Computer analysis of portal hemodynamics after small-diameter portacaval H-grafts: the theoretical basis for partial shunting

A computational model of the hepatic circulation applied to analyze the sensitivity of hepatic venous pressure gradient (HVPG) in liver cirrhosis

Measurement of hepatic venous pressure gradient (HVPG) is currently widely adopted to provide an estimate of portal pressure gradient (PPG) in the diagnosis and treatment of portal hypertension associated with liver cirrhosis. Despite the well-documented clinical utility of HVPG, it remains poorly understood how the relationship between HVPG and PPG is affected by factors involved in the pathogenesis and progression of cirrhosis. In the study, a computational model of the hepatic circulation calibrated to in vivo data was developed to simulate the procedure of HVPG measurement and quantitatively investigate the error of HVPG relative to PPG under various pathophysiological conditions. Obtained results confirmed the clinical consensus that HVPG is applicable to the assessment of portal hypertension caused by increased vascular resistance located primarily at the sinusoidal and postsinusoidal sites rather than at the presinusoidal site. On the other hand, our study demonstrated that the accuracy of HVPG measurement was influenced by many factors related to hepatic hemodynamics even in the case of sinusoidal portal hypertension. For instance, varying presinusoidal portal vascular resistance significantly altered the difference between HVPG and PPG, while an enhancement in portosystemic collateral flow tended to improve the accuracy of HVPG measurement. Moreover, it was found that presinusoidal and postsinusoidal vascular resistances interfered with each other with respect to their influence on HVPG measurement. These findings suggest that one should take into account patient-specific pathological conditions in order to achieve a better understanding and utilization of HVPG in the clinical practice.

Partial hepatectomy hemodynamics changes: Experimental data explained by closed-loop lumped modeling

The liver function may be degraded after partial liver ablation surgery. Adverse liver hemodynamics have been shown to be associated to liver failure. The link between these hemodynamics changes and ablation size is however poorly understood. This article proposes to explain with a closed-loop lumped model the hemodynamics changes observed during twelve surgeries in pigs. The portal venous tree is modeled with a pressure-dependent variable resistor. The variables measured, before liver ablation, are used to tune the model parameters. Then, the liver partial ablation is simulated with the model and the simulated pressures and flows are compared with post-operative measurements. Fluid infusion and blood losses occur during the surgery. The closed-loop model presented accounts for these blood volume changes. Moreover, the impact of blood volume changes and the liver lobe mass estimations on the simulated variables is studied. The typical increase of portal pressure, increase of liver pressure loss, slight decrease of portal flow and major decrease in arterial flow are quantitatively captured by the model for a 75% hepatectomy. It appears that the 75% decrease in hepatic arterial flow can be explained by the resistance increase induced by the surgery, and that no hepatic arterial buffer response (HABR) mechanism is needed to account for this change. The different post-operative states, observed in experiments, are reproduced with the proposed model. Thus, an explanation for inter-subjects post-operative variability is proposed. The presented framework can easily be adapted to other species circulations and to different pathologies for clinical hepatic applications.

Hemodynamic analysis for transjugular intrahepatic portosystemic shunt (TIPS) in the liver based on a CT-image

In this paper, we apply a 3-D flow model and a 1-D circulation model to the hemodynamic analysis of transjugular intrahepatic portosystemic shunt (TIPS), the therapy for treating acute portal hypertension (PH) induced diseases. Using the 3-D model we are able to simulate the blood flow within a patient-specific TIPS system which was reconstructed from a computed tomography image, and quantify such hemodynamic data as the wall shear stress and flow velocity. The 1-D model is used for the investigation of generic TIPS-induced hepatic circulation phenomena. By incorporating physiological data into the 1-D model we can reproduce some complex flow patterns such as the increased arterial flow after TIPS implantation, the formation of retrograde flow in the portal vein, etc. In particular, our model gives a quantitative analysis of the interplay between TIPS and hepatic flows. In conclusion, the presented computational model can be used for the theoretical analysis of TIPS, in which clinical decisions are often made based on contradictory considerations to balance the procedure-induced complications and the urgency of relieving acute PH symptoms.

