Review article: liver support systems in acute hepatic failure

Review article: liver support systems in acute hepatic failure

T. M. RAHMAN & H. J. F. HODGSON

Department of Gastroenterology, Imperial College School of Medicine, Hammersmith Hospital, London, UK

Accepted for publication 7 May 1999

INTRODUCTION

Hepatic failure has long been a challenge to the

physician. Both acute and chronic hepatic failure

present a spectrum of clinical problems associated with

high morbidity and mortality. This review surveys

changing perspectives in the treatment of acute hepatic

failure (AHF).

The development of hepatic encephalopathy, jaundice

and coagulopathy de®nes AHF. Following Lucke and

Mallory's description of two distinct clinical patterns

observed in acute hepatitis, Trey & Davidson1, 2 intro-

duced the terms fulminant and sub-fulminant hepatic

failure. These have since been modi®ed to make the

classi®cation more accurate and universal. Their series

and other similar reports have helped to inform and

familiarize clinicians. Early diagnosis, development of

specialist centres and better understanding has reduced

mortality. Advances in intensive care monitoring,

management and pharmacological therapy have made

a signi®cant impact on survival. Whilst liver transplan-

tation remains the only de®nitive treatment for patients,

improved techniques and immuno-suppressive regimes

have improved long-term survival post-transplantation.

Organ availability, however, remains a limiting factor.

The clinical aspects of the management of AHF have

recently been reviewed in this journal.3

Over the last 50 years there have been advances in the

understanding of the deranged pathophysiology encom-

passing the clinical features of AHF: encephalopathy,

cerebral oedema, haemorrhage, electrolyte and meta-

bolic disturbance, renal failure, cardiovascular instabil-

ity and increased risk of infection. The `toxin hypothesis'

and the `critical mass theory', originally thought to

explain the changes, have been modi®ed by the

realization that synergistic forces (endotoxin and cyto-

kines) are also involved in AHF, and these views have

led to a range of approaches to arti®cial liver support.

Advances taking place in the treatment of chronic

renal failure were trialled in patients with chronic liver

disease and began an era of `non-biological liver

support'. Areas investigated include haemodialysis,

haemo®ltration, the use of adsorbents, plasma exchange

and later plasmapheresis. With the limited impact that

these systems provided, alternative approaches were

also investigated. The use of extra-corporeal perfusion of

xenogeneic and allogeneic cadaveric organs marked the

SUMMARY

The treatment of acute hepatic failure has developed

rapidly over the last 40 years, reducing morbidity and

mortality from this syndrome. Whilst this has been

partly attributed to signi®cant improvements in the

specialist medical management of these patients,

advances in surgical techniques and pharmaceutical

developments have led to the establishment of

successful liver transplantation programmes, which

have improved mortality signi®cantly.

This review will examine the clinical impact of

alternative methods that have been used to provide

extra-corporeal hepatic support. Non-biological, bio-

logical and hybrid hepatic extra-corporeal support will

be explored, offering a comprehensive historical

overview and an appraisal of present and future

advances.

Correspondence to: Prof. H.J.F. Hodgson, Department of Gastroenterology,

Imperial College School of Medicine, Hammersmith Hospital, Du CaneRoad, London W12 0NN, UK.

E-mail: [email protected]

Aliment Pharmacol Ther 1999; 13: 1255±1272.

Ó 1999 Blackwell Science Ltd 1255

advent of `biological liver support'. The effects of

xenogeneic liver homogenate, fresh liver slices, freeze-

dried liver granules and whole organs on patients in

AHF have been reported. Subsequently, technological

advances in cell biology and biotechnology allowed

more sophisticated systems to be developed. More

recently a class of `hybrid liver support' has undergone

clinical trials involving the use of biological tissue with

non-biological materials. Several investigators have

devised complex systems that combine human and pig

cryopreserved hepatocytes, providing the synthetic,

metabolic and excretory functions of the failing liver

with advanced biotech constructs. Preliminary reports

have identi®ed the feasibility of the approach, but

effectiveness has not yet been proven.

Finally, also in its infancy, `hepatocyte transplantation'

has been shown in animal studies to be effective in the

treatment of AHF.

This review will summarize previous and current

endeavours in arti®cial liver support.

THE ROLE OF HEPATIC SUPPORT

The unique and complex architecture of the liver goes

some way to explaining its diverse involvement in

maintaining metabolic homeostasis within the body.

AHF leads to deranged intracellular metabolism and

failure of interconversion of carbohydrates, lipids and

amino acids, and reduced synthesis of plasma proteins,

coagulation factors and lipoproteins. The loss of detox-

i®cation and biotransformation increases susceptibility to

further damage and may lead to an increased incidence of

infection. These metabolic abnormalities are thought to

cause the unique clinical features seen in AHF.

Decades of investigation have focused on two hypoth-

eses that attempt to link the metabolic changes to the

clinical features of AHF. The `toxin hypothesis' suggests

that the failing organ is unable to clear toxins normally

processed by the healthy liver from the bloodstream.

Animal and human studies of AHF have identi®ed the

presence and deleterious effects of ammonia, phenols,

mercaptans, aromatic amino acids, fatty acids, cyto-

kines and nitric oxide moeities. The `critical mass

hypothesis' suggests that a profound loss of hepatocel-

lular metabolic capacity below a critical threshold leads

to end organ dysfunction and failure to support

peripheral organ function. More recent investigation

suggests that the hepatocyte itself, once injured,

contributes to the ampli®cation of liver injury. The

reduced integrity of the cell membrane, the loss of

intracellular homeostasis, imbalance of pro-oxidant and

antioxidant pathways, mitochondrial damage and de-

pletion of ATP all contribute to the generation of toxic

species, decreased cytoprotective capabilities and alter-

ations in cell-to-cell interactions. Both animal and

human studies have shown that, under extreme

circumstances, total hepatectomy improves the clinical

stability of the subject in AHF.4

The role of arti®cial hepatic support is therefore a

complex one. It must encompass several roles, i.e. the

removal of toxins incriminated in the pathobiology of

AHF, the synthesis of products such as coagulation

factors, albumin and other plasma proteins, and it must

also attempt to reverse the massive in¯ammatory

process taking place within the failing organ. Ideally,

hepatic support should provide metabolic, synthetic and

detoxi®cation functions, allowing time for recovery and

regeneration of the host organ. Where regeneration is

not possible, hepatic support may allow time for organ

transplantation to take place.

NON-BIOLOGICAL HEPATIC SUPPORT

Haemodialysis

Inspired by successful advances taking place in the

management of chronic renal failure in the 1950s,

extra-corporeal technology was applied to patients with

chronic liver disease. Haemodialysis requires a semi-

permeable dialysis membrane through which ¯uid and

small solutes may pass, whilst exchange occurs against

dialysis ¯uid. Transfer of solutes and molecules occurs

by diffusion.

