Noninvasive monitoring of cerebral perfusion pressure in patients with acute liver failure using transcranial doppler ultrasonography.

Elevated intracranial pressure (ICP) leads to loss of cerebral perfusion, cerebral herniation, and irreversible brain damage in patients with acute liver failure (ALF). Conventional techniques for monitoring ICP can be complicated by hemorrhage and infection. Transcranial doppler ultrasonography (TCD) is a noninvasive device which can continuously measure cerebral blood flow velocity, producing a velocity-time waveform that indirectly monitors changes in cerebral hemodynamics, including ICP. The primary goal of this study was to determine whether TCD waveform features could be used to differentiate ALF patients with respect to ICP or, equally important, cerebral perfusion pressure (CPP) levels. A retrospective study of 16 ALF subjects with simultaneous TCD, ICP, and CPP measurements yielded a total of 209 coupled ICP-CPP-TCD observations. The TCD waveforms were digitally scanned and seven points corresponding to a simplified linear waveform were identified. TCD waveform features including velocity, pulsatility index, resistive index, fraction of the cycle in systole, slopes, and angles associated with changes in the slope in each region, were calculated from the simplified waveform data. Paired ICP-TCD observations were divided into three groups (ICP < 20 mmHg, n = 102; 20 < or = ICP < 30 mmHg, n = 74; and ICP > or = 30 mmHg, n = 33). Paired CPP-TCD observations were also divided into three groups (CPP > or = 80 mmHg, n = 42; 80 > CPP > or = 60 mmHg, n = 111; and CPP < 60 mmHg, n = 56). Stepwise linear discriminant analysis was used to identify TCD waveform features that discriminate between ICP groups and CPP groups. Four primary features were found to discriminate between ICP groups: the blood velocity at the start of the Windkessel effect, the slope of the Windkessel upstroke, the angle between the end systolic downstroke and start diastolic upstroke, and the fraction of time spent in systole. Likewise, 4 features were found to discriminate between the CPP groups: the slope of the Windkessel upstroke, the slope of the Windkessel downstroke, the slope of the diastolic downstroke, and the angle between the end systolic downstroke and start diastolic upstroke. The TCD waveform captures the cerebral hemodynamic state and can be used to predict dynamic changes in ICP or CPP in patients with ALF. The mean TCD waveforms for corresponding, correctly classified ICP and CPP groups are remarkably similar. However, this approach to predicting intracranial hypertension and CPP needs to be further refined and developed before clinical application is feasible.

Thermodynamic considerations in solid adsorption of bound solutes for patient support in liver failure.

New detoxification modes of treatment for liver failure that use solid adsorbents to remove toxins bound to albumin in the patient bloodstream are entering clinical evaluations, frequently in head-to-head competition. While generally effective in reducing toxin concentration beyond that obtainable by conventional dialysis procedures, the solid adsorbent processes are largely the result of heuristic development. Understanding the principles and limitations inherent in competitive toxin binding, albumin versus solid adsorbent, will enhance the design process and, possibly, improve detoxification performance. An equilibrium thermodynamic analysis is presented for both the molecular adsorbent recirculating system (MARS) and fractionated plasma separation, adsorption, and dialysis system (Prometheus), two advanced systems with distinctly different operating modes but with similar equilibrium limitations. The Prometheus analysis also applies to two newer approaches: sorbent suspension reactor and microsphere-based detoxification system. Primary results from the thermodynamic analysis are that: (i) the solute-albumin binding constant is of minor importance to equilibrium once it exceeds about 10(5) L/mol; (ii) the Prometheus approach requires larger solid adsorbent columns than calculated by adsorbent solute capacity alone; and (iii) the albumin-containing recycle stream in the MARS approach is a major reservoir of removed toxin. A survey of published results indicates that MARS is operating under mass transfer control dictated by solute-albumin equilibrium in the recycle stream, and Prometheus is approaching equilibrium limits under current clinical protocols.

Intracranial pressure observations in a canine model of acute liver failure supported by a bioartificial liver support system.

Life-threatening, severely elevated intracranial pressure (ICP) is a common feature of acute liver failure (ALF). Perfusion with a bioartificial liver may serve to mitigate rising ICP. A retrospective analysis of ICP measurements in a canine ALF model prospectively supported with a bioartificial liver support system (BLSS) is presented. Animals are divided into two groups based upon care provided: (i) standard medical care (n = 6); and (ii) standard medical care plus BLSS support (n = 9). Nonparametric analysis with respect to ICP, arterial NH(3), lactate, and supportive-care parameters found BLSS-supported animals evidenced significantly less metabolic acidosis than unsupported animals. Analysis of variance/linear regression for direct dependence of ICP on arterial NH(3), lactate, and supportive care parameters irrespective of care found ICP was uncorrelated with any measured factor (P > 0.06 for all factors). Lack of correlation of ICP with the considered parameters indicates that none of these factors are predictive of the extent of ICP elevation in the D-galactosamine canine model. Blood chemistry and supportive care factors that are correlated with and predictive of ICP elevation remain to be identified.

