Early assessment of reperfusion injury by intraoperative quantification of hepatic microcirculation in patients

Wireless monitoring of liver hemodynamics in vivo

Liver transplants have their highest technical failure rate in the first two weeks following surgery. Currently, there are limited devices for continuous, real-time monitoring of the graft. In this work, a three wavelengths system is presented that combines near-infrared spectroscopy and photoplethysmography with a processing method that can uniquely measure and separate the venous and arterial oxygen contributions. This strategy allows for the quantification of tissue oxygen consumption used to study hepatic metabolic activity and to relate it to tissue stress. The sensor is battery operated and communicates wirelessly with a data acquisition computer which provides the possibility of implantation provided sufficient miniaturization. In two in vivo porcine studies, the sensor tracked perfusion changes in hepatic tissue during vascular occlusions with a root mean square error (RMSE) of 0.135 mL/min/g of tissue. We show the possibility of using the pulsatile wave to measure the arterial oxygen saturation similar to pulse oximetry. The signal is also used to extract the venous oxygen saturation from the direct current (DC) levels. Arterial and venous oxygen saturation changes were measured with an RMSE of 2.19% and 1.39% respectively when no vascular occlusions were induced. This error increased to 2.82% and 3.83% when vascular occlusions were induced during hypoxia. These errors are similar to the resolution of a commercial oximetry catheter used as a reference. This work is the first realization of a wireless optical sensor for continuous monitoring of hepatic hemodynamics.

Hepatic ischemic preconditioning increases portal vein flow in experimental liver ischemia reperfusion injury

BACKGROUND

Ischemic preconditioning (IPC) has been shown to decrease liver injury and to increase hepatic microvascular perfusion after liver ischemia reperfusion. This study aimed to evaluate the effects of IPC on hemodynamics of the portal venous system.

METHODS

Thirty-two rats were randomized into two groups: IPC group and control group. The rats of the IPC group underwent IPC by 10 minutes of liver ischemia followed by 10 minutes of reperfusion before liver ischemia, and the rats of the control group were subjected to 60 minutes of partial liver ischemia. Non-ischemic lobes were resected immediately after reperfusion. The animals were studied at 4 hours and 12 hours after reperfusion. Mean arterial pressure, heart rate, portal vein flow and pressure were analyzed. Blood was collected for the determination of the levels of aspartate aminotransferase, alanine aminotransferase, calcium, lactate, pH, bicarbonate, and base excess.

RESULTS

IPC increased the mean portal vein flow at 4 hours and 12 hours after reperfusion. IPC recovered 78% of the mean portal vein flow at 12 hours after reperfusion. IPC decreased the levels of aspartate aminotransferase, alanine aminotransferase and lactate, and increased the levels of ionized calcium, bicarbonate and base excess at 12 hours after reperfusion.

CONCLUSIONS

This study demonstrated that IPC increases portal vein flow and enhances hepatoprotective effects in liver ischemia reperfusion. The better recovery of portal vein flow after IPC may be correlated with the lower levels of transaminases and with the better metabolic profile.

Ischemic preconditioning-induced hyperperfusion correlates with hepatoprotection after liver resection

AIM: To characterize the impact of the Pringle maneuver (PM) and ischemic preconditioning (IP) on total blood supply to the liver following hepatectomies.

METHODS: Sixty one consecutive patients who underwent hepatic resection under inflow occlusion were randomized either to receive PM alone (n = 31) or IP (10 min of ischemia followed by 10 min of reperfusion) prior to PM (n = 30). Quantification of liver perfusion was measured by Doppler probes at the hepatic artery and portal vein at various time points after reperfusion of remnant livers.