Haemodialysis had been shown to improve survival in

patients with chronic renal failure and other diseases

such as porphyria.5, 6 Zysno et al.7, 8 demonstrated

improvements in EEG recordings that correlated with

clinical improvement in severely uraemic patients. This

principle was applied to a group of chronic cirrhotic

patients, demonstrating improved encephalopathy and

an associated decrease in arterial ammonia levels.9 In

1968 a patient in acute liver failure was treated with

haemodialysis and showed temporary improvement in

their clinical status following treatment.10 (Table 1

illustrates the various studies discussed.)

Success was also reported by Oules et al.,11 who

suggested that removal of soluble, uncharacterized

compounds in the `middle' molecular weight range

1256 T. M. RAHMAN & H. J. F. HODGSON

Ó 1999 Blackwell Science Ltd, Aliment Pharmacol Ther 13, 1255±1272

(400±2000 Da) improved encephalopathy in patients

with chronic liver disease and also the clinical status of

patients with chronic renal failure.12 However, these

observations were challenged by several investigators,

who suggested that the toxaemia of AHF was not due to

middle molecules, based on the results of a larger trial

showing minimal improvement in clinical state.

Application of haemodialysis became more widespread

in the early 1970s and reports of improvement in lactic

acidosis13 and of improved survival (individual case

reports) following paracetamol, salicylate and ben-

zodiazepine overdoses also appeared. Advances in

membrane technology allowed Opolon et al.12 to dem-

onstrate improved neurological status in animal models

of AHF using polyacrylonitrile (PAN) membranes. This

was followed by a clinical trial using PAN membranes

in 1975,14 in which ®ve complete neurological and two

partial recoveries were reported in a group of nine non-

decerebrate patients in coma due to `acute hepatic

atrophy'. No improvement in survival was demonstrated.

Similar ®ndings were reported by Denis et al.15 in 41

patients with fulminant hepatic failure who had

between them 180 treatment periods. Seventeen

patients had complete neurological recovery from coma

and seven had partial recovery. Of those that had

complete neurological recovery, nine survived.

Whilst PAN membranes allowed increasing solute

removal, it and other similar polymers such as polysul-

phone were unable to remove protein-bound and

lipophilic substances. An alternative approach was

developed using a liquid membrane that allowed the

removal of lipophilic toxins but prevented the removal

of physiological substances such as hormones. The

structure consisted of a hydrophobic polysulphone

membrane with large voids containing paraf®n oil.

Blood passed through the liquid membrane ®lter with a

sodium hydroxide acceptor solution on the other side.

Protein-bound toxins contacting the membrane were

released into the oily layer, diffused through the

membrane and became water-soluble due to a reaction

with the accepting solution. Application in pigs has

shown improvement in neurological function.16

Haemodialysis was further adapted by Stange &

Mitzner,17 who introduced the Molecular Adsorbents

Recirculating System (MARS) in 1993. It had been well

known that some toxins were avidly protein bound and

that standard haemodialysis membranes did not remove

these. The polysulphone membrane was impregnated

on both sides with albumin and dialysis took place

against a closed loop dialysate also containing 10±20%

albumin. In vitro studies have shown enhanced removal

of protein-bound toxins such as bilirubin, bile acids,

Table 1. Non-biological hepatic support in man

Non-biological hepatic support Subjects Outcome

Haemodialysis (Acute and chronic hepatic failure)

Kiley et al.9 Improved neurological

Keynes10 outcome. Improved

Opolon et al.14 9 patients encephalopathy

Denis et al.15 41 patients (AHF)

Klammt et al.18; Mitzner et al.19 Decreased urea, creatinine, bilirubin

Haemo®ltration

Bellomo et al.28 Animal and human sepsis Reduced IL-6, IL-1, TNFa,

Ronco & Bellomo26 C1q, C3a, C5a. Improved clinical status

Adsorbents (Acute and chronic hepatic failure)

Charcoal

Gazzard et al.35 22 patients Improved neurological status/survival

O'Grady et al.42 137 patients No bene®t in survival

Haemodiabsorption

Ash et al.57, 58 Acute and chronic hepatic failure Decreased bilirubin, lactate, creatinine. No

survival bene®t

Plasmapheresis

Larsen et al.68 12 patients (AHF) Increased survival

Clemmesen et al.20 16 AHF 11 chronics Improved ICP, CBF, CPP,

Larsen et al.67 40 AHF and chronics CMRO2, CO, MAP

REVIEW: SUPPORT SYSTEMS IN LIVER FAILURE 1257


tryptophan and fatty acids. The success of this tech-

nique is dependent on complex physiochemical processes

involving speci®c trilateral interactions between ligands,

binding proteins and the polymer. More recent clinical

studies have demonstrated improved clinical parameters

(encephalopathy and renal function) with reductions in

bilirubin, urea and creatinine in eight patients with

decompensated chronic liver disease following a total of

47 single treatments.18, 19

Despite the improvements in technology and apparent

effects on encephalopathy, there is no proof of effective-

ness in altering survival. Haemodialysis is therefore not

currently recommended for use in the treatment of

AHF. Haemodynamic instability may occur, as well as

electrolyte and ¯uid shifts, and these changes can

exacerbate cerebral oedema, leading to increased intra-

cranial pressure; they may also have adverse effects on

cardiac output and other haemodynamic parame-

ters.20, 21 High-volume haemodialysis has, however,

been used together with plasma exchange in AHF, as

discussed below.

Haemo®ltration

Haemo®ltration has been shown to be more suitable

than haemodialysis for the treatment of some patients

in renal failure following sepsis, trauma and liver

failure. Haemo®ltration uses a permeable membrane

and relies on continuous convective solute removal,

avoiding large volume and electrolyte shifts. There is no

dialysate ¯uid, only a substitution solution replacing the

ultra®ltrate. Convective removal of solutes has been

shown to be more ef®cient than haemodialysis for the

removal of molecules with molecular weights of 2000±

3000 Da. This range corresponds to the `middle'

molecules thought to be involved in encephalopathy

and liver failure.