Slow continuous ultrafiltration with bound solute dialysis.

Bound solute dialysis (BSD), often referred to as "albumin dialysis" (practiced clinically as the molecular adsorbents recirculating system, MARS, or single-pass albumin dialysis, SPAD) or "sorbent dialysis" (practiced clinically as the charcoal-based Biologic-DT), is based upon the thermodynamic principle that the driving force for solute mass transfer across a dialysis membrane is the difference in free solute concentration across the membrane. The clinically relevant practice of slow continuous ultrafiltration (SCUF) for maintenance of patients with liver failure is analyzed in conjunction with BSD. The primary dimensionless operating parameters that describe SCUF-BSD include (1) beta, the dialysate/blood binder concentration ratio; (2) kappa, the dialyzer mass transfer/blood flow rate ratio; (3) alpha, the dialysate/blood flow rate ratio; and, (4) gamma, the ultrafiltration/blood flow rate ratio. Results from mathematical modeling of solute removal during a single pass through a dialyzer and solute removal from a one-compartment model indicate that solute removal is remarkably insensitive to gamma. Solute removal approaches an asymptote (improvement in theoretical clearance over that obtainable with no binder in the dialysate) with increasing beta that is dependent on kappa and independent of alpha. The amount of binder required to approach the asymptote decreases with increasing solute-binder equilibrium constant, i.e., more strongly bound solutes require less binder in the dialysate. The results of experimental observations over a range of blood flow rates, 100 to 180 mL/min, dialysate flow rates, 600 to 2150 mL/h, ultrafiltration rates, 0 to 220 mL/h, and dialysate/blood albumin concentration ratios, beta = 0.01 to 0.04, were independently predicted remarkably well by the one-compartment model (with no adjustable parameters) based on BSD principles.

Preclinical evaluation of the Excorp Medical, Inc, Bioartificial Liver Support System.

BACKGROUND: Acute liver failure has no medically recognized effective therapy other than orthotopic liver transplantation. Development of bioartificial livers for support of patients with acute liver failure requires meaningful preclinical evaluation before clinical trials. STUDY DESIGN: Complete results from preclinical safety and efficacy evaluation of the Excorp Medical Bioartificial Liver Support System (BLSS) using a D-galactosamine (D-gal) canine liver failure model are presented. From a total cohort of 23 purpose-bred male hounds, 18 animals were administered a lethal dose (1.5 g/kg) of D-gal. The 18 animals were divided into four treatment groups: no BLSS treatment (n = 6), BLSS treatment starting at 24 to 26 hours post D-gal (n = 5), BLSS treatment starting at 16 to 18 hours post D-gal (n = 4), and "mock support" treatment with a BLSS system containing no hepatocytes (n = 3). The animals were treated until death or death equivalent, or euthanized at 60 hours. Physiologic parameters were continuously monitored. Blood chemistries were obtained every 8 hours. RESULTS: Although survival times for BLSS-supported animals were significantly greater than for the unsupported group, the greatest impact on delaying progression of liver disease was time of intervention. Intervention at 16 to 18 hours post D-gal administration showed significant delay in increasing blood ammonia, lactate, and prothrombin time as compared with untreated animals. Elevated intracranial pressure was found in two of six untreated animals, but in none of the treated animals (zero of nine). Healthy animals supported by the BLSS system evidenced no significant safety problems. CONCLUSIONS: Results suggest the BLSS impacts the course of liver failure in the animal model. Phase I clinical safety evaluation is underway.

Bound solute dialysis.

We used the thermodynamic principles governing bound solute dialysis, commonly referred to as "albumin dialysis" or "sorbent dialysis" and practiced clinically with the Molecular Adsorbent Recirculating System (MARS) and Biologic-DT approaches, respectively, to develop a comprehensive understanding of the process. Dimensionless parameters emerging from the thermodynamic analysis that govern bound solute dialysis are as follows: (1) lambda, the binding power of the solute binding moiety; (2) kappa, the dialyzer mass transfer/blood flow rate ratio; (3) alpha, the dialysate/blood flow rate ratio; (4) beta, the dialysate/blood binding moiety concentration ratio, and (5) psi, the solute/binding moiety concentration ratio in the blood. Results from a mathematical model of countercurrent bound solute dialysis for phi = 0.9 indicate that for a given binding moiety (fixed lambda), the most important parameter for achieving high removal rates is the dialyzer mass transfer ratio for free (unbound) solute. The results also show solute removal approaching an asymptote with increasing beta that is dependent on kappa and independent of alpha. More importantly, results indicate that once a dialysis membrane is chosen, solute removal is virtually independent of blood flow rate, dialysate flow rate, and amount of binding moiety in the dialysate, provided the amount is greater than approximately 90% of that required to reach the asymptote. Experimental observations over a range of blood flow rates (100-400 ml/ minute), dialysate flow rates (50-400 ml/minute), and dialysate/blood albumin concentration ratios (beta = 0-0.3) corroborate the model predictions and indicate that < 4 g/L albumin in the dialysate solution is required for effective bound solute dialysis. The experimental results also show evidence of enhanced mass transfer once the dialysis membrane pore structure surface saturates with albumin.