RESULTS: Occlusion times of 33 ± 12 min (mean ± SD) and 34 ± 14 min and the extent of resected liver tissue (2.7 segments) were similar in both groups. In controls (PM), on reperfusion of liver remnants for 15 min, portal perfusion markedly decreased by 29% while there was a slight increase of 8% in the arterial blood flow. In contrast, following IP + PM the portal vein flow remained unchanged during reperfusion and a significantly increased arterial blood flow (+56% vs baseline) was observed. In accordance with a better postischemic blood supply of the liver, hepatocellular injury, as measured by alanine aminotransferase (ALT) levels on day 1 was considerably lower in group B compared to group A (247 ± 210 U/I vs 550 ± 650 U/I, P < 0.05). Additionally, ALT levels were significantly correlated to the hepatic artery inflow.

CONCLUSION: IP prevents postischemic flow reduction of the portal vein and simultaneously increases arterial perfusion, suggesting that improved hepatic macrocirculation is a protective mechanism following hepatectomy.

Quantification of liver perfusion by dynamic magnetic resonance imaging: experimental evaluation and clinical pilot study

Changes in liver microcirculation are considered essential in assessing ischemia-reperfusion injury, which in turn has an impact on liver graft function and outcome following liver transplantation (LTx). The aim of this study was to introduce dynamic magnetic resonance imaging (dMRI) as a new technique for overall quantification of hepatic microcirculation and compare it to perfusion measured by laser Doppler flowmetry (LDF; hepatic artery/portal vein) and thermal diffusion (TD). The study included 3 groups, measuring hepatic blood flow and microcirculation with the help of TD, LDF, and dMRI. In group I (9 landrace pigs; 26 +/- 5 kg), the native liver before and after partial portal occlusion was studied; in group II (6 landrace pigs; 25.5 +/- 4.4 kg), the liver 24 hours after LTx was studied; and in group III (14 patients), the liver on days 4 to 7 following LTx was studied. A close correlation was found between dMRI measurements and TD (r = 0.7-0.9, P < 0.01) in 4 defined regions of interest. Portal blood flow and partial occlusion of the portal vein were accurately detected by LDF flowmetry and correlated well with dMRI (r = 0.95, P < 0.01). In the clinical setting, representative TD measurements in segment 4b of the transplanted liver correlated well with dMRI analysis in other segments. Quantification of the portal blood flow and imaging of the whole liver could be performed simultaneously by dMRI. In conclusion, dMRI has been proved to be a sensitive modality for the quantification of liver microcirculation and hepatic blood flow in experimental and clinical LTx. It allows for a synchronous, noninvasive assessment of macrocirculation and microcirculation of the liver and could become a valuable diagnostic tool in advanced liver surgery and transplantation.

[An experimental organ model for magnetic resonance imaging]

The investigation of reperfusion of organs after an ischaemic phase is of great interest in transplantation medicine. This work presents an experimental organ model for the examination of isolated canine livers by means of magnetic resonance imaging in reperfusion experiments. The perfusion of the organs inside a perfusate container in the scanner's head coil was performed with approximately physiological conditions for 1.5 hours. The pumps for the perfusate circulation were installed outside the scanner's room, where oxygenation and heating of the perfusate was also performed. In the MR examinations, T1- and T2-weighted sequences could be used to display the morphology of the organs and the spatial perfusion distribution after administration of contrast media. The images revealed oedema development during reperfusion and an inhomogenous perfusion distribution. Thus, the model allows for the non-invasive investigation of morphology and perfusion distribution in isolated reperfused organs.

Continuous infusion of N-acetylcysteine reduces liver warm ischaemia–reperfusion injury

Background

N-acetylcysteine (NAC) may modulate the initial phase (less than 2 h) of liver warm ischaemia–reperfusion (IR) injury but its effect on the late phase remains unclear. The present study investigated the role of NAC during the early and late phases in a rabbit lobar IR model.

Methods

Liver ischaemia was induced by inflow occlusion to the median and left liver lobes for 60 min, followed by 7 h of reperfusion. In the NAC group (n = 6), NAC was administered intravenously at 150 mg per kg over the 15 min before reperfusion and maintained at 10 mg per kg per h during reperfusion. In the IR group (n = 6), 20 ml 5 per cent dextrose was infused over the 15 min before reperfusion and continued at a rate of 10 ml/h. Animals in a sham operation group (n = 6) underwent laparotomy but no liver ischaemia. All animals were killed at the end of the experiment.