Continuous venous±venous haemo®ltration is pre-

ferred in AHF because it offers haemodynamic stability

and allows predictable and gradual control of metabolic

disturbance.21, 22 The substitution solution for AHF can

be lactate-free and bene®ts have been observed using a

bicarbonate-buffered solution.23 Venous±venous circuits

are commonly used and require anticoagulation with

heparin, low molecular weight heparin or prostacyclin if

the platelet count is low.24 Potentially, the bene®cial

effects include ef®cient removal of middle and indeed

larger molecules, allowing immuno-modulation with the

removal of vaso-active substances from the circulation,

including immunoglobulins, IL-1, IL-6, TNF-a, C1q, C3a,

C5a and platelet activating factor.25±27 Removal of these

may be advantageous in sepsis, multi-organ failure and

systemic in¯ammatory response syndrome, all variants

of that seen in AHF. Bellomo et al. described high-

volume exchange haemo®ltration of up to 6 L/h, which

has demonstrated improved survival in animal studies of

sepsis.27 This has recently been demonstrated in a

clinical setting.28

Haemodia®ltration

There are advantages also in haemodia®ltration, which

combines the advantages of both convection (removal of

larger molecules, 2000±3000 Da) and diffusion (re-

moval of smaller molecules, 400 Da or less). Yoshiba

et al.29 used this technique in an uncontrolled series of

patients with AHF, sub-acute hepatic failure and late

onset hepatic failure, demonstrating survival rates of

85, 54 and 50%, respectively. Adverse effects included

complement activation, activation of the coagulation

cascade and release of vaso-active and chemo-attractant

fragments.

Adsorbents

There has been considerable focus on the use of different

adsorbents as a part of non-biological hepatic support

systems to remove toxins or middle molecules thought

to contribute to the clinical manifestations of AHF. Most

work in this area has involved the study of three major

types of sorbents, including activated charcoal in

various forms, synthetic neutral resins (XAD-2,4,7)

and anion exchange resins (Dowex-1). This work has

recently been further developed with the combination of

charcoal and a cation exchange resin in the form of the

Biologic-DT system.55±58

Charcoal. Yatzidis30 demonstrated the adsorbent prop-

erties of coconut charcoal in a haemoperfusion circuit.

This provided effective removal of creatinine, uric acid

and guanidine from uraemic dogs. Removal of pheno-

barbitone in dogs and in vitro and in vivo work in pigs

demonstrating the removal of paracetamol using char-

coal haemoperfusion led to isolated clinical case

reports.31±33 Problems with biocompatibility of charcoal

encouraged adaptations of its use. Chang34 used

microencapsulated charcoal for haemoperfusion in

AHF, recording complete recovery of consciousness



from hepatic coma in a patient with alcoholic hepatitis.

Microencapsulation stops ®ne particles of charcoal

escaping into the circulation, thus improving its bio-

compatibility. Gazzard et al.35 reported the use of

charcoal haemoperfusion in 22 cases of AHF with

grade IV hepatic coma in an uncontrolled trial.

Recovery of consciousness and improved survival was

demonstrated. Subsequently several groups attempted

to repeat these observations, but were unsuccessful,

encountering problems such as intractable hypotension,

charcoal particle emboli, and thrombocytopaenia.36, 37

To improve the biocompatibility of charcoal Weston

et al.38 examined the advantages of coated and uncoated

charcoal. However, little difference in leucocyte and

platelet loss was demonstrated. Gelfrand et al.39 exam-

ined the use of charcoal coated with acrylic hydrogel,

which resulted in improved clinical tolerance to

haemoperfusion. Uncontrolled clinical trials showed

improved levels of consciousness in 9 of 10 patients in

hepatic coma and complete recovery in 40%.

Gimson et al.40, 41 used prostacyclin to prevent plate-

let activation in clinical trials of charcoal haemoper-

fusion. Two groups were studied, those in grade III and

those in grade IV encephalopathy. The use of charcoal

haemoperfusion in the grade III group led to a better

rate of survival (65%) and a reduced incidence of

cerebral oedema (49%) as opposed to those in the

grade IV group (20% survival, 78% cerebral oedema).

Contrasting ®ndings were reported in the largest

controlled trial of charcoal haemoperfusion undertaken

by O'Grady et al.42 One hundred and thirty-seven

patients were entered into two controlled trials run

concurrently. Trial A randomized 75 patients in grade

III encephalopathy to 5 or 10 h of charcoal haemo-

perfusion. There was no signi®cant bene®t seen in

survival, incidence of renal failure or cerebral oedema

in the two groups. Trial B randomized 62 patients in

grade IV encephalopathy to charcoal haemoperfusion

or no haemoperfusion. Survival rates were similar

(39.3 and 34.5%, respectively). The conclusion was

that charcoal haemoperfusion had little in¯uence on

survival or incidence of cerebral oedema and renal

failure. The additional conclusion was reached that the

apparent effectiveness of the approach, noted in

uncontrolled trials during the 1970s and early

1980s, was due to the use of historical controls and

in particular re¯ected the progressive general enhance-

ment of intensive care of patients in severe liver

failure.

Other adsorbents. The properties of other adsorbents

have been explored in an attempt to overcome the

biocompatibility problems encountered with charcoal.

The use of Dowex 50-X8, a cation exchange resin, was

reported in dogs with hepatic coma.43 Blood ammonia

levels were reduced by 50% and concurrent clinical

improvement encouraged its use in humans. Small

uncontrolled trials showed reversal in coma and

reduction of ammonia levels in 20% of patients.44

Dowex 50-X8 and Amberlite IR 120 (another cation

exchange resin) were trialled in 13 patients with

hepatic failure, with reports of improvement in con-

scious level in 54% of patients.45±47 Amberlite XAD-7,

an albumin-coated macro-reticular resin, was shown to

reduce plasma total bilirubin in 19 patients with AHF, of

whom eight left hospital.48 Its ability to remove TNF-a,

IL-6 and IFN-a, as compared with charcoal, was also

demonstrated.49, 50

More speci®c resins have been developed and tested

with recorded improvement in clinical parameters and

survival. Examples include polyamine triglycidylisocy-

anurate (PAT) resin, polylysine-immobilized chitosan

beads for removal of bilirubin, and polymyxin B

immobilized on polystyrene ®bres for the removal of

endotoxin.51±54 The last two in particular have been

associated with biocompatibility problems and despite,

or perhaps re¯ecting, the multiple types trialled, no

adsorbents have come into widespread clinical usage.

Haemodiabsorption

The Biologic-DT system55 is currently in use at a small

number of centres. This system combines charcoal and

cation exchanger in a system called `haemodiabsorp-

tion'. Blood is dialysed across a parallel plate dialyser

with a cellulose membrane that has sorbent present at

the membrane surface. The sorbent contains powdered

charcoal, sodium and a loaded cation exchange resin.