Plasma versus whole blood perfusion in a bioartificial liver assist device.

The ramifications of using whole blood or plasma for perfusion off an hepatocyte containing bioartificial liver bioreactor in which the hepatocytes are separated by a membrane or other physical barrier from the perfusate stream on the rate of change of patient blood concentrations are explored through dynamic modeling of whole blood perfusion as a two compartment system (patient tissue and blood compartments), and plasma perfusion as a three compartment system (patient tissue and blood compartments, and a plasma reservoir). The whole blood perfusion model is described by three dimensionless parameters: the Damkohler number, Da, which represents the ratio of the rate of conversion by the bioreactor to the rate of perfusion; kappa, which represents the ratio of the rate of internal reequilibration between the tissue and blood compartments and the rate of perfusion; and Vtb, the tissue/blood volume ratio. The plasma perfusion model has three additional dimensionless parameters: f, the fraction of plasma withdrawn from the blood in a plasma separator; alpha, the ratio of the plasma perfusion rate in the bioreactor to the blood draw rate; and Vbr, the blood/plasma reservoir volume ratio. Within the physiologic range of parameters, the rate of reduction in blood concentration in both the whole blood-perfused and plasma-perfused systems are sensitive to Damkohler number up to Da approximately 2. Neither system is sensitive to variations in kappa, and the plasma perfusion system has little sensitivity to alpha. Given bioreactors of equivalent activity, a greater rate of blood concentration reduction and lower endpoint blood concentration at equivalent perfusion times will be achieved with whole blood perfusion. There are two physical reasons for this. The first is that the plasma perfused system is only processing a fraction, f, of the blood compared with the whole blood perfusion system. The second reason is that, although the blood-perfused system is limited by overall bioreactor performance, the plasma-perfused system is mass transfer limited to the rate of blood concentration dilution into the plasma reservoir rather than limited by the overall bioreactor performance.

D-galactosamine based canine acute liver failure model.

BACKGROUND: Appropriate preclinical evaluation of a bioartificial liver assist device (BAL) demands a large animal model, as presented here, that demonstrates many of the clinical features of acute liver failure and that is suitable for clinical qualitative and quantitative evaluation of the BAL. A lethal canine liver failure model of acute hepatic failure that removes many of the artifacts evidenced in prior canine models is presented. METHODS: Six male hounds, 24-30 kg, under isoflurane anesthesia, were administered 1.5 g/kg D-galactosamine intravenously. Canine supportive care followed a well-defined management protocol that was guided by electrolyte and invasive monitoring consisting of arterial pressure, central venous pressure, extradural intracranial pressure (ICP), pulmonary artery pressure, and end-tidal CO2. The animals were treated until death-equivalent, defined as inability to sustain systolic blood pressure >80 mmHg for 20 minutes despite maximal fluids and 20 microg/kg/min dopamine infusion. RESULTS: The mean survival time was 43.7+/-4.6 hours (mean+/-SE). All animals showed evidence of progressive liver failure characterized by increasing liver enzymes (aspartate transaminase from 26 to 5977 IU/L; alanine transaminase from 32 to 9740 IU/L), bilirubin (0.25 to 1.30 mg/dl), ammonia (19.8 to 85.3 micromol/L), and coagulopathy (prothrombin time from 8.7 to 46 s). Increased lability and elevations in intracranial pressures were observed. All animals were refractory to maintenance of cerebral perfusion pressure even with only moderately elevated intracranial pressure. Severe neurologic obtundation, seen in 2 of 6 animals, was associated with elevations of ICP above 50 mmHg. Post-mortem liver histology showed evidence of massive hepatic necrosis. Postmortem blood and ascites microbial growth was consistent with possible translocation of intestinal microbes. CONCLUSIONS: The improved lethal canine liver failure model presented here reproduces many of the clinical features of acute liver failure. The model may prove useful for qualitative and quantitative evaluation of BALs.

Transport advances in disposable bioreactors for liver tissue engineering.