Modeling the hepatic arterial buffer response in the liver

In this paper we present an electrical analog model for the hepatic arterial buffer response (HABR), an intrinsic regulation mechanism in the liver whereby the arterial flow counteracts the changes in portal venous flow. The model itself is a substantial simplification of a previously published model, with nonlinear arterial and portal resistors introduced to account for the dynamic HABR effects. We calibrate the baseline model using published hemodynamic data, and then perform a virtual portal occlusion simulation where the portal vein is half or fully occluded. The simulation results, which suggest that the increased arterial flow cannot fully compensate lost portal perfusion, are consistent with clinical reports and animal model findings. Since HABR functions in both the whole liver and liver graft after transplantation, we also simulate blood flow in a virtual right-lobe graft by adjusting the electronic component parameters in the electric circuit, and our model is able to reproduce the portal venous hyperperfusion and hepatic arterial hypoperfusion conditions due to the HABR effects.

Hemodynamic simulation for an anatomically realistic portal system

In this paper we present a computer model that simulates blood flow in the portal system, the vascular network that delivers nutrients and hormones to the liver from other digestion organs. Firstly the vascular geometry of a patient-specific portal system was digitised from a 3D CT image. Then blood flow in this system was solved using a set of 1D partial differential equations coupled with a bifurcation model. Some preliminary results are presented and compared with published ultrasonography and phase contrast MRA data. We further simulate a surgical procedure (portacaval shunting) that connects the portal vein with the inferior vena cava (IVC). The simulation confirms that the high pressure gradient between the portal vein and IVC leads to substantially reduced portal perfusion or even reversed flow in the portal veins, thus makes the shunting graft a flow 'highway' in directing portal flow.

Liver function and encephalopathy after partial vs direct side-to-side portacaval shunt: a prospective randomized clinical trial

BACKGROUND

The aim of this study was to determine, in a prospective randomized clinical trial, whether the partial portacaval shunt offers any advantage in terms of liver function and encephalopathy rate when compared with direct side-to-side direct portacaval shunt.

METHODS

Forty-six "good risk" patients with cirrhosis and with documented variceal hemorrhage were randomly assigned to either a partial shunt procedure (achieved by 10-mm diameter interposition portacaval H-graft) or direct small-diameter side-to-side portacaval anastomosis.

RESULTS

Operative mortality was zero in both groups. During the follow-up period, encephalopathy developed in 3 patients in the partial shunt group and 9 in the direct shunt group (P =.04). Kaplan-Meier analysis demonstrated that encephalopathy-free survival was significantly longer in the partial shunt group (P =.025). Direct shunt patients had significant hepatic functional deterioration postoperatively compared with the partial shunt group.

CONCLUSIONS

The partial portacaval shunt effectively controls variceal hemorrhage. Compared with direct side-to-side portacaval shunt, partial shunt preserves long-term hepatic function and minimizes postoperative encephalopathy. We conclude that the partial portacaval shunt is the preferred approach over direct shunts for patients with cirrhosis and with variceal bleeding.

Narrow-diameter portacaval shunts for management of variceal bleeding

Over the past decade, we have developed and refined a method for partial portosystemic shunting for the control of bleeding esophageal varices in alcoholic cirrhotic patients. The narrow-diameter interposition portacaval H-graft using 8 mm polytetrafluoroethylene has been performed in 32 patients at our institution with low operative mortality (6.3%) and nearly complete cessation of variceal bleeding (96.7%) over a mean follow-up period of 43 months. In comparison with total shunts, diminished rates of postshunt encephalopathy (13% versus 40%) have been observed. Prograde portal blood flow has been preserved in 90% of 30 patients studied by perioperative portography. Shunt patency with continued prograde flow has been demonstrated at up to 9 years of follow-up. Investigators at three other centers have studied partial shunting using substantially similar techniques, with similar findings. Based on these results, we conclude that narrow-diameter shunts provide effective, long-lasting treatment for variceal hemorrhage due to portal hypertension in the alcoholic.

Influence of portal hemodynamics on long-term survival of alcoholic cirrhotic patients after small-diameter portacaval H grafts

Estimating postoperative survival rates after portasystemic shunt procedures has concerned surgeons during the last 40 years. The relationship between survival and Child's classification has clearly demonstrated the importance of preoperative hepatic functional reserve. Maintaining hepatic portal perfusion has been proposed as an additional protective factor but has never been proved clinically. Our analysis of survival after partial shunting with small-diameter portacaval H grafts has shown that both hepatic functional reserve and postoperative portal perfusion correlate with postoperative survival in alcoholic patients, but the latter was a stronger correlate of long-term survival. A predictive model based on both factors has been described for estimating the overall survival rate of alcoholics after partial shunting with small-diameter portacaval H grafts.