Animal studies have examined the effect of the

Biologic-DT system in dogs with AHF induced by a

two-step devascularization procedure. Following 6 h of

treatment the treated animals were more physiologically

stable, developed less lactic acidosis, had reduced

transaminase increases and also maintained higher

blood glucose levels than the untreated controls.56

Uncontrolled clinical trials in patients with hepatic

failure have recorded improved neurological status and

normalization of diastolic blood pressure in treated

patients.57, 58 Controlled trials, however, have shown



no bene®t in survival in AHF but plasma lactate,

creatinine and bilirubin were reduced. Increased

thrombocytopaenia, decreased ®brinogen and an

increase in the activated clotting time were, however,

seen in the Biologic-DT treated group. Neurological

improvement has been recorded in chronic decompen-

sated patients.59

Plasma exchange and plasmapheresis

The removal of toxins from plasma and replacement

with plasma from healthy individuals was ®rst performed

by Lepore et al.60, 61 They reported neurological im-

provement prior to death in two patients out of nine

with AHF. This followed one to 12 treatments with 10±

83 L of plasma exchanged. Patient survival was dem-

onstrated by Buckner et al.,62 who treated four patients

with 10 L exchanges per day for 3±36 days. Three

patients survived in this uncontrolled trial. The devel-

opment of membrane separators led to several groups

claiming increased survival.63, 64

In 1985, Winikoff et al.65 suggested that plasmaphere-

sis was only a bridge to liver transplantation (OLT).

Berk53 demonstrated the theoretical advantage of high-

volume plasmapheresis, suggesting that the volume of

distribution of toxins would correspond to the extracel-

lular space. The development and subsequent use of

high-volume plasmapheresis (HVP) for the treatment of

AHF has since been re®ned.

Kondrup et al.66 investigated the effect of HVP in 11

patients with AHF, all initially in grade III or IV

encephalopathy. An average of 2.6 exchanges, each

with a mean volume equivalent to 16% of body weight,

were performed. Five of the 11 patients, all with

acetaminophen poisoning, survived. The six non-survi-

vors remained haemodynamically stable during treat-

ment for a mean of 6.9 days. The conclusion was that

HVP could be used as a bridge to transplantation, and

also that patients who have residual liver function may

be supported until their own organs recover.

A larger series of 40 patients (mixed aetiologies)

receiving HVP reported 28 survivors (54%). Seventeen

patients received OLT, of whom three later died. The

documented improvement was in parameters of cerebral

blood ¯ow (CBF), cerebral perfusion pressure (CPP),

cerebral metabolic rate for oxygen (CMRO2), and mean

arterial pressure (MAP). No episodes of raised intracra-

nial pressure (ICP) were reported. Systemic vascular

resistance (SVR) increased and cardiac output (CO)

appropriately decreased, an effect lasting 12 h. These

changes imply improved tissue oxygen extraction

systemically and in the brain.67 Similar haemodynamic

®ndings were reported by Clemmesen et al.20 who

suggested that the removal of a humoral factor by

HVP may explain these results.

The effect of HVP on intracranial parameters, CBF,

CMRO2 and oxygen extraction has been reported in 12

patients with AHF. Encephalopathy was seen to im-

prove in four patients and improvements in CBF and

CMRO2 were statistically signi®cant.68 This improve-

ment was thought to re¯ect partial removal of neuro-

inhibitory plasma factors.

There is therefore currently some evidence that HVP

improves haemodynamic parameters, reduces cerebral

oedema and prolongs survival in those awaiting OLT. It

may be that it can be effectively used to sustain life in a

small group of patients that have some recovering

hepatic function, and might be of bene®t in patients

who are not awaiting transplantation.

BIOLOGICAL HEPATIC SUPPORT

Xenogeneic and allogeneic extra-corporeal support have

been investigated for 50 years. Several techniques have

been used, all of which have had only limited success

(Table 2). Approaches that have been explored include

cross-haemodialysis with animals, and extra-corporeal

haemoperfusion with pig, baboon and human livers.

Haemoperfusion has been extended to pass blood over

liver fragments and also liver cells. All techniques

performed have shown some improvement in clinical

and/or physiological parameters but not improvement

of survival in controlled trials.

Xenogeneic support has the disadvantage that naturally

occurring antibodies in the human recipient will react

with endothelial expressed antigens in the perfused liver.

This rapidly leads to complement activation, activation of

the clotting cascade, and haemodynamic instability.

Cross-haemodialysis

Kimoto performed a technique called cross-heterohae-

modialysis in 1957, which involved haemodialysis in a

man in hepatic failure against the circulation of a live

dog.85 This resulted in improvement of ammonia levels.

This technique was further adapted with the addition of

an ion exchange column by Hori et al.69,70 who

performed cross-haemodialysis between a man in AHF



and a dog separated by a semi-permeable membrane.

This was associated with recovery from hepatic coma

and a corresponding reduction in blood ammonia.

Extra-corporeal haemoperfusion

Eiseman et al.71 in 1965 described the ®rst experience of

heterologous liver perfusion in the treatment of liver

failure. This technique demonstrated bile ¯ow, galactose

elimination, clearance of ammonia and bilirubin in a

pig liver extra-corporeally perfused with human blood.

Abouna72, 73 repeated this technique using pig livers on

10 patients with AHF, seven with acute hepatitis and

liver cell necrosis and three with pre-existing liver

disease. Improvements in conscious state and survival

greater than 4 days was recorded, but only two patients

survived.73 Waldschmidt et al. haemoperfused nine

patients with AHF using isolated pig livers and reported

one survivor, and transiently improved conscious levels

of the other patients who later died.74

Isolated baboon livers were also used to provide extra-

corporeal support in the treatment of 14 grade IV

encephalopathic patients. Thirteen patients had viral

hepatitis and one had developed drug-induced AHF.

Thirty perfusions were performed (29 baboon livers and

1 human cadaveric liver) each lasting 5±27 h, with 1±4

perfusions per patient. Bile excretion peaked at 7±8 h

and was used as a marker for successful perfusion. Lie

et al.75±77 claim a 50% survival rate of patients with

AHF, when perfused by this technique. Baboon livers

were also used by other groups who have claimed

survival rates of 34±45%. No controlled trials exist to

compare treatment regimes.78

Extra-corporeal liver perfusion using human livers

procured and unused by the United Network for Organ

Sharing was reported in three patients with AHF in

1993.79 All patients showed improved serial serum

bilirubin and arterial ammonia values, while two of three

patients also showed marked neurological improvement.

These patients were later successfully transplanted. The

third patient failed to show clinical improvement and died

7 days after the treatment was discontinued.

Common problems encountered with xenogeneic

haemoperfusion were: activation of the clotting cascade,

antibody and immune complex formation, and the

expected sequelae. This limited the length of perfusion

time per session and its repeated use. Technical

problems also include the requirements of large num-

bers of trained surgical, nursing, technical and ancillary

staff and the supply of animals for each haemoperfusion

session. In the case of cadaveric human liver extra-

corporeal perfusions, organs and transplant surgeons

are of limited availability.

