P2Y-purinoceptors in the rabbit hepatic arterial vasculature

1. Responses to adenosine 5'-triphosphate (ATP) and its agonists were studied in the isolated liver of the rabbit dually perfused through the hepatic artery and the portal vein. 2. In the hepatic arterial vascular bed at basal tone, ATP and its agonists elicited vasoconstrictor responses with the rank order of potency alpha,beta-methylene ATP greater than 2-methylthio ATP greater than ATP, consistent with their action at the P2X-purinoceptor. 3. When tone was raised with noradrenaline (10(-5) M), vasodilator responses were produced with ATP and 2-methylthio ATP; alpha,beta-methylene ATP produced only further constriction. The rank order of vasodilator potency was 2-methylthio ATP greater than ATP much greater than alpha,beta-methylene ATP, consistent with their action at the P2Y-purinoceptor. 4. Methylene blue (10(-5) M) antagonized vasodilator responses to acetylcholine and ATP, but not those to adenosine or sodium nitroprusside. Addition of 8-phenyltheophylline (10(-5) M) antagonized responses to adenosine but not those to sodium nitroprusside. Responses to ATP remaining after antagonism with methylene blue were not further antagonized by 8-phenyltheophylline. 5. These results present evidence for discrete P2X- and P2Y-purinoceptors in the rabbit hepatic arterial bed which mediate vasoconstrictor and vasodilator responses respectively. 6. Vasodilatation produced by ATP was entirely due to direct action at the P2Y-purinoceptor, and not at a P1-purinoceptor following breakdown to adenosine. The antagonism of these responses by methylene blue is consistent with the view that vasodilatation by ATP takes place largely via endothelial P2Y-purinoceptors that lead to release of endothelium-derived relaxing factor. However, we cannot exclude the possibility that P2y-purinoceptors located on the vascular smooth muscle play a contributory role in ATP-induced vasodilatation.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

Quantitation of the hepatic arterial buffer response to graded changes in portal blood flow

Hepatic arterial blood flow changes inversely in response to altered portal blood flow. The hepatic arterial capacity to buffer portal flow changes was studied over a wide range of portal flow with arterial pressure held steady (the active buffer response) or uncontrolled. The active component of the buffer response led to nearly full dilation of the hepatic artery at low portal flows as shown by inability to dilate further in response to adenosine infusion; at high portal flows the hepatic artery was nearly fully constricted as shown by lack of further constriction to norepinephrine. With pressure uncontrolled, active and passive effects combined to produce an increased compensation with similar efficiency (44% +/- 4%) over the full range of portal blood flows. Thus, although the active component of the hepatic arterial buffer response becomes less efficient at very high and low portal flows, the combination of active and passive effects leads to a larger buffer capacity which is equally efficient over a wide range of portal blood flow changes.

The effect of propranolol on the hyperaemic response of the hepatic artery to portal venous occlusion in the dog

1. It has been reported that activation of beta-adrenoceptors may be responsible for the hyperaemic response of the hepatic artery to portal venous blood flow reduction. 2. The effect of beta-adrenoceptor blockade on the hepatic arterial response to portal vein occlusion was investigated in 6 anaesthetized dogs. A side-to-side portacaval shunt was established to prevent loss of venous return and arterial blood pressure during periods of portal occlusion. Measurements of hepatic arterial and portal venous blood flows were made by use of electromagnetic flow probes. 3. Intravenous propranolol injection, at a dose sufficient to block the vasodilator effect of low doses of exogenous adrenaline, did not alter the magnitude of the hyperaemic response of the hepatic artery. Propranolol also produced no change in baseline portal venous pressure. 4. It is concluded that hepatic beta-adrenoceptors are unlikely to be involved in the arterial response to portal occlusion. The absence of any reduction in basal portal venous pressure by propranolol is of interest in view of the current application of the drug in the treatment of patients with portal hypertension.

Effect of portal vein occlusion on liver blood flow in normal and cirrhotic dogs

The purpose of this study was to demonstrate that galactose clearance (GC) can measure acute changes in liver blood flow (LBF) in normal and cirrhotic dogs. Ten dogs were studied. GC was measured preop. At laparotomy, GC, hepatic artery (HA) flow, portal vein (PV) flow, and cardiac output (CO) were measured at baseline, 50% portal vein occlusion (PVO), and portal vein release. HA and PV flows were measured using a flow probe (FP). Common bile duct ligation was then performed to cause cirrhosis and all measurements were repeated in 7 weeks. Statistical analyses showed that on PVO in both normal dogs (n = 10) and cirrhotic dogs (n = 5) the GC, HA flow, and CO were significantly different from their baseline values. In both groups PVO caused HA flow to increase, thus keeping FP-LBF unchanged while GC-LBF was significantly reduced compared to baseline. The possible explanations for this are discussed in the text. PVO also caused a significant reduction in CO due to splanchnic pooling in both normal and cirrhotic dogs. In both groups PVO results in an increased percentage of CO going to FP-LBF, while the percentage of CO going to GC-LBF remains unchanged. We conclude that GC can measure acute changes in LBF caused by a 50% PVO in both normal and cirrhotic dogs.