Results

Intracellular tissue oxygenation was improved after the second hour of reperfusion in animals treated with NAC compared with that in the IR group (P = 0·023). Hepatic microcirculation improved after 5 h of reperfusion (P = 0·036) and liver injury was reduced after 5 h, as indicated by alanine aminotransferase activity (P = 0·007) and indocyanine green clearance (uptake, P = 0·001; excretion, P = 0·032).

Conclusion

The main protective effect of NAC becomes apparent 5 h after hepatic ischaemic injury.

Assessment of renal graft function by perioperative monitoring of cortical microcirculation in kidney transplantation

BACKGROUND

We evaluated the significance of perioperative cortical microperfusion for graft function and long-term prognosis after renal allotransplantation. Thermodiffusion technology was clinically applied for the first time, after previous validation for perfusion monitoring of the renal cortex in pigs.

METHODS

A thermodiffusion probe was inserted into the renal cortex in 30 transplant recipients after graft reperfusion. Real-time measurements were recorded until the end of the operation. In 14 patients perfusion was measured daily until postoperative day 7. Microcirculation was correlated to serum creatinine level, scintigraphic findings, and long-term outcome.

RESULTS

In primary graft function, intraoperative perfusion was 85+/-7 mL/100 g per min compared with significantly lower values in cases with subsequent graft dysfunction. The best discrimination was defined for a level of 70 mL/100 g per min with a positive predictive value of 88% for detection of good graft function and 86% for nonfunction. Intraoperative perfusion was significantly different in patients with normal grafts, delayed function, and graft loss. Postoperatively, lower perfusion was found in acute tubular necrosis; a significant correlation could be noted between microcirculation and perfusion index measured by nuclear scanning (r=0.78, P<0.01). Living-related grafts were characterized by higher intraoperative perfusion and superior graft quality.

CONCLUSION

Thermodiffusion could be clinically applicable for the perioperative monitoring of renal graft perfusion. Intraoperative reduction of cortical microcirculation has a high predictive value with respect to detection of delayed renal function. Postoperatively, impaired renal microperfusion is associated with acute tubular necrosis. Living-related donor grafts show less microcirculatory alteration than cadaveric kidneys.

Characterization of hepatic parenchymous perfusion heterogeneity and regional flow kinetics after porcine liver transplantation

BACKGROUND

In clinical practice, a heterogeneous hepatic tissue microperfusion (MC) is often observed after liver resection or transplantation (LTx). Nevertheless this hepatic perfusion phenomenon has never been really quantified with respect to its anatomic distribution and time course in detail. The aim of the study was to characterize liver perfusion heterogeneity and local flow kinetics both in the physiological situation and after standardized ischemia and reperfusion using an established model of porcine LTx.

METHODS

Regional distribution of hepatic MC in healthy native porcine livers (control group; n = 8) was analyzed in comparison with data derived 60 min, 24 h, and 72 h after porcine LTx (transplantation group; n = 8 each subgroup; cold ischemia time: 5.7 +/- 1.2 h). MC was measured with implanted thermal diffusion electrodes (TD). Flow in hepatic artery and portal vein was continuously detected by ultrasonic probes. For standardization of measurement localizations, porcine liver lobes were divided anatomically into three horizontal layers (cranial, medial, caudal), defining 12 distinct hepatic measurement regions.

RESULTS

In the control group, a homogenous liver MC with a mean flow of 81.6 +/- 13.9 ml/100 g/min was detected in all regions. After LTx, a marked MC heterogeneity was noted 60 min after reperfusion. MC rehomogenization was first documented within horizontal liver planes 24 h later. Comparison of MC between planes showed persisting heterogeneity with a significant intralober drop of mean MC in the cranio-caudal direction. Complete MC rehomogenization (both between horizontal and vertical liver planes) was detected 72 h after reperfusion. Still, an overall reduction of mean liver perfusion by about 15% was existent.