Transgenic organ transplantation

Despite problems with organ availability, immunolog-

ical incompatibility, hyperacute rejection and ethical

dilemmas, the role of xenogeneic transplantation is an

area of current interest. The development of trans-

genic animals with genetically modi®ed immune

expression to reduce incompatibility for use in organ

transplantation is now the subject of ethical debate

and approval. Manipulation of the animals' genome

involves the insertion of human anti-complement

genes and modi®cation of their xenoantigens. Prelim-

inary studies have perfused human blood into isolated

transgenic pig livers expressing human complement-

ary±regulatory protein, human CD59 and human

decay-accelerating factor (hDAF). Results show

Table 2. Biological hepatic support in man

Biological hepatic support Technique No. of patients Outcome

Hori et al.70 Cross-heterohaemodialysis 1 patient Improved neurological status

Dog/man Decreased NH3

Eiseman et al.71 Heterologous liver perfusion 10 patients Bile ¯ow, galactose elimination,

Pig/man decreased NH3

Abouna72 Heterologous liver perfusion 10 patients Improved neurological status and

Pig/man survival

Lie et al.75 Heterologous liver perfusion 14 patients 50% survival

Baboon/man

Fox et al.79 Extra-corporeal human cadaveric 3 patients Decreased NH3, bilirubin.

Survival 2/3



reduced complement and TNF-a expression and

reduced activation of the hyperacute rejection pro-

cess.80±82 Expression of porcine MHC class II is a

potent stimulator of human CD4+ T-cells and is

thought to be dependent on class II transactivator

(CIITA), a bi- or multifactorial domain protein.

Manipulation of this protein has successfully demon-

strated potent suppression of MHC class II expression

and may lead to prolonged graft survival.83

Practical use at present still remains some way off.

Even without the complexities that xenotransplantation

introduces (such as the potential for spread of viruses

from animal to man), there is much that needs to be

addressed with respect to perfusion of whole livers

extra-corporeally, necrosis, apoptosis, ischaemia and

reperfusion injury, and allogeneic rejection.84

HYBRID HEPATIC SUPPORT

Hybrid hepatic support combines the use of biological

tissue with the use of non-biological materials. The use

of hepatic tissue may provide synthetic, excretory and

biotransformational functions, which combined with

membranes removing cytokines and other toxins, is

thought to be bene®cial in AHF.

Recent advances in the isolation and characterization

of hepatocyte function and growth, and the require-

ments for in vitro function, have allowed the use of

colonies of cells in hybrid liver support. Bioreactor

designs have changed to allow for optimum integrity

and function of cells taking into account the complex

biophysics involved in oxygenation, removal of waste

products and ¯ow dynamics.

The concept of hybrid support was introduced in the

last section, and was developed by the pioneers Kimoto

and Hori.85, 86 Their techniques were used by others

and cleverly adapted by Mikami et al.87 and Nose

et al.88 An extra-corporeal circuit was used to assess

the function of liver tissue homogenate, fresh liver slices

and freeze-dried granules of liver tissue in clinical trials.

Patients were connected to a dialysis circuit, and blood

was then passed to a `metabolic circuit' which con-

tained a gel-type cellulose membrane, a bubble oxygen-

ator and a chamber or `bioreactor' containing either

liver homogenate, liver slices or freeze-dried granules.

Limited clinical application demonstrated stable glucose

and lactate levels and a reduction in ammonia levels.

The use of liver fragments has been further developed

and now colonies of cells are being used in similar but

more sophisticated arti®cial liver support systems.

Parallel advances have been taking place in biotech-

nology, study of cell growth, function, integrity and

manipulation. Cells may now be genetically manipulat-

ed, cell lines immortalized and cryopreserved.

Hepatocytes

Ideally, liver cells maintained in an extra-corporeal

environment should express the full functional reper-

toire of the normal liver. As the liver contains various

sub-populations, not only the major metabolically

active cells, i.e. the hepatocytes, but also Kupffer cells,

sinusoidal endothelial cells and stellate (mesenchymal-

derived) cells providing extracellular matrix and growth

factors should ideally be present. This is clearly a major

challenge. However, there is general consensus that the

most important functions of an extra-corporeal liver

circuit, i.e. detoxi®cation and synthesis, are largely

(although not totally) ful®lled by the hepatocytes.

Calculations indicate that there are of the order of

1±2 ´ 1011 hepatocytes in a normal adult human liver.

Data from surgical resections indicate that probably

one-third of this number is suf®cient for normal survival,

but of course a larger number may be required to reverse

the changes of hepatic failure. Of course, data from

surgical experience also de®nes the numbers of cells that

are adequate when they are in the optimum, i.e. natural,

con®guration as components of a normal liver.

The ®rst major issue in providing hepatocytes for such

systems is that the mature adult hepatocyte is a non-

dividing cell. It is capable of entering DNA synthesis and

dividing in vivo, but in vitro there are only specialized

circumstances in which limited rounds of cell division

can be achieved. The option of taking a limited number

of adult human hepatocytes and letting them proliferate

in culture to provide the number required, is currently

not available. Scienti®c approaches to the limited

replicative potential of adult hepatocytes in vitro involve

identifying growth factors and media that will allow

proliferation, and/or genetically manipulating cells to

remove the normal checks to division in the adult cell.89

The use of foetal or neonatal hepatocytes with a greater

proliferative rate is also being explored.90 However,

until such techniques are truly successful, workers have

to use one of two optionsÐeither utilizing cell lines from

human livers, or using primary cells from other species.

Current evidence indicates that any proliferating

hepatocyte cell line from a human liver falls short in



some respects from a fully functional repertoire. On the

other hand the use of xenogeneic cells has the same

problems as have been alluded to under xenogeneic

haemoperfusion, due to naturally occurring antibodies.

It is possible, however, with the use of diffusion barriers

to prevent direct exposure of xenogeneic cells to human

antibodies and cells. This should delay immediate

deleterious immune responses whilst a patient is

connected to an extra-corporeal circuit, but the diffu-

sion barriers which prevent exchange of proteins and

protein-bound small molecules will also limit the

effectiveness of functional replacement.

The second set of issues in providing adequately

functioning hepatocytes derives from our expanding

knowledge of the requirement for maintenance and

expression of fully differentiated function. Primary

hepatocytes in the most straightforward con®guration

of tissue cultureÐas a monolayerÐrapidly lose differ-

entiated function. A variety of techniques can maintain

function: co-culture with non-parenchymal cells, cul-

ture as multilayer spheroids and exposure of cells to

extracellular matrix proteins. In essence, maintaining

hepatocytes with normal occupancy of their surface

integrin receptors,91 and normal cell-to-cell contact, is

probably the critical feature for maintaining function.

This can be achieved by, for example, encapsulating

cells in substances such as alginates;92 such techniques

also tend, by producing compact cell masses, to reduce

the volume required to hold 3 ´ 1010 cells and thus

render extra-corporeal circuits practicable. However,

too compact a cell mass imperils transfer of nutrients,

cell products, and, perhaps most importantly, oxygen.