Acute liver failure (ALF) is a devastating diagnosis with an overall survival of approximately 60%. Liver transplantation is the therapy of choice for ALF patients but is limited by the scarce availability of donor organs. The prognosis of ALF patients may improve if essential liver functions are restored during liver failure by means of auxiliary methods because liver tissue has the capability to regenerate and heal. Bioartificial liver (BAL) approaches use liver tissue or cells to provide ALF patients with liver-specific metabolism and synthesis products necessary to relieve some of the symptoms and to promote liver tissue regeneration. The most promising BAL treatments are based on the culture of tissue engineered (TE) liver constructs, with mature liver cells or cells that may differentiate into hepatocytes to perform liver-specific functions, in disposable continuous-flow bioreactors. In fact, adult hepatocytes perform all essential liver functions. Clinical evaluations of the proposed BALs show that they are safe but have not clearly proven the efficacy of treatment as compared to standard supportive treatments. Ambiguous clinical results, the time loss of cellular activity during treatment, and the presence of a necrotic core in the cell compartment of many bioreactors suggest that improvement of transport of nutrients, and metabolic wastes and products to or from the cells in the bioreactor is critical for the development of therapeutically effective BALs. In this chapter, advanced strategies that have been proposed over to improve mass transport in the bioreactors at the core of a BAL for the treatment of ALF patients are reviewed.

Oxygen consumption in a hollow fiber bioartificial liver--revisited.

Oxygen consumption dynamics in a hollow fiber, hepatocyte-loaded bioartificial liver are investigated both theoretically and experimentally. The theoretical model is based upon the Krogh cylinder, which approximates the bioreactor as a collection of cylindrical elements comprised of an inner fiber lumen for media perfusion, the fiber wall through which oxygen can diffuse, and an annular region of hepatocytes surrounding the fiber. The primary non-dimensional parameters that describe the system are: (i) the Peclet number, Pe, which is the ratio of convective oxygen transport through the lumen to diffusive oxygen transport to the fiber walls; (ii) the hepatocyte saturation parameter, theta, which is the ratio of the inlet oxygen partial pressure to the Michaelis-Menten half-rate oxygen partial pressure; (iii) the Thiele modulus, phi2, which is the ratio of oxygen consumption rate to oxygen diffusion rate in the hepatocyte annulus; (iv) the hepatocyte permeability ratio, beta31, which is the ratio of oxygen permeability in the hepatocyte cell mass to oxygen permeability in the perfusing lumen medium; and (v) the hepatocyte annular thickness, rho3, which is the ratio of the exterior hepatocyte annular radius to the fiber lumen radius. Only Pe and theta are easily manipulated operating variables. phi2, beta31, and rho3 are engineering design parameters that are set when a bioreactor is fabricated. The model results are expressed as the effective hepatocyte utilization ratio, Vratio, which is the ratio of the observed oxygen consumption rate to the intrinsic hepatocyte oxygen consumption rate. Large regions of Vratio > 0.9, which is deemed an acceptable effective hepatocyte utilization are found for parameter values consistent with standard hollow fiber cartridges used in bioartificial liver fabrication. The extent of the Vratio > 0.9 region increases to a plateau with increasing Pe, increases with increasing theta, decreases with increasing phi2, increases with increasing beta31, and decreases with increasing rho3. The theoretical results indicate that Vratio > 0.9 is found whenever the experimentally observed fractional oxygen consumption from the perfusing medium, is less than 0.25. Combination of the theoretical and experimental results indicate that intrinsic, per cell oxygen consumption in the hollow fiber system may decrease as hepatocyte cell density increases and that this decrease may be due to lower intrinsic oxygen requirements in denser suspensions and not due to diffusion limitations in oxygen transport in the hollow fiber system as might be expected from two-dimensional, monolayer culture oxygen consumption measurements.

First clinical use of a novel bioartificial liver support system (BLSS).

The first clinical use of the Excorp Medical Bioartificial Liver Support System (BLSS) in support of a 41-year-old African-American female with fulminant hepatic failure is described. The BLSS is currently in a Phase I/II safety evaluation at the University of Pittsburgh/UPMC System. Inclusion criteria for the study are patients with acute liver failure, any etiology, presenting with encephalopathy deteriorating beyond Parson's Grade 2. The BLSS consists of a blood pump; a heat exchanger to control blood temperature; an oxygenator to control oxygenation and pH; a bioreactor; and associated pressure and flow alarm systems. Patient liver support is provided by 70-100 g of porcine liver cells housed in the hollow fiber bioreactor. The patient exhibited transient hypotension and thrombocytopenia at initiation of perfusion. The only unanticipated safety event was a lowering of patient glucose level at the onset of perfusion with the BLSS that was treatable with intravenous glucose administration. Moderate changes in blood biochemistries pre- and post perfusion are indicative of liver support being provided by the BLSS. While the initial experience with the BLSS is encouraging, completion of the Phase I/II study is required in order to more fully understand the safety aspects of the BLSS.