CONCLUSIONS

A homogenous tissue perfusion was observed in healthy porcine livers. In contrast, marked heterogeneity of hepatic MC was detected after LTx. Heterogeneity presents as a very dynamic and temporary phenomenon. Early horizontal flow rehomogenization and reconstitution of normal blood flow, particularly primarily in the cranial liver layers, appear to be characteristic features during early flow reconstitution after postischemic reperfusion. Due to heterogeneity and time-dependent flow dynamics, measurement of MC volumes at single hepatic regions may not always allow a valid characterization of liver perfusion quality during the first 24 h after postischemic reperfusion.

N-acetilcisteína diminui a congestão hepática na lesão de isquemia e reperfusão: estudo experimental

OBJETIVOS: A isquemia tem sido utilizada na cirurgia hepática desde o início do século. Embora possibilite a diminuição da perda sangüínea durante as ressecções e a manutenção do órgão à espera de um transplante, a ausência de perfusão traz como conseqüência um dano ao órgão, que se amplifica por ocasião da sua reoxigenação. A N-Acetilcisteína é uma droga capaz de repor os estoques celulares de glutationa, um antioxidante fundamental no controle das lesões resultantes do restabelecimento da perfusão sangüínea, esperando-se dessa forma que diminua a lesão acima descrita. Com o propósito de avaliar a capacidade da N-Acetilcisteína reduzir o dano hepático, utilizou-se um modelo murino de isquemia e reperfusão normotérmica. MÉTODO: Foram utilizados vinte ratos Wistar fêmeas, divididos em dois grupos. No grupo tratado, 400mg/kg de N-Acetilcisteína foram administrados pela via intravenosa, 15 minutos antes do clampeamento do pedículo do lobo esquerdo por 90 minutos. No grupo controle foi administrado o volume equivalente de solução fisiológica. Foi estabelecido um período de quatro horas de reperfusão, após o qual os animais foram sacrificados para a realização de análise histopatológica do lobo esquerdo com coloração de Hematoxilina-Eosina. A lesão tecidual foi quantificada quanto à presença de congestão, esteatose e necrose. RESULTADOS: O estudo evidenciou a capacidade de a N-Acetilcisteína diminuir significativamente a congestão. Não houve diferenças quanto à presença de esteatose e necrose. CONCLUSÃO: Os resultados obtidos permitem-nos concluir que o uso prévio da N-Acetilcisteína nos processos de isquemia e reperfusão, em normotermia, é capaz de diminuir a congestão hepática. A N-Acetilcisteína não diminui a presença de esteatose e necrose.

Validation of real-time continuous perfusion measurement

Perfusion, the rate at which blood in tissue is replenished at the capillary level, is a primary factor in the transport of heat, drugs, oxygen and nutrients. While there have been many measurement techniques proposed, most do not lend themselves to routine, continuous and real-time use. A minimally invasive probe, called the thermal diffusion probe (TDP), which uses a self-heated thermistor to measure absolute perfusion continuously and in real time, was validated at low flows with the microsphere technique. In 27 rabbits, simultaneous TDP measurements were made in liver from 0 to 60 ml min-1 100 g-1. The TDP perfusion correlated well with the microspheres (R2 = 0.898) and the agreement between techniques is very good with a slope close to unity (0.921) and an intercept close to zero (0.566 ml min-1 100 g-1). Variability between the two techniques was primarily due to the sampling error from the microsphere 'snap shot' of periodic blood flow when compared with the continuous TDP perfusion measurement. The ability to quantify local perfusion continuously and in real time may have a profound impact on patient management in a number of clinical areas such as organ transplantation, neurosurgery, oncology and others, in which quantitative knowledge of perfusion is of value.