Thus the micro-design of the cell aggregates, and the

macro-design of the bioreactor, are of fundamental

importance.

Bioreactor design

The successful development of the arti®cial liver is

dependent on the hepatocyte component, matrix sup-

port and bioreactor design. Much has been invested in

the design of the `bioreactor', which will allow optimum

cell culture, storage and cell integrity allowing its

practical use in the circuit designed for support.

Three general forms of hepatocyte culture have been

identi®ed: suspension culture, attachment culture and

hepatocyte spheroid culture. Suspension culture is the

least effective method as hepatocytes lose function

rapidly.93 Advantages, however, include low gradients

of nutrients, metabolites, toxins and oxygen, allowing

high transfer ef®ciency.94, 95

Attachment culture has been shown to maintain cell

integrity and function.96 This has been used extensively

in arti®cial liver support systems that use hollow ®bre

ultra®ltration cartridges. These devices rely on trans-

membrane diffusion for exchange of metabolites and

this may lead to reduced ef®ciency.

Hepatocyte spheroid culture allows optimal distribu-

tion of media around hepatocytes, allowing high mass

transfer ef®ciency. This may be reduced when cells have

been encapsulated and/or coated.93, 97, 98

The choice of culture suspension will in¯uence the

design, materials and construction of the bioreactor as

ef®cient attachment and growth will be required. Three

basic designs have emerged; bioreactors for use with

suspension culture, bioreactors based on cell immobili-

zation and those with membranes.

Most bioreactors have used capillary membranes

within a cartridge for cell attachment. Capillary mem-

branes allow a number of other functions to take place

(gas exchange, substrate supply and waste removal)

ef®ciently and with practical ease. Hepatocytes may be

seeded, cultured and grown within capillary mem-

branes and perfused in the extra-capillary space pro-

viding mechanical and physiological protection from

toxic blood or plasma. This approach has been used by

Nyberg et al.99, 100 in the three-compartment gel

entrapment bioreactor. This entraps porcine hepato-

cytes in a collagen matrix inoculated into the capillary

lumen spaces of two 100 kDa molecular mass cut-off

hollow ®bre bioreactors.

An alternative approach is to construct a bioreactor

with hepatocytes in the extra-capillary space with

capillary membranes providing the in-¯ow and out-

¯ow of media, oxygen, nutrients, toxins and waste.

Capillary membrane constructions rely on transmem-

brane diffusion for mass transfer and so choice of

materials is also of paramount importance.

Construction of the three arti®cial liver support devices

that have had clinical exposure will be reviewed:

Sussman's Extra-corporeal Liver Assist Device (ELAD),

Demetriou's Bioarti®cial Liver (BAL) and Gerlach's

hybrid liver support system.

ELAD, developed by Sussman et al.101 in 1992, incor-

porates the C3A cell line. This is a highly differentiated

clonal population isolated from a human hepatoblas-

toma cell line, HepG2. Two hundred grams of cells were

originally seeded and grown in the extra-capillary space



of a haemodialysis cartridge containing » 10 000 hollow

®bres with a surface area of 2 m2. Two to four weeks is

required for adequate numbers of cells to have grown for

clinical use. They may then be stored and studies have

shown good function (glucose, albumin secretion) after

8 months. The intra-capillary space is used in the growth

period for culture medium and oxygen supply and later in

clinical use this space is used for perfusion of blood.

Diffusion gradients and mass transfer is felt to be ef®cient

as ®bres are approximately four cells apart, allowing

adequate oxygenation and waste removal. The mem-

brane has a molecular weight cut-off of 70 000 kDa,

protecting hepatocytes from ¯ow trauma, white cells and

immunoglobulins, but allowing middle molecules and

ammonia to pass across the membrane.

Recent modi®cations include two main design chan-

ges, i.e. increased cell numbers used per cartridge

(700 g) and adaptation of the circuit so that it can be

perfused with plasma and not blood (1999).

The BAL system, developed by Demetriou and

co-workers, is conceptually very similar to ELAD but

originally had three major differences: the cell source,

i.e. primary pig hepatocytes rather than a human

tumour derived cell line; the perfusate, which is plasma

rather than blood; and the presence of a charcoal

column ®ltering the plasma prior to its entry to the

bioreactor.102, 103

Hepatocytes are isolated from pigs and attached to

collagen-coated dextran microcarriers. More recently

cryopreserved cells have been used. The hollow ®bre

bioreactor consists of a polycarbonate cylinder contain-

ing cellulose nitrate/cellulose acetate porous ®bres.

Fibres have a pore size of 0.2 lm and a total internal

surface area of about 6000 cm2. The total extra-

capillary surface area is 7000 cm2.

The BAL system comprises a plasma separator, gener-

ating plasma (80±105 mL/min) from venous blood and

passing this to the charcoal column. The plasma is then

directed across the bioreactor at high ¯ow rates (220±

500 mL/min) which allows several passes before it is

passed back to the individual.

Gerlach et al.97, 98, 104 described a more sophisticated

hybrid liver support system. The structure is housed in a

polyurethane PUR 725 case. The bioreactor is made up

of several interwoven, independent polyurethane capil-

lary systems, entering and leaving the bioreactor in four

discrete bundles and each serving a different function.

The four capillary bundles provide plasma in-¯ow,

oxygen supply and carbon dioxide removal, plasma

out-¯ow, and sinusoidal endothelial co-culture. Hepa-

tocytes (pig hepatocytes) are seeded in the extra-

capillary space and ®nd all types of bundles locally,

thus reducing transmembrane diffusion gradients. The

design can be adapted by the addition of further

bundles, allowing additional functions to be incorpo-

rated. The design is such that many identical capillary

units supply only a few hepatocytes, thus allowing small

diffusion gradients.

Trials

Both ELAD and BAL have been trialled extensively

in vitro, in vivo animal work and also in clinical trials.

(Table 3 illustrates the clinical trials.)

ELAD. Kelly et al.105 performed the ®rst in vivo trials in

1992. Six male beagles had portacaval shunts inserted

and then underwent a total hepatectomy. Dogs were

kept sedated and were given sodium bicarbonate and

glucose maintenance infusions to avoid hypoglycaemia.

ELAD was perfused by vascular access via the internal

carotid artery and the external jugular vein. Blood was

driven through ELAD solely by arterial pressure and

maintained at a ¯ow rate of 80±100 mL/min. The

circuit was heparinized to avoid clot formation. Of the

six beagles, three control dogs lived for 3±5 h after

surgery but did not awake from the anaesthesia. Two of

the three hepatectomized dogs also died within 3±5 h of

surgery but some did require extra sedation. The third

hepatectomized dog survived 125 h post-surgery. Plas-

ma ammonia levels were seen to increase following

surgery in control animals, and this was felt to be a pre-

terminal event. This was not seen in the ELAD-treated

dogs.

Further work was carried out on an improved model of

AHF using intravenously administered acetaminop-

hen.106 Control animals developed AHF with worsening

encephalopathy, hypoglycaemia, prolonged prothrom-

bin time and severe transaminitis. Death occurred in

all controls at 15±30 h. Dogs treated with ELAD for

42±48 h had a survival rate of 80%.

Technical modi®cations were made before the system

underwent trials in man. These included a switch to a

venous±venous circuit and also an increase in ¯ow rate

that allowed the formation of an ultra®ltrate, which

would then bathe the cells within the extra-capillary

space. Filters were inserted to stop any cell debris

entering the patient's circulation, as some concern was



raised over the use of a hepatoblastoma cell line

metastasizing in the patient.

Initial trials between 1991 and 1993107±109 were

carried out in four centres and 11 AHF patients were

treated with ELAD. All patients were in grade III/IV

encephalopathy, nine of whom required mechanical

ventilation. One patient had been made anhepatic

following primary graft failure. The age range was

9±63 years old and treatment times ranged between 9 h

and 144 h. Encephalopathy improved in eight out of 11

patients and renal function was preserved in those who

were not anuric at the start of treatment. Improvement

in the galactose elimination capacity was demonstrated.

Of the 11, six patients died, four received liver trans-

plantation and one survived without transplantation.

Following the initial studies, a controlled clinical trial

containing 24 patients in two risk-strati®ed groups was

initiated. Groups I and II had predicted survivals of 50%

and less than 10%, respectively. Patients were then

randomly allocated intensive care only or intensive care

and ELAD treatment within each group. Patients were

treated for a mean period of 72 h (3±168 h). Results

showed improved galactose elimination time, and

encephalopathy, but no survival changes; Group I had

78% survival compared with 75% in the control group;

group II also showed no difference, with 33% survival

compared to 25% in controls.110

BAL. Rozga et al. reported the use of BAL in the

treatment of an animal model of AHF. Dogs were

ventilated, had portacaval shunts inserted and then

hepatic and gastroduodenal arteries were occluded.

They were divided into three groups. Group 1, the

control, received treatment with BAL containing no

cells. Group 2 received BAL containing dog hepatocytes

and group 3 received BAL with pig hepatocytes. Within

5 h of devascularization all animals in the control group

demonstrated increases in lactate, transaminases, lac-

tate dehydrogenase and a signi®cant decrease was seen

in the blood glucose, pH and mean arterial blood

pressure as compared to the BAL/hepatocyte treated

animals.102

This study demonstrated that there was no signi®cant

difference in the use of allogeneic or xenogeneic cells

Table 3. Hybrid hepatic support in man

Hybrid hepatic support Bioreactor Subjects Parameters Outcome

BAL (Uncontrolled trials)

Chen et al.113 Plasma perfused

cryopreserved pig

hepatocytes (6 ´ 109) attached

to dextran-coated microcarrier

beads packed into

extracapillary space of a

hollow ®bre bioreactor

Group 1: 12 patients

(AHF)

Group 2: 8 patients

(chronics)

Improved ICP, CPP

Improved NH3, bilirubin

and glucose

12 patients to OLT

6/8 died

Watanabe et al.114 Group 1: 18 patients

(AHF)

Improved ICP, CPP 16/18 to OLT

Group 2: 3 patients

(Primary graft

non-function)

Improved NH3, bilirubin

and glucose

3 patients to OLT

Group 3: 10 patients

(chronics)

2 patients to OLT

ELAD (Pilot controlled

trial)

Survival

Group 1: 50% chance

of survival on

admission Blood perfused.

Group 1: ELAD 9

patients, control

5 patients

1 patient decreased ICP

Galactose elimination

after the ®rst 6 h was

8/9 patients (89%)

4/5 patients (80%)

Ellis et al.110 200g of C3A

hepatoblastoma cells

no different between

groups, as was NH3,

Group 2: met criteria for

OLT on admission

in attachment culture

on outer surface of

hollow ®bre membranes

Group 2: ELAD

3 patients, control

3 patients

factor V levels and

arterial ketone

body ratios

1/3 patients (33%)

1/3 patients (33%)



within the BAL bioreactor. However, only single

treatments lasting 4±6 h were performed and the effect

of xenogeneic hepatocytes might have become more

apparent upon subsequent use. BAL and charcoal

perfusion were compared in a canine model of hepatic

ischaemia in 1993. Group 1 (n � 7) was treated with

BAL; group 2 was plasmapheresed and treated via a

charcoal column only. After 6 h group 1 had signi®-

cantly lower blood ammonia and normalized prothrom-

bin times. Blood glucose was also signi®cantly higher in

group 1 than in group 2.103

BAL was soon clinically applied and an early case

report demonstrated signi®cant improvements in hae-

modynamic stability, increased presence of clotting

factors, reduction in serum ammonia levels and also

improved mental state following a single period of BAL

treatment lasting 6 h in a patient with severe decom-

pensated alcoholic liver disease.111 This demonstrated

that BAL could be used safely in man and in 1993, 10

patients were treated with BAL for AHF. Eight patients

showed improvement in ICP, CPP, transaminases,

ammonia levels and clinical encephalopathic state

leading to OLT.112

In 1996 Chen et al.113 examined the effect of BAL on

two groups of patients. Group 1 consisted of 12 patients

with AHF all of whom were bridged to OLT. Biochemical

parameters, blood glucose, ammonia and bilirubin

improved signi®cantly in this group, as did ICP and

CPP. Group 2 contained eight patients with acute

decompensation of chronic liver disease of whom six

patients died.

Watanabe et al.114 presented similar ®ndings with three

groups of patients treated with BAL: group 1, patients

with AHF (n � 18); group 2, three patients with primary

non-function of transplanted grafts; and group 3, 10

patients with decompensation of chronic liver disease

(not regarded as candidates for OLT). Analysis of the

results revealed signi®cant improvements in bio-

chemical and clinical parameters as seen previously

(Chen et al.113). All but two patients in groups 1 and 2

went on to have OLT. Two surviving patients from group

3 also went forward for OLT at a later date.

A multicentre controlled trial of the use of BAL in AHF

is currently taking place, the results of which are

eagerly awaited.

Limitations. The use of isolated primary cryopreserved

hepatocytes presents some immunological and micro-

biological concerns. Immunological attack will come

from the host, by both cellular and humoral mecha-

nisms. Naturally occurring antibodies will attack the pig

cells. The synthesis of foreign plasma proteins, coagu-

lation factors and transport factors by the pig hepato-

cytes have signi®cant immunological effects on the host,

including antibody formation. Patients have been

shown to develop type III hypersensitivity (serum

sickness). Coagulation factors produced by the pig

hepatocytes used in the BAL have been shown to cause

immune complex deposition in animal and human

studies and may contribute to end organ dysfunc-

tion.115 Some investigators have avoided this problem

by manipulating cell lines and abrogating protein

production.116, 117

The use of xenogeneic material also raises the possibility

of the transfer of viral or prion disease to the host. Recent

reports of pig endogenous retrovirus (PERV) infections

in man led to concern over the use of pig hepatocytes in

bioarti®cial support systems. Retrospective studies of

patients treated with BAL have shown no evidence

of PERV DNA in blood samples. Similar results have been

found in patients who have received other forms of

porcine tissue (pancreatic islets and heart valves).118±120

However, the long-term aetiology of viral or prion

transfer in immuno-compromised patients is still an area

that is unclear and highly controversial.

HEPATOCYTE TRANSPLANTATION

Injection of hepatocytes into an individual with AHF

has the potential to replace detoxifying and synthetic

function, provided cells access blood or tissue ¯uid in

equilibrium with plasma. Sutherland et al.121 in 1977

demonstrated improved survival following transplanta-

tion of 1.5±2.0 g hepatocytes intra-portally or intra-

peritoneally after dimethylnitrosamine toxicity in rats as

compared to controls. Sommer et al.122 used D-galactos-

amine in rats as the AHF model and showed similar

survival. However, the bene®ts of transplantation could

also be obtained by injection of irradiated hepatocytes,

hepatocyte supernatants and homogenates, suggesting

that substances other than the hepatocyte itself may be

eliciting bene®cial effects.

Surgical models of AHF (90% hepatectomy) in animals

were also used to demonstrate improved survival.123, 124

As with previous experiments, improved survival was

evident if transplantation was carried out prior to or at

the same time as the original insult. Advances in

biotechnology have led to the concept of encapsulation



of hepatocytes. This protects the hepatocytes by reducing

immunogenicity, mechanical trauma and also allows

three-dimensional structures to develop within the semi-

permeable capsules.125 Improved results are also found if

the hepatocytes are co-cultured and transplanted with

non-parenchymal cells. These developments may en-

hance cell-to-cell interactions, improve cell viability and

may reduce the incidence of host rejection.

The clinical use of hepatocyte transplantation has been

limited due to the dif®culty of providing adequate and as

yet unknown quantities of hepatocytes. These trans-

planted cells are required to function optimally in a toxic

environment. Both Mito and Kusano126, 127 demon-

strated little bene®t from intrasplenic transplantation of

hepatocytes isolated from nine chronic hepatitics and

cirrhotics (their own left lateral segments). Habibullah

et al.128 delivered 60 ´ 106 cells/kg body weight of

human foetal hepatocytes (isolated from 2 to 34 week

gestational foetuses) intra-peritoneally into seven AHF

patients. Overall survival was 43% compared with 33%

in matched controls. Further small studies have been

reported. Soriano et al.129 reported three children in

AHF who were treated with intra-portal injection of

cryopreserved hepatocytes taken from unused donor

segments. One out of three children survived and some

biochemical parameters improved post-transplantation.

Bilir et al.130 showed improved encephalopathy, serum

ammonia levels and prothrombin times, following

percutaneous cryopreserved human hepatocyte trans-

plantation in three patients with AHF. Strom et al.131

performed a prospective controlled trial of transplanted

isolated fresh and cryopreserved human hepatocytes as

a bridge to transplantation. Five hepatocyte transplant

recipients with grade IV encephalopathy and multi-

organ failure and four patients of equal illness severity

with AHF were studied. Medical treatment resulted in

signi®cant improvements in the biochemical markers of

AHF, including blood ammonia. No improvement was

seen in haemodynamic stability or cerebral stability and

all died within 3 days. Those receiving hepatocyte

transplants maintained normal cerebral perfusion and

haemodynamic stability with signi®cant reductions in

blood ammonia and liver injury markers. All were

transplanted within 2±10 days.

This area requires further re®nement and investigation

to demonstrate de®nite clinical impact. Overall, it seems

challenging to develop techniques for acute failure, if

the estimate of » 3 ´ 1010 fully functioning hepatocytes

are required is correct, due to the dif®culties of

establishing the conditions for effective function imme-

diately. On the other hand the technique may be of

greater potential in chronic disease, and can also be

modi®ed for gene transfer.

CONCLUSION

The clinical sequelae of AHF re¯ect the multi-faceted

dysfunction that takes place once the liver fails.

Recognition of the extreme nature of these problems

has led to the development of specialist centres, which

have been instrumental in reducing the mortality

associated with this devastating syndrome. Treatment

of clinical problems in AHF such as cerebral oedema,

systemic hypotension and renal failure may be tempo-

rarily valuable until either transplantation takes place

or the organ begins to regenerate. Greater understand-

ing of the biochemistry of systemic in¯ammatory

response syndrome, sepsis and AHF have led to

evidence-based treatment of clinical problems. The

complexities of liver function and dysfunction are still

not completely understood and therefore it is naive to

expect arti®cial support systems to correct and/or

replace the synthetic, metabolic and excretory functions

that are associated with a healthy organ.

Currently, arti®cial support systems demonstrate im-

provements in some clinical and biochemical parame-

ters. Effects on survival are still not clear and the results

of multicentre randomized controlled trials are awaited.

The design of such trials poses several problems. AHF is

a rare disease and therefore to recruit the appropriate

number of patients into both the control and active

treatment arms will take some time unless the multi-

centre approach is used. Problems will arise in the

matching of patients within the trial, as the syndrome is

both disparate and varied in its pattern and presenta-

tion. Present treatment for AHF is liver transplantation,

which is well established with low morbidity and

mortality. Following patient recruitment, it would be

unethical to deny or delay transplantation if available.

This may result in signi®cant loss of numbers. The

varying aetiology of AHF has a great in¯uence on

patient survival. Therefore groups will have to be

subdivided according to aetiology and controlled studies

will have to examine the differences within each group.

Separate studies will have to take place to look at the

decompensated chronic liver disease group. These

patients may also have to be studied according to the

aetiology of their disease.



The nature of international multicentre trials raises

issues such as the fact that AHF classi®cation differs

between countries, as does the incidence, epidemiology

and treatment of different aetiologies. Intensive care

units and specialist centres have differences in medical

and nursing standards as well as marked differences in

management of the critically ill. Evidence exists that

genotypic variation amongst patients may introduce

degrees of susceptibility to the immune and physiological

response seen in AHF and sepsis. This may have an as

yet unexplained effect on individual patient survival.132

It is clear that precise end-points are required for such

a trial. Design of a multicentre controlled trial will be a

dif®cult task, the results will take time to accrue and

must in the ®rst instance be carefully interpreted.

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Review article: liver support systems in acute hepatic failure

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